Just as feathers characterize birds, so hairs are integumental hall-marks of mammals. Such apparently hairless animals as whales and sea-cows even are clothed in part before birth with embryonic hair, while the bare thick-skinned rhinoceros and the hippopotamus have sparse bristly hairs about the snout, and the big, apparently naked elephant, whose skin upon close examination is about as forbidding as a chestnut burr, has a supplementary mammalian passport in the form of a tuft of hair at the end of its ridiculous tail.
Hair serves a variety of uses besides its obvious benefit of affording general protection. The air-imprisoning pelt of a fur-bearing animal retains the bodily heat and sheds the rain; the thick mane of a wild horse is a specially placed buffer against carnivorous enemies that would pounce upon its otherwise unprotected neck; the squirrel’s frisky tail is a portable blanket which conveniently enwraps the owner when at rest; the anteater’s bushy tail is a diverting and confusing barrier to the armies of ants that swarm forth in defense when the anteater makes a foray upon their citadel; the long hairs of the horse’s tail form an effective brush to ward off pestering insects; the stiff sinus hairs, or vibrissae, that supply the snouts of many mammals, are sensitive “feelers” (Fig. 197); and lastly, the color schemes carried out on the bodies of mammals, whatever uses they may serve, are due principally to hairs as color bearers. Transient sinus hairs on the inner or contact side of the forearm in the human embryo (Fig. 198) hark back to arboreal lemur-like life.
In structure a single hair is an epidermal shaft projecting, usually at an acute angle like an exaggerated leaning tower of Pisa, from a pit or depression in the skin. The projecting dead part of the shaft is typically cylindrical, with the root concealed at the base in the pit expanding into a club-shaped bulb that derives nourishment from the corium through the living Malpighian cells producing it from below. Directly beneath the bulb and in intimate contact with it, is an upward-projecting dermal papilla containing capillaries and nerve endings which supply the hair root. Corneal cells, surrounding the root of the hair, constitute the inner root sheath. Other epidermal cells that form the outer root sheath line the walls of the pit. The root of the hair with its sheaths make up the hair follicle.
Opening into the pit from the sides are sebaceous glands, which produce an oily secretion that renders the dead hair shaft less dry and brittle.
In cross section a hair shaft ordinarily shows three kinds of cells, namely, those of an inside core, or medulla (absent in human body hair); a surrounding ring, the cortex, making up the bulk of the hair; and a thin outer single layer of shingling cells, the cuticle.
Although the root of the hair may be embedded deep in the corium, the entire structure is epidermal in origin except the papilla, which is dermal. The shaft of the hair usually tapers towards the tip and does not branch, although bristles sometimes split distally. Frequently hairs taper also towards the root end, particularly near the point where they emerge from the skin, so that they tend to bend easily when stiff or give way instead of breaking off upon contact with external objects. The exquisite softness of the fur upon a mole skin when it is stroked either way is due to this adaptation of hairs thinned down at the surface of the skin, which enables the animal to go forward or backward in its burrow with a minimum of frictional injury to the pelt.
Each follicle is supplied with an involuntary muscle, arrector pili, running from near its base diagonally to the superficial region of the corium, on the side towards which the hair slants. When this smooth muscle shortens, it pulls upon the base of the follicle causing the hair to “stand on end”. The action of the arrectores pilorum is particularly noticeable upon the scruff of an angry dog’s neck, or upon the tail of a frightened cat, when these animals take on a more terrifying aspect as the result of this reflex. The ghostly remains of this apparatus in man is the cause of “goose flesh,” which appears when the skin contracts somewhat upon exposure to cold. “Then a spirit passed before my face; the hair of my flesh stood up.”
The slant at which hairs emerge from the skin varies in such a way that in their direction the hairs taken together form vortices and streams as they lie over the surface of the body (Fig. 199). This is particularly apparent on a horse or short-haired dog.
Convergent vortices form around the base of projecting structures, such as horns, the tail, and the umbilical cord. These hair whirlpools persist even after the structure around which they converge has disappeared, for example in man about the umbilicus, and at the focus in the coccygeal region where the vanished embryonic tail was formerly located (Fig. 200).
