310 



REPTILIA. 



brain occupy their usual position, but there 

 is no commissura mollis. The cerebellum is 

 generally extremely small, and in some cases 

 is even reduced to a simple transverse la- 

 mella, which does not entirely cover the 

 fourth ventricle, formed as usual by the 

 separation of the posterior columns of the 

 Fig. 224. 



c, lateral view of the brain of a turtle ; 1, optic 

 tract ; 2, crus cerebri. 



d, the same : a portion of the optic nerve has been 

 removed to show the crus cerebri passing upwards. 

 {After JSwan.) 



spinal chord. The inferior surface of the brain 

 is almost smooth, presenting no other ele- 

 vations than those formed by the union of the 

 optic nerves and by the tuber cinereum. As 

 there are no lateral lobes to the cerebellum, 

 of course no traces of a pons varolii exist. 

 As in birds, a vascular inflation, which seems 

 to represent the fissure of Sylvius, separates 

 each hemisphere into two lobes, into the 

 posterior of which the lateral ventricle pe- 

 netrates. The pineal and pituitary bodies are 

 met with in all reptiles. 



In the Ckeloman reptiles the cerebral he- 

 mispheres contain as usual a ventricle, in 

 which may be perceived a body analogous to 

 the corpus striatum, presenting an arrange- 

 ment very similar to what is observed in 

 birds, only it is much less voluminous, sj as 

 to occupy a small portion comparatively of 

 the interior of the ventricle. The crura" ce- 

 rebri, when they reach the lobes of the hemi- 

 spheres, do not, as in mammalia and in birds, 

 immediately dilate into large ganglia, but 

 curving upwards and backwards, expand on 

 each side into a tubercle, which is the corpus 

 striatum. The optic thalami are very small, 

 but the pineal gland which lies upon them 

 is of considerable bulk. 



The bigeminal tubercles are of a rounded 

 form, and instead of being separated from 

 each other by a slight groove, as in the mam- 

 malia, a deep fissure is interposed between 

 them which penetrates to the roof of the 

 aqueduct, and contains a fold derived from 

 the pia mater. 



The cerebellum is nearly hemispherical in its 

 form, consisting of an arched layer of nervous 

 substance of equal thickness throughout, which 

 spreads over a portion of the fourth ventricle. 

 The remainder of that cavity is covered by a 

 vascular plexus, derived from the sides of the 

 medulla oblongata, which forms a sort of 

 valve, and by becoming united to the margin 



of the cerebellum, completes the roof of the 

 fourth ventricle, which is large and prolonged 

 very far back. The anterior columns of the 

 spinal chord form a very distinct projection 

 into the floor of this ventricle, as they advance 

 upwards towards the brain. A similar dispo- 

 sition exists in the crocodiles, and in the Sau- 

 rian reptiles generally; the principal differences 

 being that the hemispheres are proportionall} 

 larger, and do not separate from each othei 

 so as to display the optic thalami; the olfactory 

 bulbs also are less closely approximated to 

 the cerebral hemispheres, with which they 

 are sometimes connected by the intervention 

 of a narrow pedicle, in which, however, a 

 canal is always to be detected, communicating 

 between the ventricles and the cavity of the 

 bulb. The corpus striatum is comparatively 

 larger than in the Chelonians, occupying a 

 considerable proportion of the base of the 

 Fig. 225. 



Brain and Nerves of Boa Constrictor. {After Swan.) 



a, anterior lobe of the brain ; b, optic lobe ; c, 

 cerebellum ; d, schneiderian membrane of the nose ; 

 1, olfactory nerve ; 2, optic nerve ; 3, third or 

 common oculo-muscular nerve cut short ; 4, fourth 

 nerve given to the superior oblique muscle of the 

 eye ; 5, first trunk of the fifth ; 6, second trunk of 

 the fifth ; 7, third trunk of the fifth ; 8, hard portion 

 of the seventh nerve ; 9, auditory nerve ; 10, glosso- 

 pharyngeal nerve; 11, trunk of the par vagum ; 

 12, ninth nerve; 13, ganglion of the sympathetic 

 nerve, as fa fig. 22G ; 14, a branch of the sympathetic 

 nerve passing to the palatine nerve, as fa fig. 226. 



