Production of Motion and Locomotion - Muscles

Muscles are characteristic of animals rather than of plants. When danger is near animals may make an escape but plants must stand still wherever fate has placed them. Animals employ muscles in the daily business of food-getting, and also by means of them go afield in quest of mates. Muscular movements, which play an important part in courtship, enable animals to exercise some degree of choice in the altruistic matter of reproducing their kind. With non-muscular plants, on the other hand, “forced marriage” without choice, and by some outside agency such as wind, water, or officious insect, is the universal rule.

Not only in locomotion, or bodily movement from place to place, but also in motion, or the movement of the organs of the body, do muscular animals have a great advantage over plants, for plants must depend upon such physical agencies as wind, internal turgor, or unequal growth in order to effect the rustling of their foliage, twining of their tendrils, and closing their leaves in “sleep.”

From the insignificant flatworms up to man evolutionary development is particularly mirrored in the musculature, which has dragged with it all the other systems. The bobolink on the wing over a sunny meadow sings a cheerful hymn of praise to the muscles, and conquering man by means of muscles is able to go forth to the four corners of the earth.

By the agency of musculature man lies down, stands up, sits, walks, runs, jumps, creeps, dances, skates, skis, writes, drives, paints, makes a machine, and plays a violin. What other animal can do all this!

The actions of animals, which “speak louder than words,” are brought about by the triple agency of nerve, skeleton, and muscle. Of this trinity of parts the muscles, or “flesh,” constitute the greatest bulk, making up in man approximately half of the total weight. Although the skeleton, or scaffolding, is the primary factor in determining the form of the body, the muscles that drape the skeleton are chiefly responsible for the characteristic contours which give it grace and beauty of outline.

It is very essential for the surgeon to possess detailed knowledge of the separate muscles of the human body. Consequently careful training in Myology, or the science of muscles, is a part of the indispensable discipline that every well-equipped medical student must undergo, and this knowledge can be attained only by careful and repeated dissections, rather than by reading descriptions of muscles in books. A superficial comparative survey of the muscular system in different vertebrates, which is all that will be attempted in this chapter, may, however, be profitably undertaken by any student, whether or not he is looking toward a medical career.

Muscular Activity

Muscular activity, or the way animals expend their energy, has its morphological basis in cellular units, which, unlike inorganic substances, possess the power to increase with use. Muscle cells, although presenting nothing fundamentally new in cell structure, are specialized in the matter of contractility, which is one of the universal properties of protoplasm.

Elastic myofibrils, that shorten in only one direction, appear within the muscle cell, and these are capable of causing movement not only within the cytoplasm of the muscle cell itself, but also outside of it, thus effecting both motion and locomotion.

Muscular movement is always the result of muscular contraction. When, for example, a leg is bent by shortening one set of muscles, it is restored to its original position not by the relaxation of these muscles but by the contraction of an antagonistic set on the other side of the leg (Fig. 578). Usually the group of muscles that flexes, or brings the appendages close to the body, is stronger than its opponents. In the chewing muscles of the jaws those that effect the bite are stronger than their opponents which open the jaws for another bite.

Antagonistic muscles of the human leg

Whether this fundamental difference in opposing muscles has any deep evolutionary significance or not is open to speculation, though natural selection is always on the lookout for whatever may contribute to individual preservation. In any case the centralized drawing in of the arms by flexion is better adapted for defensive purposes than the opposing gesture of wide-flung extension of the appendages.

Aside from accomplishing countless necessary movements of different parts of the body, and serving as the engine of propulsion for the animal machine, muscular activity contributes in a variety of ways to the welfare of the animal organism. For example, the exercise of the muscles constantly changes the character of the blood and lymph, since more carbon dioxide is given off from the blood during exercise than at other times; vascular congestion is decreased by the muscular dilation of the arteries, and at the same time the ventilation of the lungs is increased; the lymph flow is promoted; peristalsis is stimulated; and the heart is trained not only to continuous performance but to meet emergencies.

The muscles of the living body are never entirely relaxed, but maintain a tonus, that is, a condition of balanced tension between opposing sets. In standing erect there is ceaseless employment of opposing muscles, and when the body wall is pierced by a bullet the resulting hole is more slit-like than round, showing that the muscles involved are always somewhat taut even when not effecting motion. Considered in all aspects the muscular work done by an ordinary ambulatory human being during a lifetime amounts to a dizzy total.

The duration of contraction and the interval elapsing before the next contraction vary greatly in the muscles of different animals. It has been reckoned, for example, that the duration of contraction in cold-blooded turtles and frogs is one second and one fifth of a second respectively, while in man it is one tenth of a second. A violinist can make ten muscle movements per second in executing a trill with his fingers, but a buzzing house fly has a record of 330 muscular movements per second. What a Paganini a fly would make!