Perhaps the most familiar instance of divergent whirlpools is on the human scalp at the vertex of the crown, where the hairs are centrifugally arranged. Other divergent vortices appear in the axillae. The coarse hair of the sloth is divergently parted along the midline of the belly instead of down the back as in most mammals. This adaptation, as in other divergent streams and whirlpools wherever found, is useful for shedding rain. The unusual arrangement in the case of the sloth is due to the fact that this mammal customarily hangs suspended upside down from the horizontal branches of trees.
Although hairs are not arranged in definite patches like the pterylae ol feathers, they do emerge from the skin embryonically in orderly array with reference to each other (Fig. 201). In man they appear in groups of twos, threes, and fours with the largest hairs in the middle of each row, these rows in turn being spaced in such a way as to suggest that each one is homologous with an interscale area. This hypothesis is further borne out by the arrangement of hairs in similar groups in other mammalian skins, particularly those of the armadillo and scaly anteater where scales are actually present with a definite group of hairs behind each scale.
In mammals other than man, localized masses of hair appear as fetlocks, tufts, manes, and modifications of the tail. A horse is enabled to brush away annoying flies with a “swish” of long hairs on the tail, while a cow accomplishes the same result, as every farm boy who has ever milked a cow knows, with a “flip” of the terminal tassel.
Hairs occur in various shapes and forms all the way from hard rigid spines, like those of the porcupine, European hedgehog, spiny mouse, and spiny anteater, to the soft delicate wool of sheep and goats. The bristles of swine are stiff elastic hairs, sometimes with split ends, in which the outer layer of cuticle predominates. They are more numerous on the dorsal side of a hog than elsewhere on the body and tend to make the wild animal look somewhat larger and more formidable.
Fur is composed of dense soft hairs, frequently lacking the medulla, with a few long coarser hairs interspersed. In the process of transferring the skin of a seal to milady’s back in the form of a sealskin coat, the long stiffer guard hairs are carefully removed, leaving the soft thick-set fur-hairs making a uniform surface.
The cuticle of wool hairs is usually rough and scaly, and since the hair shafts are somewhat twisted they spin well into yarn because the separate hairs interlock easily.
Sinus hairs, or “feelers” (Fig. 197), that radiate from the inquisitive noses of nocturnal prowlers, such as cats, rats, and weasels, are each seated in a large papilla especially well provided with nerve endings, so that any chance contact which disturbs the stiff outstanding dead shaft is communicated at once to sensory headquarters through the mechanical agitation of the basal papilla.
The unusual beard on the faces of goats and men is the very latest evolutionary style in hair decoration. That the human beard is not so much a relic of the past as a prophecy of the future, is evident not only by its sharp differentiation in the male sex, and its delayed appearance in the individual, but also by the fact that it is much less apparent in the more primitive races.
Sexual differentiation of human hair is largely controlled by hormone action as demonstrated when sexual hormones are prevented from normal occurrence by the removal of ovaries or testes. The distribution of adult female hair over the body is intermediate between that of the embryonic or infantile condition and the arrangement in the adult male.
Although man is one of the least hairy of the mammals, with the exception of the aberrant whales and sea-cows, an examination of his embryonic development shows his close relationship to other members of the order of Primates.
The first evidence of hairs in the mammalian skin is found in the form of concentrations of epidermal cells which, because of displacement resulting from their rapid multiplication, grow down like plugs into the corium (Fig. 202), and become hair follicles. The bulb of each follicle, with its surrounding inner and outer root sheaths, soon differentiates, and the newly formed lengthening shaft pushes out towards the surface, loosening the temporary epitrichium which at this stage covers the body like a gauzy envelope. The hairs in man first emerge at about the fifth fetal month, in the region of the forehead and eyebrows, eventually becoming a transient coat of delicate embryonic fur called lanugo (Fig. 203), which clothes the entire body with the exception of the lips, palms, soles, nails, and spaces around the apertures of the external genitalia. The lanugo usually reaches its highest development during the eighth fetal month, when it begins to be shed into the amnionic fluid that surrounds the embryo, and is replaced by the permanent hair, at least over certain parts of the body. It remains longest on the shoulders and in many instances is still in evidence at birth.
The permanent hair in attaining its growth becomes localized in distribution, and differentiated for various uses, as already pointed out. It is thickest on the top of the scalp, since it was originally adapted to shed the rain which fell alike on our just and unjust hatless arboreal ancestors. In apes, which assume a semi-erect posture with, the crown of the head projecting somewhat forward instead of upward, the hair, as would be expected, is thicker on the scruff of the neck than on the top of the head.
