(1976). The principal difference observable between the brain of Reptiles and of Fishes is the increased proportionate size of the cerebral hemispheres (fig. 347, b); but they are still extremely small when compared with the bulk of the body. The appended figure (fig. 348), which represents the brain of the Tortoise in three different aspects, may easily be compared with that of the fish already given. The olfactory lobes (c) might now be mistaken for prolongations of the anterior extremity of the hemispheres; they contain distinct ventricles, and of course give origin to the olfactory nerves (oo.) The hemispheres (b) are much more developed than in the last class; their surface is always smooth and without convolutions; and they are hollowed out into capacious ventricular chambers, in which are contained the corpus striatum and choroid plexus (fig. 348, c), and the two sides are moreover brought into communication by an anterior and posterior commissure.

(1977). The optic lobes (e) are as yet uncovered by the extension of the hemisphere backwards, and each, when laid open, is found to enclose a ventricle (fig. 348, c.) The cerebellum (a) is still small, and consists but of the median portion: behind it is a supplementary lobe (g), extending over the fourth ventricle, as in Fishes. The student will easily recognize the pituitary body (f); but neither this nor the origins of the nerves present any peculiarity worthy of more particular description.

Brain of the Tortoise.

Fig. 348. Brain of the Tortoise.

(1978). Taking the cerebral nerves in the order in which they arise, we will now proceed briefly to trace their general distribution; and this we shall find to correspond most exactly in all essential points throughout the different classes of Vertebrata.

(1979). The olfactory nerves leave the olfactory lobes of the brain as single round cords, and are not, as in the Mammalia, divided into numerous filaments; consequently there is no cribriform plate to the ethmoid bone, but the nerve of each side (fig. 350, e) is received into a simple canal, partly osseous and partly cartilaginous, through which it is conducted to the cavity of the nose.

(1980). The nasal apparatus of Reptiles differs from that of Eishes in one important particular. Breathing air as these creatures do, the sense of smell now becomes connected with the respiratory function; and a communication being established between the nasal cavities and the larynx, the air which passes through this channel into the lungs must necessarily come in contact with the sentient surface formed by those portions of the lining membrane of the nose to which the nerves of smell are distributed; and in proportion as the extent of that surface becomes developed, the power of appreciating the presence of odorous particles in the atmosphere will necessarily be increased. The physiologist is thus enabled to estimate with great exactness the relative perfection of the sense of smell in different classes, or even in different families of the air-breathing Vertebrata, simply by observing the complication and extent of surface presented by the lining membrane of the olfactory organ.

(1981). Taking this as our guide, we must suppose that in all reptiles the sense in question is extremely obtuse, since in these creatures there are neither turbinated bones nor ethmoidal plates as yet distinguishable, - a few folds of the membrane lining the nose, even in those species which are most highly gifted in this particular, being the only provision for extending the olfactory surface; and in many cases, as for example in the Amphibia, the nose seems merely a simple canal leading into the mouth.

(1982). On reaching the nasal cavity, the olfactory nerve spreads out into delicate filaments (fig. 350, d), which are distributed to the Schneiderean membrane covering the septum and upper part of the nose.

(1983). The optic nerves of Reptiles (fig. 348, n), soon after their origin, become confounded together by a commissure, in the same way as in the human subject; and again separating, they are continued through the optic foramina to the eyes.

(1984). The eyeball itself presents few peculiarities in its structure. In the Tortoise, and many Lizards, the sclerotic contains a circle of bony plates imbedded in its substance, and surrounding its anterior margin: these are obviously the rudiments of that osseous zone which in the class of Birds, as we shall find, performs a very important office. The ciliary processes of the choroid are generally very feebly developed. The pupil is frequently round; but it is sometimes of a rhomboidal figure, as for example in the Gecko; and in the Crocodile and some Serpents the pupillary aperture is a vertical fissure like that of a Cat.

(1985). The optic nerve enters the eye in the same way as in quadrupeds, and, having passed the choroid, it terminates in a round papilla, from the margin of which the retina spreads out. As to the rest, the eye of a Reptile differs so little in any essential circumstance from that of Man as to render any more elaborate description superfluous.

(1986). The eyeball is moved by six muscles, disposed as in Fishes, - the four recti arising from the margin of the optic foramen, while the two obliqui are derived from the anterior margin of the orbit.

(1987). In Fishes, from the circumstances under which they live, there is no occasion for the presence of any lacrymal apparatus, or for eyelids adapted to defend and moisten the surface of the cornea; but in the class before us, especially in the more elevated tribes, these appendages to the eye make their appearance, and gradually assume a complexity of structure even greater than that which they present in the human subject.

(1988). In Serpents, and in some of those Lizards which are most nearly allied to the Ophidians, there are still no eyelids; and consequently in such genera there can be neither any lacrymal apparatus nor a conjunctiva, properly so called; the skin of the head merely passes like a delicate film over the transparent cornea, offering no fold worthy of the name of an eyelid.