Fatigue following muscular activity results from the release of lactic acid into the blood stream. Muscles always react in the same manner, by a change of form accompanied by the release of lactic acid and the production of heat, regardless of the category into which the stimulus applied may fall, whether it is nervous, chemical, physical, or mechanical. Consequently a muscle cannot be described as exclusively a thermo-dynamic, a chemo-dynamic, or an electro-dynamic apparatus.

Muscles as Tissues

Muscles may be considered from the point of view of tissues or of organs. There are three sorts of muscle tissues (already briefly described in Chapter VII as smooth, striated, and cardiac), which differ from each other in origin, histological appearance, physiological action, and distribution. Each muscle cell connects with a nerve ending that controls its behavior.

Smooth muscle cells, which have to do with the events that go on inside the body, are derived from mesenchyme, are comparatively short, have a single nucleus, and are frequently isolated or combined in thin sheets. For example, every hair is provided with an individual pomade tube in the form of a sebaceous gland, administered by its own private smooth muscle in the skin which makes the hair “stand on end” and also presses the secretion from the gland.

In the walls of blood vessels smooth muscle cells are arranged mainly in circular fashion, thus decreasing the bore of the passage-way upon contraction, while the enlargement of the lumen is brought about by blood pressure. They occur not only in the walls of blood vessels and in the skin but also in the walls of the digestive tract, the urogenital passages, and ducts of various kinds, thus forming parts of other organs rather than whole organs in themselves. Since they are supplied from the autonomic nervous system they are comparatively slow in action and are involuntary.

Striated muscle fibers are elongated cells which constitute most of the musculature of the body, particularly the muscles with skeletal connections. They differ from smooth muscle cells in at least the following points, being (1) anatomically more complicated; (2) embryonically younger; (3) connected with the voluntary nervous system and consequently under the control of the will; (4) quicker in action; (5) more easily tired; (6) less stretchable; and (7) weaker in effect. The enormous power of extension of the smooth muscles is shown in the great capacity for dilation possessed by the stomach, for example, or the urinary bladder, while the remarkable strength exerted by them in contrast to skeletal muscles is demonstrated by the extraordinary expulsive power exerted by the walls of the gravid uterus. In general the strength of striated muscles is proportional to their thickness, while the degree of contractility is dependent upon their length.

Cardiac muscle cells or fibers, which are intermediate between smooth muscle cells and striated muscle fibers, are of mesenchymal origin, being modified from the tunica media of an embryonic blood vessel. They are short multinucleate structures, usually branched or anastomosing together and showing faint striations. Unlike true striated muscle cells, they are involuntary in action. They occur in the walls of the heart, as well as of the pulmonary arteries and veins, and also in the roots of the aorta.

Muscles as Organs

While smooth and cardiac muscle cells form mainly tissues, which are parts of various organs, striated muscle cells combine to form organs with more or less morphological and physiological unity. Connective tissue sheaths, while making possible the action of a muscle as a whole, separate single parts of muscles so that their identity is not always easy to determine. A further complication is encountered by the comparative anatomist in determining the homologies of muscles in different vertebrates, since the names applied to them are borrowed from human anatomy, although frequent changes in function, liable to throw the investigator off the evolutionary track, have occurred.

Anyone with experience in the dissecting rooms of a medical school is well aware of the difficulty in trying to make a dissection invariably meet the expectation of the manual, for there is great individual variation in separate muscles, and even in homologous muscles on the two sides of the same individual. As a matter of fact muscles are not nearly as conservative as bones, teeth, and nerves. There is consequently a difference of opinion as to how many muscles are to be accounted for in man. One authority lists 639, of which 5 are unpaired and 317 are paired, with the following distribution: head, 53; neck, 32; back, 180; breast, 54; belly, 15; legs, 124; arms, 98; and viscera, 83. Testut has written an impressive tome of 900 pages, Les anomalies musculaires chez I’homme, 1884, which deals mostly with variations found in the muscles of the human body.

None of the paired muscles oversteps the midline which divides the body into right and left halves. It is not unusual, however, for a pair of muscles to develop more on one side than on the other, resulting ordinarily in right and left handedness. Statistics show that nearly 96 per cent of humankind are “righthanded,” and consequently we may be said to have a righthanded civilization. This is recognized in the way we make our screws, scissors, firearms, watches and clocks, buckles and buttonholes, coffee mills and hand-organs, violins and flutes, and in the way we shake hands. Some famous lefthanders who made a notable impression in a righthanded world are Leonardo da Vinci, Michelangelo, Holbein, Menzel, and Napoleon.

