NERVOUS CENTRES OF REPTILES. 493 



surfaces are shown in Figs. 5 and 6. In the lateral strands of the 

 Medulla Oblongata, close to the fourth ventricle, there is a pair of 

 ganglionic centres (characterized by the presence of vesicular matter), 

 in which the Auditory nerve terminates ; and these are sometimes deve- 

 loped as distinct ganglionic enlargements. Other separate ganglia, 

 sometimes of considerable size, are very commonly found at the origin 

 of the Par Yagum. It is curious to notice the very large comparative 

 size of the Pineal gland (/), and of the Pituitary body (A), in this class ; 

 the functions of these organs are entirely unknown. 



870. The analogy of the Optic lobes of Fishes to the Corpora Quad- 

 rigemina and Thalami Optici of the fully-formed brain of the higher 

 Vertebrata, is not so complete as it is to certain parts which occupy their 

 place at an earlier period. In the Human Embryo, at about the 6th 

 week, the Encephalon consists of a series of vesicles arranged in a line 



Fig. 153. 



** 



Human Embryo of sixth week, enlarged about three times : a, vesicle of corpora quadrigemina ; 6, vesicle 

 of cerebral hemispheres ; c, vesicle of thalami optici and third ventricle ; d, vesicle for cerebellum and medulla 

 oblongata; e, auditory vesicle; /, olfactory fossa; h, liver; ** caudal extremity. 



with each other ; of which those that represent the cerebrum (5, Fig. 

 153) are the smallest, whilst that which represents the cerebellum (d) is 

 the largest. Between the cerebral and the cerebellic vesicles, are two 

 others, (<?, and a), of which the posterior one is the Optic ganglion, and 

 answers to the Tubercula Quadrigemina ; whilst the anterior contains 

 the Third Ventricle, and corresponds in some degree to the Thalami 

 Optici. This condition is precisely represented in the Lamprey ; but in 

 most Fishes, the optic ganglia, and the parts surrounding the third ven- 

 tricle, form but one lobe ; so that the third ventricle seems hollowed 

 out of the optic ganglia, as shown in Fig. 7, c, (Plate II.) 



871. The Encephalon of Reptiles does not show any considerable 

 advance in its general structure, above that of the higher Fishes. The 

 Cerebral Hemispheres (Plate II. Figs. 8, 9, 10, 5) are always much 

 larger than the Olfactive and Optic ganglia ; and they generally cover- 

 in the latter (<?, c) in part, by their posterior extremities. The Cere- 

 bellum is almost invariably of small proportionate dimensions ; and this 

 is especially the case in the Frog, in which it does not even cover-in the 

 fourth ventricle. This low development of the Cerebellum in Reptiles, 

 is what might be anticipated from the general inertness of those ani- 

 mals, and the want of variety in their movements. The Spinal Cord is 



