232 NERVOUS SYSTEM. 



parts predominate over the cineritious covering, but they 

 present no convolutions nor ventricles. By the rapid growth 

 of the dorsal vertebrae, and the obliteration, anchylosis, and 

 absorption of the coccygeal vertebrae, the spinal chord ap- 

 pears to recede from behind forwards, within the vertebral 

 canal and the cauda equina to lengthen. The anterior extre- 

 mity of the chord is enlarged from the first, as it gives origin 

 to most of the cranial nerves, and the posterior end is en- 

 larged for the cauda equina, as is even perceptible in fishes 

 and serpents. Here, as in other classes, where the spinal 

 chord by the progress of development, is retracted within 

 its osseous sheath, and the cauda equina is thus length- 

 ened in the adult state, there is a greater distance ob- 

 served between the origins and the places of junction of 

 the motor and sensitive roots of the nerves which com- 

 pose it, and consequently between the intevertebral ganglia 

 and the spinal chord ; and this is most manifest at the pos- 

 terior end of the column, which has been most influenced by 

 the metamorphosis. The long narrow cerebral hemispheres 

 of the adult frogs taper to the olfactory nerves which com- 

 mence with cineritious tubercles, and the optic nerves are 

 observed distinctly to cross each other before the optic tuber- 

 cles. The changes effected in the nervous system by the 

 metamorphosis of the higher amphibia closely resemble those 

 produced by development in the human embryo. Their 

 sympathetic nerves and ganglia are more distinct than in 

 fishes. 



In the class of reptiles the small cavity of the cranium 

 nearly corresponds with the dimensions of the enclosed 

 brain, as in some fishes, and the superficial cineritious sub- 

 stance still predominates over the internal white fibrous 

 matter, though to a less extent than in fishes and amphibia. 

 The cerebellum is remarkable for its proportionate smallness 

 in this class, and the cerebral hemispheres, containing each a 

 distinct ventricle, now always exceed the optic lobes. The 

 spinal chord of serpents, from their want of arms and legs, is 

 still destitute of enlargements, as in the apodal fishes, but the 

 medulla oblongata is of considerable size, and the fourth 

 ventricle, still uncovered by the small cerebellum, descends 

 into the spinal chord. From the smallness of the brain and 

 cranial cavity in the centre of the head of reptiles, the rela- 



