272 



ANIMAL PHYSIOLOGY. 



[LESSON 84. 



FIG. 886. FIG. 887. 



most of the osseous fishes ; but the cerebellum is generally smaller in 

 amphibia and reptiles than in all the other vertebrata. 



1224. As in the lower fishes, the spinal chord in these inferior 

 forms of amphibia is prolonged, small, and tapering, without distinct 

 enlargement where the nerves usually come off to the arms and to 

 the legs. The medulla oblongata is yet broad and lobed, the cere- 

 bellum in form of a very small median transverse lobe without hem- 

 ispheres, the optic lobes large, gray, smooth without, hollow within, 

 and quite exposed, and the cerebral hemispheres extending longi- 

 tudinally, without internal ventricles, and smaller than the optic 

 lobe. 



1225. The metamorphosis of the animals of this class, prone to 

 such phases, changes the condition of their nervous system, from that 

 of the lower fishes, to nearly that of the reptiles above them, in which 



no metamorphosis occurs ; and these changes in the 

 nervous system are effected so rapidly, that we can 

 perceive a marked advancement in the development 

 of the nervous system of the Tadpole produced in 

 one day. 



1226. In the Tadpole of the common frog, on 

 the fourth day (Fig. 386), the spinal chord is per- 

 ceptibly enlarged at its posterior part (a), and also 

 the medulla oblongata. The cerebellum (b) is 

 scarcely visible, extended across the median plain ; 

 the optic lobes (c), and the cerebral hemispheres (e) 

 are small, narrow, and so far separate longitudinal- 

 ly as to expose the optic thalami (d). 

 On the following, or fifth day (Fig. 387), besides the general in- 

 crease of the spinal chord (o), the cerebellum (b) is perceptibly en- 

 larged, the optic lobes (c) are proportionately broader and shorter 

 and the cerebral hemispheres (e), increased in every direction, begin, 

 to extend backwards over the optic thalami (d). As the tadpole ad- 

 vances in its development, and the legs and arms are extended from 

 the sides, the posterior and middle enlargements of the spinal chord 

 are proportionally increased, the cerebral hemispheres enlarge, but 

 they present no convolutions or ventricles. The anterior extremity 

 of the chord is enlarged from the first, as it gives origin to the crani- 

 al (head) nerves, and the posterior end is enlarged for the cauda 

 equina (the termination of the spinal chord in man, and other ani- 

 mals, is supposed to represent a Horse's tail, and is thus named 

 cauda, a tail ; equina, a Horse). 



Nervous systems of 



Frog on fourth and 



fifth days. 