The biceps muscle may be taken as a typical representative (Fig. 579) of a muscle. It consists of an enlarged middle portion, the belly, with tapering ends, and is surrounded by a connective tissue sheath which at the ends becomes continuous with tendons, that in turn merge into the periosteum ensheathing the bones, thus securing anchorage for the muscle. One end of the muscle, the punctum fixum, where it is attached to the most stationary part of the skeleton, is the origin. The other end, the punctum mobile, where it connects with the part of the skeleton which it moves, is the insertion. Upon the contraction of a muscle the insertion is always pulled towards the origin. There may be several insertions, as in the serratus muscles along the back, or there may be more than a single origin, for example, in the biceps there are two present, giving it the name “biceps.”

The biceps muscle, showing origin and insertion

Sometimes, as in the trapezius muscle, which moves the head and shoulder, the punctum fixum may become the punctum mobile, according to the movement to be effected.

The wide range of variation in form (Fig. 580) is a necessary adaptation in the accomplishment of different movements. The original form of embryonic muscles shows sheets of fibers, or myotomes, extending between partitions of connective tissues, similar to the arrangement of muscles on the sides of the body of a fish. Out of this primitive alignment modifications are initiated (1) by delamination, or splitting flatwise; (2) by splitting lengthwise; (3) by proximo-distal division; and (4) by various degrees of fusion.

Superficial muscles of the back. The tendon of Achilles

The shapes that muscles assume also exhibit a great variety. The word itself (musculus, little mouse) indicates an originally rather compact form that has undergone much adaptive streamlining in successfully accomplishing different movements. The muscles of the limbs are often fusiform, like the shape of a single smooth muscle cell, since this type is less bulky for the amount of muscular tissue involved than some other shapes. Triangular muscles appear where there are broad origins and narrow insertions, such as are found in the deltoid and pectoralis groups. Sheet muscles occur in situations like that occupied by the diaphragm, where the work to be performed is best served by this morphological form.

It is not always easy, as already pointed out, to delimit a muscle, because of the changes brought about through functional necessity. The best criterion, however, for homologizing a muscle is its nerve supply. A nerve once assigned to do duty with a particular muscle follows it through all its vicissitudes, just as a faithful dog, trotting behind its master, serves to identify him, regardless of the different costumes or disguises which the master may assume.

A striking illustration of the constancy of nerves to transforming muscles is furnished by the phrenic nerve that supplies the diaphragm, which is a migratory muscle laid down originally far anterior in the neck region. With the backward shifting of the heart the diaphragm finally assumes an abdominal position remote from the neck, yet the phrenic nerve, although made up from the third, fourth, and fifth cervical nerves, goes out of its way to retain connection with it and to proclaim its origin.

As already indicated tendons, or sinews, are means by which muscles are attached to bones, and in this capacity they serve a double purpose. In the first place they enable soft, delicate, contractile muscles to gain a firm, tenacious grip upon solid skeletal parts, whereby motion may be effected. They also enable large muscles that make up the bulky part of the body to be packed in out-of-the-way situations, sometimes at considerable distances from the work to be performed, where they will not interfere by their bulk with the free movement of the joints. The “tendon of Achilles” (Fig. 581), like the electric cable that transmits power generated at Niagara Falls to industries in the city of Buffalo, for example, is the most efficient arrangement for the ankle to have, because muscles connected with it are largely concentrated out of the way at the “calf of the leg,” rather than around the ankle itself, where they would interfere with the freedom of the movements they are detailed to perform.

Muscles generally are encased in connective tissue sheaths, or epimysium, which are more or less continuous by means of connecting tendons with similar periosteal sheaths surrounding skeletal parts. Connective tissue elements, perimysium, also extend even between groups of muscle fibers themselves, separating them into irregular bundles, or fasciculi.

Sometimes ossifications occur, embedded within tendons at points of friction. These are called sesamoid bones, the most conspicuous example being the kneecap, or patella (Fig. 581), mentioned in the preceding chapter. Sometimes an entire tendon ossifies, as may be seen on the sides of a turkey’s “drumstick.”

The topographical relationship of neighboring muscles to each other and to the bone foundation which they surround is typically indicated by Figure 582, in which is represented a diagrammatic cross section of a cat’s front leg above the elbow. Blood vessels and nerves are shown between the muscles.

Muscles of the front leg of the cat, in cross-sectional view

Movements brought about by muscles are described by a variety of terms, including: flexion, extension, adduction, abduction, elevation, depression, rotation, constriction, and dilation. Flexion is the bending of a part, as of the arm at the elbow, while extension is the opposite action, the bringing of two parts into line with one another, as in straightening the arm. Adduction is the swinging of an arm or leg ventrally, usually toward the median plane; the opposite action, abduction, is the swinging of the appendage dorsally. Elevation is the pulling of a part dorsally, for example the “drawing back” of the shoulder-blade (scapula) or the raising of the lower jaw to close the mouth; depression is the drawing of a part ventrally and includes the “dropping” of the lower jaw in opening the mouth. Rotation of parts on one another includes supination, or the turning of the palm of the hand to face upward, and pronation, or the turning of the palm to face downward. Constriction is the function of sphincters, such as those which close the mouth or anus or reduce the size of the pupil of the eye; dilation is the opposite action, by muscles which enlarge the opening.

