Although equilibration and hearing are two quite distinct functions, the sense organs concerned have much in common in vertebrates and are often described as stato-acoustic organs. The same is not true of invertebrates generally, since in them the sense organs regulating equilibrium, when present, are distinct and independent of those receiving sound waves.
The function of equilibrium is unnecessary and absent in plant-like stationary animals, like hydroids and corals that remain attached in one position, although stationary plants possess devices which are comparable to gravity receptors, enabling stems and roots to assume a position, positive or negative, with reference to the center of the earth.
This response, geotaxis, is not fundamentally different from equilibration in moving animals, that maintain their bodies by their own efforts in various positions with reference to gravity. Geotaxis is brought about by the arrangement of the component parts within the responsive cells, the heavier parts “settling” to the bottom of the growing geotactic cells, thereby orienting them.
There are three general types of equilibrium, namely, indifferent, stable, and labile.
Indifferent equilibrium is uncommon and involves no specific receptors. It may be seen, for example, in the heliozoans, which are spherical microscopic protozoans that hang suspended any side up without contact with solid objects, and also in the green globular colonies of Volvox, which roll about in pond water without definite orientation to gravity.
Stable equilibrium, that likewise is not dependent upon special sense organs, applies not only to inert forms but also to living bodies, such as a flounder that comes to rest on one side, like a bicycle laid flat upon the ground. In this case the pull of gravity allows the animal to come to rest without individual muscular effort.
When, however, the center of gravity is too high to allow balancing without muscular effort, like a dog “standing on all fours,” or a man balanced upon two legs, then labile equilibrium is demonstrated.
This latter type involves biological factors within the body, as well as the physical pull of gravity from without, for not only must a tension be maintained between opposing muscles to prevent collapse, but a coordinating nervous apparatus, including sense receptors, is also necessary to mediate between the body of an organism and the environmental setting in which it finds itself.
In lower free-swimming animals the sense organs of equilibrium are usually statoreceptors of some sort, depending for their action upon differences in the pressure of a comparatively heavy secreted mass, the statolith, which is brought into contact with surrounding sensory receptor cells whenever there is a change in the position of the body.
Around the margin of a floating jellyfish at the base of the tentacles that hang like a fringe from the edge of the “bell,” there may be arranged a row of statocysts, each consisting of a hollow chamber lined with sensitive cells and containing a pendulous “clapper,” which is made heavy by the deposition of limy salts in its end (Fig. 685). Whenever these fragile medusae in the course of their aimless voyagings upon the surface of the ocean are tipped up at an unusual angle by the force of the waves or by any other outside agency, the tiny loaded clappers within the statocysts swing over and bump the sensory cells that are on one particular side, with the result that the jellyfish rights itself sufficiently to bring the swinging clappers to a standstill, thus restoring equilibrium.
If the statocysts half way around the margin of a jellyfish are carefully dissected off or destroyed in any way, the animal is no longer able to right itself, but floats about in an unnatural attitude in the water. Anyone who has ever heard the clang of a bell-buoy and seen it rock back and forth when the surrounding water is agitated can understand perfectly the way the statoreceptors of jellyfishes operate.
Equilibratory mechanisms, presenting various modifications embodying this fundamental idea, occur both in free-swimming coelenterates and arthropods, as well as in various other invertebrates.
Among vertebrates the function of equilibration is accomplished not so much by static organs alone, as through the cooperation of several sense organs, primarily devoted to other purposes.
Thus a man maintains his upright posture without collapsing through the interaction of (1) tangoreceptors upon the soles of the feet that are in contact with the ground, acting as thigmotactic pressure organs; (2) the sense of sight, which informs and reassures him of his relation to his surroundings; (3) the proprioceptive “muscle sense,” that attends to the proper tension of the opposing muscles; and finally (4) nerve endings from the acoustic (VIIIth) nerve, connecting with the semicircular canals of the ears, which have a gravity function much like that of the statoreceptors of the invertebrates.
If for any reason any one of these four factors is withdrawn, equilibrium is either upset or maintained with difficulty, as anyone may observe by watching a young child that is learning to navigate a toppling body upon wobbly legs.
The semicircular canals are embryologically inseparable from the hearing portions of the vertebrate ear. Further the essential sensory cells of both parts of the ear are so-called “hair-cells” comparable to those which are found in neuromasts. Consequently all of these parts may be grouped together as the acoustico-lateral system.
As the origin and arrangement of the semicircular canals will be more fully considered in connection with phonoreceptors, it is only necessary to point out here that they are typically three in number on each side of the body and are placed approximately at right angles to each other in the three planes of space (Fig. 686). Near one end of each fluid-filled canal there is an enlargement, or ampulla. The ampullae are static sense organs, responding to the stimulus of differential pressure. In order that the body movements may not upset these biological seismographs, they are buried deep in the temporal bone, which is the densest bone of the skull.
A group of delicate receptor cells, bearing stiff hairs, projects into the fluid-filled cavity of the ampulla (Fig. 687) and connects with the vestibular branch of the VIIIth nerve, forming a device which receives stimuli from the movement of the fluid (endolymph) contained within the tube, whenever the apparatus is tipped into a new position. Each of these sensory areas is known as a crista ampullaris.
Since the semicircular canals occupy three different planes, the combined stimuli received by the three ampullae make it possible to detect any shift in position, thereby initiating muscular responses appropriate for the maintenance of equilibrium.
Otoconia, which are usually either crystals or concretions of calcium salts, may be secreted in the ampullae and become attached to the projecting hairs of the receptor cells, especially in land vertebrates. In selachians sand grains, instead of tiny concretions, serve as the otoconia.
Within the statoreceptor apparatus of the ear of most fishes the limy concretions may acquire considerable size and solidity, when they become known as otoliths, or “lucky bones” of fishermen. These otoliths are used to determine the approximate age of individual fishes, since they show lines of growth, resembling the rings of growth in the woody trunk of a tree.
The statoreceptor mechanism of the semicircular canals does not present any great amount of variation throughout the vertebrate series, except in cyclostomes. The hagfish, Myxine, equipped with a single semicircular canal bearing an ampulla at each end, and the lamprey, Petromyzon, with two canals, the third one being missing, can no more sense a three-dimensional world than we, with three canals, can understand the “fourth dimension.”
The abnormal semicircular canal system of cyclostomes is probably more a degenerate than a primitive system, dependent upon the semi-attached habit of these parasitic vertebrates.
The peculiar erratic circling movements that are characteristic of Japanese waltzing mice are correlated with defective statoreceptive organs, or semicircular canals.
A problematical structure called Weber’s organ is found in connection with the stato-acoustic apparatus in the ear of certain fishes. The two internal ears become connected by a transverse canal from which a row of small ossicles extends to the anterior end of the swim bladder. This connection, known as Weber’s organ, is said to transmit movements of the anterior wall of the swim bladder to the ear. Its real function is unknown.