Hairs are also conspicuously specialized in man in the form of eyebrows, eyelashes, and as guardians of nasal and external ear passages against dust invasion. At the pubes and axillae cushions of hair that perhaps tend to lessen friction develop at puberty, while the remainder of the human body, which normally appears to be comparatively bare, is supplied in varying degree with hairy reminders of other days.
Hairs of the head are straight, wavy, curly, or kinky. In cross section the series varies from nearly round in the straight head hair of Indians and Mongolians, to elliptical in the kinky hair of the Hottentot. The shaft of curly or kinky hair, growing more rapidly on one side than on the other, emerges from the skin in a curve. Pubic and axillary hairs usually curl, even in straight-haired people, and straight hair tends to curl in wet weather while curly hair tends to straighten. True “permanent waves,” like poets, are born not made.
Growth of hair varies individually, in health and sickness, seasonally, with quantity and quality of food, with climate, and with the region of the body on which it occurs. The beard hairs may easily grow a millimeter in twenty-four hours. Dr. W. W. Keen reckoned that with the production per millimeter of approximately 500 cells of hair, there would need to be only 5000 hairs on the head to produce 40,000 hair cells per minute. This is what keeps barbers busy.
According to the manner of growth, hair is either definitive or angora in character. Definitive hair grows until a certain length is attained, when it becomes pinched off from its base of supplies in the papilla of the bulb (Fig. 204), and the lifeless hair shaft loosens and is shed. A new hair then starts to grow. The interruption in growth at the root of an angora hair does not occur either as often or as completely as in definitive hair, so that the shaft continues to lengthen as long as the follicle remains intact. In man the body hairs are definitive, while those of the scalp are angora in character. In apes those of the scalp are also definitive.
The color of hair is due to pigment deposited during growth in the intercellular spaces of the cortex. When hairs “turn gray” there is a reduction in the amount of pigment present and an increase in the number and size of the light-reflecting air-spaces between the cells. Gray hair in man appears first at the “temples,” situated over the temporal bones, so-called because here the flight of time is marked. In dogs the graying of hair usually begins on the snout, while in mice and rats it may be anywhere on the body. Some animals such as the varying hare, Lepus americanus, for example, show a seasonal whitening of the hair coat, that brings them into harmony with their snowy habitat, thus insuring them a degree of protection against their enemies.
Data on particular differences in human hair have been gathered in certain cases. For instance the head hair of blondes is usually finer, longer and more dense than that of brunettes. Someone has made an estimated census, after a partial count, of the number of head hairs on four females with the following result: blonde, 140,000; brown, 109,000; black, 102,000; red, 88,000. A mathematical moment with a pencil and a pad of paper reveals the fact that if the blonde lady in question should have her hair bobbed, supposing that it was originally two feet long, she might thereby dispose of something over fifty linear miles of hair.
Ecdysis, or molting, which is so universal a phenomenon of other epidermal structures, occurs at intervals also in the hair coat. With most mammals shedding the hair is more pronounced in spring and early summer than at other seasons, but with man it is a continuous process, involving a normal daily loss which may be increased under pathological conditions. A single head hair, according to Lewis, ordinarily lasts from four to five years, while eyelashes are normally replaced in as many months. Failure in the replacement of hairs of the scalp results in baldness of which there are two general types, both evidently hereditary. In one the divergent whirlpool of hair about the vertex of the crown is the first to go, when the subject comes to resemble a tonsured monk. In the other case the hair retreats from the forehead with the passing of the years, leaving an increasing expanse of apparent intellectuality. When both types of baldness descend upon the same individual the polished dome of the skull may be as bereft of hair as a billiard ball. Baldness, even if the truth be told, is very much more common in men than in women.
Under pathological conditions, unusual abundance of hair, hypertrichosis, or abnormal absence of hair, atrichosis, may occur. The latter condition is frequently associated with defective development of the teeth.
When embryonic lanugo persists it is spoken of as pseudohypertrichosis, as distinguished from hypertrichosis vera. The latter is exemplified by the presence of superfluous hair in the case of bearded women and shaggy men.