Embryology of Muscles

When mesoderm enters the the scene in vertebrate development, there is much more of it along the dorsal side of the embryo, epimerically, than on the ventral side, hypomerically. This distribution is not true of invertebrates, such as annelid worms, in which the mesoderm is about equally distributed all around the body. The reason for the excess of mesoderm along either side of the notochordal level in vertebrates is that this region, after giving off dermatomes and sclerotomes, becomes the myotomes, or muscle plates, from which nearly all of the striated muscles of the body are derived. While the major parts of the myotomes remain alongside the developing vertebral column, portions grow ventrally into the region between the parietal mesoderm of the hypomere and the integument until only a thin sheet of connective tissue, the ventral septum, lies between the ventral ends of the myotomes derived from opposite sides of the body.

During these embryonic changes the muscle cells, originating in the epimeric myotome, become rearranged so that their long axes come to parallel the long axis of the body, while between the myotome masses are developed mesenchyme partitions of connective tissue, myocommata, to which the muscle fibers are primarily attached. The end result, as well shown in amphioxus, is a series of shaped muscle plates, extending along the sides of the body and separated from each other by myocommata. These muscle plates are destined to form the axial muscles of the body from which the appendicular muscles secondarily bud forth, as soon as the skeletal parts of the appendages themselves arise and furnish a foundation for muscular attachment.

Scheme to illustrate the disposition of the myotomes in the human embryo

Even in man (Fig. 583) the embryonic arrangement of body muscles is metameric at first, from which primitive arrangement the complications of adult musculature are subsequently derived. Involuntary muscles generally are mesenchymal in origin, but the iris muscles, as well as those associated with the skin glands, are ectodermal.

Kinds of Voluntary Muscles

In The History of the Human Body, 1923, by H. H. Wilder an excellent analysis of the comparative anatomy of the muscular system classifies the voluntary muscles under three groups, namely, metameric, branchiomeric, and integumental.

The metameric group, which includes the axial and appendicular muscles, in reality takes in most of the muscles of the body. Branchiomeric muscles are associated with the primitive splanchnocranium and its derivatives, while the integumental group consists of muscles that have split off secondarily from the two preceding groups, and have taken up major associations with the integument rather than the skeleton.

The voluntary striated muscles will be briefly considered in the following order:

1. Metameric
  (1) Axial
    (a) Head
      1. Eyeball
      2. Hypoglossal
    (b) Trunk
      1. Ventral
      2. Dorsal
    (c) Diaphragm
  (2) Appendicular
    (a) Extrinsic
    (b) Intrinsic
2. Branchiomeric
  (a) Vth cranial nerve group
  (b) VIIth cranial nerve group
  (c) IXth cranial nerve group
  (d) X-XIth cranial nerve group
3. Integumental

Metameric Muscles

Axial muscles. - Primarily the axial muscles, which are the first to appear both embryonically and phylogenetically, are arranged with regularity in myotomes down the side of the body, fitting into each other like a nest of spoons. There may be sixty or more pairs of these myotomes in amphioxus, varying somewhat in symmetry on both sides of the body. When their longitudinal fibers contract, they exert a pull upon the myocommata that separate the myotomes from each other, and secondarily by this means upon the stiff notochord, the outer sheath of which is continuous with the myocommata.

The myotomes are all practically alike, though diminishing in size toward the tail. With the advent of the head in fishes the first real modification of this primitive form of musculature appears in the fusion of some of the myotomes and a consequent breaking up of the regular metamerism of the axial muscles.

Head. - Skull muscles are few in number, being reduced in correlation with the increasing absence of movable skeletal parts in this region. The three anterior pairs of myotomes (Fig. 583) become the six pairs of muscles that move the eyeballs within their sockets (Fig. 584). As these muscles are quite conservative, they exhibit only minor modifications throughout the vertebrate series. The most anterior identifiable myotome, which is supplied by the third cranial nerve (oculomotor), gives rise to the superior rectus, the internal rectus, the inferior rectus, and the inferior oblique muscles.

Extrinsic muscles of the left eyeball of man

The myotome next posterior, which is supplied by the fourth cranial nerve (trochlear), becomes the superior oblique muscle, and is followed, after a gap that probably represents a missing myotome, by one supplied by the sixth cranial nerve (abducens), that is responsible for the external rectus muscle, and also the retractor bulbi which first appears in amphibians.

