(2397). The plan adopted is simple and efficacious: - The external meatus of the ear is reduced to the smallest possible diameter, the canal being barely wide enough to admit a small probe: this is the hydro-phonic apparatus, and is all that is exposed for the reception of aquatic sounds. The Eustachian tube, on the contrary, is very large, and opens into the blow-hole through which the Whale respires atmospheric air: if, therefore, the Cetacean comes to the top of the water to breathe, it is the Eustachian tube that conveys aerial sounds to the ear. And thus it hears sufficiently under both conditions.

(2398). So far, as the student will have perceived, the different portions of the encephalon to which we have adverted correspond most exactly to similar parts met with even in the brain of a reptile. Where, then, are we to look for those grand differences whereby the Mammi-ferous brain is peculiarly characterized? The peculiarities of the brain of a Mammal are entirely due, first, to the increased proportional development of the cerebral hemispheres; and secondly, to the existence of lateral cerebellic lobes, in connexion with both of which additional structures become requisite.

* Vide Sir Anthony Carlisle "On the Physiology of the Stapes," Phil. Trans, for 1805.

(2399). In those Marsupial tribes that form the connecting links between the Oviparous and Placental Vertebrata, the brain still exhibits a conformation nearly allied to that of the Bird, and the great commissures required in the more perfect encephalon are even yet deficient; but in the simplest brain of a Placental Mammifer the characteristic differences are at once apparent.

(2400). In the Rabbit, for example (fig. 408), the cerebral hemispheres (b) are found very materially to have increased in their proportionate dimensions; and although, even yet, convolutions upon the surface of the cerebrum are scarcely indicated, additional means of intercommunication between the hemispheric masses become indispensable. The corpus cal-losum, therefore, or great transverse commissure of the hemispheres (fig. 408, c), is now superadded to those previously in existence; while other medullary layers, called by various ridiculous names, bring into unison remote portions of the cerebral lobes.

(2401). In proportion as intelligence advances, the surface of the cerebral hemispheres, becoming more extensive, is thrown into numerous convolutions separated by deep sulci; until at length in the Carnivora, as, for instance, in the Lion (fig. 411), the cerebrum (e e) attains such enormous dimensions that the other elements of the encephalon are, as it were, hidden among its folds.

(2402). But, in addition to this increased complexity of the cerebrum, the cerebellum likewise has assumed a proportionate importance. In the Oviparous races this important element of the brain consisted only of the mesial portion, so that no cerebellic commissure was requisite: but in the Mammal it exhibits in addition two large lateral lobes (fig. 411, c c); and coexistent with these the pons Varolii (d) makes its appearance, embracing the medulla oblongata and uniting the opposite sides of the cerebellum.

Brain of the Lion.

Fig. 411. Brain of the Lion.

(2403). The structure of the spinal cord and the origins of the spinal nerves throughout all the Mammalia are precisely similar, and exactly correspond with what occurs in the human body; neither does the anatomical distribution of the individual nerves derived from this source require any special notice, since, generally speaking, it differs in no important particular from the arrangement with which every anatomist is familiar.

(2404). The sense of touch in Mammalia is diffused over the whole surface of the body, - its perfection in different parts being of course influenced by the nature of the integument, and the number of sentient nerves appropriated to any given region. All the nerves derived from the sensitive tract of the spinal medulla, and the three divisions of the fifth pair of encephalic nerves, are equally susceptible of tactile impressions; so that, in a class so extensively distributed as that before us, we need not be surprised to find a special apparatus of touch developed in very different and remote parts adapted to particular exigencies. Thus the whiskers of the Seal and of nocturnal Carnivora, the lips of the Horse, the trunk of the Elephant, the hands of Man, the hind feet of the Quadrumana, and even the extremity of the tail where that organ is prehensile, are all in turn made available as tactile instruments, and exercise the sense in question with the utmost delicacy.

(2405). In the Bats, where the sense of vision becomes inadequate to guide them through the dark recesses where they lurk, that of touch assumes its utmost development, and every part of the body that could by possibility be furnished with it has been abundantly provided for in this respect. Not only is the broad expanse of the wing acutely sensible, but the very ears have been converted into delicate feelers; nay, from the tip of the nose in some species, membranes of equal sensibility have been largely developed; so that the Bats, as was ascertained by Spallan-zani, even when deprived of sight and hearing, will fly fearlessly along, and avoid every obstacle with wonderful precision, guided apparently by the sense of touch alone.

(2406). The sympathetic system of the Mammifera differs in no important particular from the human, the arrangement of the ganglia and the distribution of the plexuses being in all respects the same.

(2407). In the conformation of the genito-urinary apparatus in Mammalia the physiologist will find many circumstances of extreme interest.

(2408). Even in Birds, as the reader will remember, the secretions of the testes and of the kidneys were both poured into the common cavity of the cloaca, and discharged through the anal orifice. No bladder was provided for the reception of the urine; and a simple, grooved, but imperforate penis, even where that organ was most fully developed, was sufficient for the purpose of impregnation.