NERVOUS SYSTEM. 233 



tive size of these parts does not influence that of the whole 

 head at different periods of life, and the head preserves the 

 same proportions to the rest of the body through life also in 

 amphibia and fishes. From the still imperfect development 

 of the cerebral parts in this class, the vital functions of rep- 

 tiles are less immediately dependent on them than in hot- 

 blooded animals, and they longer survive their mutilation. 

 In the saurian and chelonian reptiles the posterior and mid- 

 dle enlargements are obvious on the spinal chord at the ori- 

 gins of the nerves of the extremities. The wide medulla 

 oblongata within the cranium is marked longitudinally by the 

 limits of its three component fasciculi on each side, and the 

 decussating bands of its corpora pyramidalia are more dis- 

 tinct than in fishes. The nerves of the body bear a large 

 proportion to the size of the cerebral centres, and corres- 

 pond in their distribution to those of the higher air-breathing 

 classes. The great ganglia and plexuses of the sympathetics 

 now more closely accompany the arterial trunks, as we see to 

 become more exclusively their distribution from the articu- 

 lated and the moUuscous classes up to man. The twelve 

 pairs of cranial nerves are seen here as in birds and mamma- 

 lia, and they chiefly arise from the enlarged medulla oblon- 

 gata, as seen in the annexed figures of the brain of the tor- 

 toise, emys europaa, Fig. 97. C. D. Immediately before the 

 anterior roots of the first cervical nerves (97- D. ,) are seen 

 on the inferior surface of the medulla oblongata, the origins of 

 the hypo-glossal, or twelfth pair of cranial nerves, and on 

 the sides, the numerous branches of the spinal-accessory, or 

 eleventh pair. Before these are seen on the sides, two por- 

 tions of the pneumo-gastrics and the glosso-pharyngeal, or 

 ninth pair. Close together are the acoustic and the facial 

 nerves, and towards the median plain below are the sixth 

 pair, or abducentes oculorum, (97- D. b.) The small motor es 

 oculorum are seen anterior to these last, and on the sides the 

 large trunk of the trigeminus. The short and broad cere- 

 bellum (97. C. c,) not extending backwards to cover the 

 fourth ventricle, that cavity is found protected externally by 

 a highly vascular membrane (97. C. b.) The small trans- 

 versely developed cerebellum consists only of the median 

 lobe, without hemispheres, and consequently still is without 

 the corpus dentatum in its interior ; it is smooth and cineri- 