Incidentally this latter muscle has a unique function, for in addition to pulling down the eyeball of a frog, which pops up like a jack-in-the-box as soon as the muscle is relaxed, the retractor bulbi, in the absence of a hard palate, enables the eyeball to aid in gripping whatever struggling prey may be captured within the mouth cavity. The prehension of food by the eyeballs is a unique adaptation not generally employed in the animal kingdom!

Extrinsic muscles of the human tongue

Included among the derivatives of the myotomes of the head, in all vertebrates except fishes, are the muscles supplied by the XIIth cranial nerve (hypoglossal). This nerve, with its muscle segments that have to do with the tongue (Fig. 585) probably represents the fusion of several neuro-muscular units that have been incorporated into the head region from the metameric vertebral series at the anterior end of the spine. In most vertebrates these muscles, namely, the hyoglossal, styloglossal, and geniohyal, are extrinsic, that is, not forming a part of the tongue itself, but in mammals an additional muscle, the intrinsic lingualis, makes up the bulk of the fleshy tongue.

Trunk. - In amphioxus and cyclostomes, without a definite lateral line, the muscles of the trunk and tail are undifferentiated, but in fishes they are divided as a rule by the horizontal skeletogenous system into the epaxial and hypaxial regions which are supplied respectively by the dorsal and ventral rami of the spinal nerves. These two general muscular regions undergo different fates in the course of further evolution and development.

The ventral trunk musculature of urodeles becomes delaminated into four sheets of muscle with the gradual obliteration of the myocommata that separate the myotomes from each other. Along the midventral lines on both sides of the linea alba, the muscle fibers still retain their original longitudinal arrangement, together forming a flat band of muscle, the rectus abdominis. Laterally, however, the body wall is composed of three layers, with the fibers of each layer assuming different directions. Next to the peritoneum the innermost layer, or transversus abdominis muscle, has fibers tending to run around the body at right angles to the long axis. Outside the transversus are two diagonal layers, the internal and external oblique muscles, having fibers at right angles to each other (Fig. 586).

The musculature of a salamander

In reptiles such as lizards and alligators, the ventral axial muscles are still further modified in the anterior half of the trunk region by the introduction of encircling ribs. Posteriorly in the belly region the original three layers of muscles remain unchanged, but anteriorly in the thoracic region the oblique muscles become broken up into external and internal intercostal muscles, which extend from rib to rib and aid in respiratory movements. In the neck region in front of the ribs the oblique layers furnish material for the scalenus muscles.

Among birds the oblique muscles are poorly developed, the transversus is absent, and the rectus abdominis, reduced in size, is posteriorly unsegmentated. This sacrifice of the ventral axial muscles is compensated, however, by an excessive elaboration of the appendicular muscles of flight, which come to overlie the muscles of the body wall. A familiar example of them is the “white meat” on the breast of roast chicken.

The oblique muscles in mammals give rise to the intercostal and serratus muscles, which come to assume more of a dorsal than ventral position, although still supplied from the ventral branch of the spinal nerves (Fig. 587). Other mammalian muscles of ventral axial origin are the psoas muscles of the posterior abdominal wall, and the colli muscles of the neck region.

The muscles of a cat

Furthermore, even in the reptiles the rectus abdominis muscle loses its primitive character by the introduction of the neosternum, so that it is broken up into presternal and poststemal parts. Some of the prestemal muscles are the sternohyoid, sternothyroid, and thyrohyoid of the neck region. The poststemal musculature retains somewhat more of its primitive character, even up as far as the mammals, where traces of the myocommata in the form of the inscriptiones tendineae may still be seen through the skin as depressions of transverse connective tissue interrupting the broad flat abdominis muscle. These depressions are represented by sculptors of classical times on the abdomens of many Greek heroes, a curious incidental evidence of the retention of these telltale ancestral marks in man until comparatively recent times (Fig. 588).

The rectus abdominis muscles in man, showing transverse inscriptiones tendineae

The myotomes of the tail region are of great importance to fishes, enabling them to swim. Although little modified these primitive myotomes function adequately because the lateral movements of the tail are not complicated. In higher forms the caudal myotomes lose their original significance, and become reduced into muscles modified sufficiently to control the varied tail movements of such land animals as support a caudal appendage. Others become entirely freed from skeletal connection in the anal region and are transformed 'into' sphincter muscles.

The dorsal axial musculature differentiated more slowly, but eventually, with the development of vertebral processes offering places of attachment, it gives rise to a greater array of separate muscles than is found below the lateral line. In the consideration of these dorsal axial muscles care must be taken to avoid confusion with the appendicular muscles with which they are intimately involved.

