This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(2384). All the nerves derived from the medulla oblongata and from the spinal cord are throughout the Mammiferous class exactly comparable to those met with in our own species, and therefore will require but brief notice.
(2385). The third, fourth, and sixth pairs are destined to the muscles of the eye, and their distribution is the same as in Man.
(2386). The fifth pair, or trigeminal nerves, consist of both motor and sentient fasciculi, both of which are distributed to the different parts of the face exactly as in the human subject, allowance of course being made for the varying form of the jaws, and for the proportionate size of the different organs connected with mastication.
(2387). The seventh, or facial nerve, as also the glossopharyngeal, the pneumogastric, and the lingual, have the same origin and general distribution throughout the whole class.
(2388). The eighth pair of nerves are here, as in all the Vertebrata, devoted to the sense of hearing, which in the Mammifera attains its highest development and perfection. The sensitive portion of the auditory apparatus, or the internal ear, is now enclosed in the petrous portion of the temporal bone, and imbedded in osseous substance of such stony hardness that, except in very young subjects, it is by no means easy to display its different parts.
(2389). As in Fishes and Reptiles, it consists of several membranous chambers or canals, filled with a limpid fluid, over which the filaments of the auditory nerve spread out. The whole apparatus, indeed, except in its proportionate size, very accurately resembles the auditory organ of the lower Vertebrata. The semicircular canals exhibit nearly the same arrangement, and in like manner communicate with the vestibule by five orifices. The vestibule itself is small, and no longer contains any chalky concretions: it communicates on the one hand with the cavity of the tympanum, by means of the foramen ovale; and on the other sends off a canal (scala) to form the cochlea, an organ which in the Mammifer assumes its full development and perfection.
(2390). In Reptiles and Birds, as the reader will remember, the cochlea was a simple canal bent upon itself (fig. 367, e), one end of which (scala vestibuli) opened into the vestibule, while the other (scala tympani) terminated at the tympanic cavity, from which it was separated by the membrane of the fenestra rotunda; but in the Mammalia the two scalge of the cochlea are considerably elongated, and wind in a spiral direction around a central axis (modiolus), so as very accurately to resemble the whorls in the shell of a snail, whence the name of the organ is derived *.
(2391). It is in the increased complexity of the cochlea, therefore, that the chief character of the labyrinth of the Mammal consists. But in the tympanic cavity the differences between the Mammiferous ear and that of the Bird are still more striking and decided.
(2392). The cavity of the tympanum in the class before us is very-extensive, and not unfrequently its extent is considerably enlarged by the addition of capacious mastoid cells. By means of the Eustachian tube it communicates freely with the throat. Upon its inner wall it offers the fenestra ovalis and the fenestra rotunda, closed by their respective membranes; and externally is the membrana tympani, the vibrations of which are to be conveyed to the labyrinth.
(2393). In Reptiles and Birds the communication between the drum of the ear and the membrane of the fenestra ovalis was effected by the interposition of a single ossicle, called the "columnella;" but in Mammals a chain of four ossicles, named respectively the malleus, the incus, the os orbiculare, and the stapes, intervenes between the labyrinth and the membrana tympani: these ossicles, both in their disposition and connexions, are precisely similar to those of Man, and, moreover, are acted upon by little muscles in every respect comparable to those of the human subject.
(2394). However remote the structure of the tympanic chain of ossicles in the Mammal may appear to be from that of the simple columnella of the Bird, it is interesting to see how gradually the transition is effected from one class to another even in this particular of their economy; for in the Ornithorhynchus, the Echidna, and the Kangaroo, so bird-like is the form of the stapes, that it might easily be mistaken for the ossicle of one of the feathered tribes *, and every intermediate shape is met with as we advance from this point towards the stirrup-shaped bone of the most perfect quadrupeds.
* In Man, and by far the greater number of Mammals, the scalae of the cochlea make two turns and a half around the modiolus; but in a few Rodent quadrupeds, as for example in the Guinea Pig, the Cavy, and the Porcupine, there are as many as three turns and a half.
(2395). It is in the class under consideration that for the first time an external ear properly so called makes its appearance; for the feathered appendages of the Owl or of the Bustard (§ 2089) are scarcely entitled to such an appellation. In the Mammifera, however, with a very few exceptions, such as the Cetacea, Moles, and the Seal tribe, a moveable cartilaginous concha is appended to the exterior of the head, adapted by its form and mobility to collect the pulses of sound and convey them inwards towards the drum of the ear. The basis of this external auricle is composed of fibro-cartilage covered with a delicate skin, and its cavity is moulded into various sinuosities, so disposed, no doubt, as to concentrate sonorous impressions. In Man, as the anatomist is aware, numerous small muscles act upon the auricular cartilages; but in quadrupeds possessed of moveable ears the number and size of these muscles are prodigiously increased, and the ears are thus directed with facility in any required direction.
(2396). More minutely to describe the structure of the auditory apparatus in the Mammiferous class would be foreign to our present purpose; nevertheless we must not omit to notice one most remarkable provision whereby the Whales, strangely circumstanced as those creatures are, are permitted to hear either through the medium of the air they breathe, or of the water in which they pass their lives. The reader will at once appreciate the difficulties of the case: the ear of a fish, without any external communication, although best adapted to receive the stunning concussions conveyed through the denser element, could never appreciate the more delicate vibrations of the air; and the ordinary Mammiferous ear would be perpetually deafened by the thundering of the water. How is the Whale to hear what is going on in either the sea or the atmosphere?