Fishes and urodeles show little differentiation of dorsal-axial musculature, but reptiles have gone far enough in the elaboration of the neural arches of the vertebral column and its processes to give foothold to a large number of small muscles that, with their tendons and ligaments, tie together the different vertebrae.

The largest dorsal muscle is the longissimus dorsi, which extends lengthwise between the neural spines and the transverse processes. Groups of shorter muscles, metamerically arranged, extend (1) from one transverse process to another, intertransversales; (2) from the neural spine of one vertebra to the transverse process of the one in front of it, transversospinales; (3) from the transverse process to the ribs, transversocostales; and (4) from one spine to another, interspinales.

In birds the dorsal muscles of the rigid thoracic area are largely atrophied, except in the neck region where they are very much elaborated, as may be readily seen in the neck of a roast chicken.

The dorsal axial muscles of mammals resemble those of reptiles rather than those of birds, but are influenced more than in either of these groups by the appendicular musculature which increases much in importance.

Diaphragm. - The lung-filling diaphragm, ordinarily unseen and unsung but described by one appreciative biologist as the indispensable “creator of all human acts,” is an organ that plays a necessary part in the release of energy in every mammal.

It is a dome-shaped muscular structure coming from the ventral axial elements in the neck region and assuming, after gradual displacement and growth, a transverse position across the bottom of the thoracic basket. It is pierced by the esophagus and by large arterial, venous, and lymphatic trunks, as well as by the tenth pair of cranial nerves (vagus), and the sympathetic trunk. Its own supply, as already stated, comes from the cervical region in the form of the phrenic nerve.

The non-muscular transverse septum, which partitions off the pericardial cavity from the body cavity in lower vertebrates, probably contributes to the median ventral quarter of the muscular diaphragm of mammals.

Appendicular muscles. - Muscles of the appendages are derived from myotomes of the axial musculature. In the case of elasmobranch fishes, for example, at the region where the pectoral and pelvic fins are to become established, the ventral ends of the lateral myotomes sprout out myotome buds, two for each myotome, which later become the fin muscles. Not only do the myotomes which are exactly opposite the future fin produce these buds, but several others immediately anterior and posterior to them also crowd together, adding contributions. As a result a generous number of muscular elements takes part in the formation of the fin musculature, the accompanying augmented nerve supply being sufficient to form a plexus of nerves which adequately insures effective performance of the fins.

Most of the appendicular muscles of mammals are considered to be primarily of myotome origin despite the fact that during embryonic development they arise, not from myotomic buds as in fishes, but from mesenchyme cells which migrate into the limb-buds.

In general the locomotor muscles may be classified into two groups, extrinsic and intrinsic, although the distinction is not always unmistakable.

Extrinsic Muscles. - Extrinsic muscles serve the girdles and the proximal ends of the appendages. They connect the appendages with the axial skeleton and move the limbs as a whole. Intrinsic muscles, on the other hand, have both origin and insertion within the appendages, effecting movements only in parts of them.

Among fishes the extrinsic group is most in evidence, consisting of elevators and depressors, which lift and lower, and abductors and adductors, which extend the fins and hug them next the body respectively. All of these muscles move the fin as a unit, and since there is little occasion, in the performance of mass movements useful in a water medium, for the niceties of movement effected by intrinsic muscles, these are absent.

The extrinsic muscles of the pectoral appendages are better developed than those of the pelvic appendages, owing to the difference in attachment of the respective girdles to the axial skeleton. The girdle of the pelvic appendages is anchored securely to the backbone, and so they require fewer intermediary muscles than the pectoral appendages, which are often entirely unattached by direct skeletal elements. In the pectoral appendages, particularly of birds, the extrinsic muscles reach a high degree of development, being packed for the most part on either side of the keeled sternum, and quite covering over and obscuring the axial muscles. The muscles of flight in a pigeon, for example, may equal as much as one fifth of the entire body weight.

Intrinsic Muscles. - The intrinsic muscles of the appendages, which increase in number with the establishment of systems of appendicular levers in connection with land life, include pronators, supinators, and other rotators as well as extensors and flexors of the forearm, shank, hand, foot, and digits. In primates, including man, these muscles are more primitive than in many other vertebrates. This is because the generalized pentadactyl skeletal framework is still retained in primates, while in many other vertebrates there is a reduction of distal skeletal parts and a consequent modification of the musculature. The generalized pentadactyl appendages of man are among the factors contributing largely to his dominance. More possible avenues of diversified activity are by this means left open than when extreme specialization takes place, as in the flipper of a whale, the wing of a bird, or the leg of a horse.

Branchiomeric Muscles

The muscles of the splanchnocranium are derived from the ventral hypomeric part of the mesoderm which remains after the myotomes have become established. They are associated with the skeletal parts representing a “boom that failed,” in the evolutionary sense. Consequently they undergo many makeshifts with the modification or disappearance of their original skeletal connections. These muscles of the splanchnocranium, although visceral in origin, are striated and voluntary in action.

The branchiomeric musculature is most evident in elasmobranch fishes, where four groups according to nerve supply may be clearly distinguished, namely, the trigeminal, facial, glossopharyngeal, and vagus. Despite all of the changes in the branchial arches from elasmobranchs to mammals the same four groups of muscles are retained in higher forms.

To the trigeminal-nerve (V), or mandibular-arch, group belong such muscles of mastication as the temporalis and masseter, which raise the lower jaw, and the anterior belly of the digastric, which depresses this structure. In man the masseter, which runs from the lower edge of the malar bone to the outside of the lower jaw, may be easily demonstrated by placing the finger tips upon the cheeks in front of the ears and biting the teeth together. In the same way the temporalis may be located by pressing upon the temples and biting. Another member of this group in mammals is a tiny middle-ear muscle, the tensor tympani, with its insertion on the malleus, a derivative of the embryonic mandibular arch, as we have seen in Chapter XVII.

The facial-nerve (VII), or hyoid-arch, group includes the stylohyoid, extending between two parts derived from the embryonic hyoid arch, and the posterior belly of the digastric, as well as the muscles of facial expression described in the section on integumentary muscles which follows. The stapedius, which is inserted on the stapes, a derivative of the embryonic hyomandibular, is probably the smallest striated muscle in the vertebrate body.

The glossopharyngeal-nerve (IX) group, associated with the first-functional-gill arch of elasmobranchs, contributes to the pharyngeal musculature of mammals.

The vagus-nerve (X) group includes the muscles which spread open and close the last four gill arches in respiration in elasmobranchs, and so long as they remain in other groups. In higher forms, when the spinal accessory nerve (XI) becomes admitted to the fraternity of cranial nerves, it joins with the vagus. In mammals these two nerves together innervate most of the pharyngeal muscles as well as those of the larynx. In addition they supply two muscles connected with the pectoral girdle, the trapezius and the sternocleidomastoid.

Integumental Muscles

The integumental, or dermal, muscles split off embryomcally from the underlying skeletal muscles. While in many cases retaining their skeletal origins at one end and inserting under the skin at the other, they sometimes, as in sphincter muscles, lose all skeletal connection.

The tight-skinned fishes are without any dermal musculature, and amphibians have only a trace of anything of the sort in the tiny muscles that open and close the lids of the nostrils.

Snakes among reptiles use integumental muscles in locomotion, for these muscles enable the scales to get a grip on the ground. This fact can be easily demonstrated by placing an active snake upon a level surface of glass and observing the difficulty it encounters when the dermal muscles are thus made ineffective on a surface that cannot be gripped by scales.

Birds fluff the feathers by means of integumental muscles, in this way changing the thickness of the layer of warm air held next the body to regulate the body temperature. The so-called patagial muscles in the web of a bird’s wing, that assist in flight, belong to the integumental group and are derived from the pectoralis muscles of the breast, together with various muscles of the shoulder and arm.

It is in mammals, however, that integumental muscles reach their greatest differentiation, serving a wide range of uses from defence to the expression of the emotions.

European hedgehog, Erinaceus, with the skin removed, showing the manner in which it rolls up

Under defensive skin muscles may be mentioned (1) those which cause hairs and bristles to stand on end in terrifying fashion, as on the tail of a frightened cat or on the scruff of an angry dog’s neck; (2) those which erect defensive spines or quills, as in the skin of the “fretful porcupine,” Erethizon; (3) those which enable animals like the armadillos and the European hedgehog Erinaceus (Fig. 589), to roll up into an impregnable ball; and (4) muscles which tend to dislodge annoying insects by causing the skin to shudder or twitch, as on the neck, shoulders, and the anterior sides of a horse, but which are less evident or necessary on the hips and flanks within reach of the swishing tail (Fig. 590).

The integumental muscles of a horse, by means of which the skin may be shuddered for dislodging annoying insects

Integumental muscles play a leading role in the expression of the emotions, particularly in man. This phase of biology has been elaborated into a subscience by Lavater (1741-1801) and his disciples under the name of Physiognomy. Charles Darwin had something to say on this theme in his book entitled The Expression of the Emotions in Man and Animals.

Generally speaking there is no great expression of the emotions by means of facial muscles in the lower animals. The “state of the mind,” whether it be fear, anger, or excitement from any cause whatsoever, is usually shown by movements and attitudes assumed by the body, rather than by the action of the skin muscles of the face. Most animals may be said to have a “poker face,” which does not reveal what may be mentally happening behind the facial mask. Whatever expression shows is usually centered in the eyes. In the rigid face of a fowl, for instance, the lively eye gleams like a jewel.

Among carnivorous animals and primates the dermal muscles of the face take on character, until in man it is the evanescent expression wrought by facial, or “mimetic,” muscles, so difficult to analyze and to image by the artist, that remains in the memory long after the form of the body is forgotten.

The facial muscles that are most expressive in man and least developed in brutes are those located at the inner extremities of the eyebrows and at the angles of the mouth.

Voluntary integumental muscles fall into two general groups according to their derivation: first, the panniculus carnosus group from the latissimus dorsi and pectoralis muscles, and second, the sphincter coli group, from the branchiomeric musculature of the hyoid region, under the dominance of the seventh pair of cranial nerves.

The panniculus carnosus is particularly evident in mammals, although somewhat degenerate in man, and with only primitive traces showing in the lower vertebrates. It is a thin sheetlike muscle that tends to wrap about the body under the skin. In monotremes it extends over the entire bodv as far as the cloaca, and includes a sphincter marsupii, and a sphincter cloacae. In the rorqual, Balaenoptera, it likewise extends from the mouth to the anus, while in the “right whale,” Balaena, it is restricted to the head region.

Fragments of the enveloping panniculus carnosus remain longest in the axillary, inguinal, and sternal regions. These fragments make up the shuddering muscles of the horse, already referred to, and the muscles by which a wet dog shakes itself. There is also occasionally a sternalis muscle under the skin and superficial to the pectoralis, which is sometimes visible in man when well enough developed.

The progressive sphincter colli group of integumentary muscles, originally associated with the hyoid arch, is supplied by the facial (VII) nerve. In such animals as turtles and birds it is a well-developed group of muscles enwrapping the neck. During its evolution it migrated forward and expanded so as to spread over the head and down onto the shoulders, becoming differentiated into a superficial sheet of muscle designated as the platysma and the deeper lying sphincter colli proper.

With the upgrowth of the cranium that part of the platysma extending over it becomes divided into an occipital and a facial part, separated by a broad sheet of connective tissue, the galea aponeurotica, that stretches over the top of the cranium under the skin. The facial parts of the platysma and sphincter colli may be classified into four groups of muscles in close association with the underlying muscles of mastication. These are the muscles of the external ears, eyebrows, nostrils, and lips and cheeks (Fig. 591).

Mimetic muscles (muscles of facial expression) in man

The muscles of the external ear, namely, auricularis anterior, posterior, and superior, enable an animal to turn the pinna of the external ear toward the source of sound without changing the position of the head. They are better developed in animals like dogs and horses than in man, although fragments still remain enabling some individuals to entertain their friends by wiggling their ears.

The eyebrow group takes in four muscles: the frontalis; orbicularis oculi; levator palpebrae superioris; and corrugator supercilii or brow-wrinkler. The eyebrow has been poetically described as “the rainbow of peace and the bended bow of discord.”

Three more or less well-developed muscles of the nostril group are the levator labii superioris et alae nasi, by means of which man as well as beast sneers and snarls; the dilatores naris and the compressor naris, by means of which rabbits and men wiggle their noses.

Finally, the lips and cheeks group consists of a strong sphincter muscle, the orbicularis oris around the mouth opening, from which radiate several other muscles. Of these the risorius muscle, attached at the corners ol the mouth opening and pulling laterally in opposite directions, and the triangularis, pulling down the comers of the mouth, serve in humankind to express the diverse emotions of laughter and tears. The buccinator makes up much of the cheek wall.

The facial muscles of expression of the ear group may be regarded as regressive or degenerate, so far as man is concerned, having disappeared entirely in a considerable percentage of individuals, while the “psychological muscles” of the face that accompany increasing intelligence and reach their highest differentiation in man, are progressive muscles, the evolution of which is by no means yet completed.

Electric Organs in Fishes

In a few exceptional instances among elasmobranchs and teleosts, for example, the electric ray Torpedo; the electric eels, Electrophorus and Gymnotus (Fig. 592); the stargazer Astroscopus; and the African Malapterurus, certain muscles have become transformed into electric organs, which in the emergency of an attack can deliver a shock, more or less effective, to other animals. The voltage of the discharge by adult specimens of Electrophorus has been found to average 370 volts but one case has been reported of a maximum discharge of 550 volts.

Electric organs of an electric eel, Gymnotus

Structurally these electric organs consist of a number of regularly arranged elements, called electroplaxes, put together in histological layers that resemble alternating plates in a storage battery. It is not difficult to imagine how these structures have developed from muscles, since normally muscles in action discharge a certain amount of electricity. The fact that the mechanism, although developed in different parts of the body in different species, is always derived from muscular tissue, indicates that it is a physiological adaptation and not a morphological inheritance of one species from another.