THE FROG. 97 



mediately behind the above-named second spinal 

 nerve. Examine in water, under a hand lens. 



a. The myelon ; subdivided, dorsally and vent- 

 rally, by median longitudinal fissures ; en- 

 closing a small cefitral canal. 



^. The 7ierve roots ; arising dorsally and ventrally, 

 and passing outwards and downwards through 

 the inter-vertebral foramina to form, on each 

 side, the trunk of the nerve. The dorsal 

 root bears an enlargement or ganglion of the 

 root^ immediately on leaving the neural canal. 



y. The investing membranes (cf. supra) ; seen to 

 merge into each other and the periosteum, in 

 the vicinity of the inter-vertebral foramina. 



2. The anatojny of the brain. 



i. Expose from the dorsal aspect, as directed in § i, 

 preferably in a frog which has been previously 

 hardened in spirit. Examine in situ. 



The anterior half of the brain consists of two 

 elongated masses, each marked off by a slight 

 transverse depression into a smaller anterior and 

 a larger posterior portion. 



a. The cerebral he7?iispheres {prosencephalon); the 

 posterior of the above-named, separated by a 

 deep cleft. 



b. The olfactory lobes {rhinencephalon) ; the anterior 

 of the above-named, confluent with each other 

 at their bases. Each passes into a rounded 

 trunk (commonly termed the olfactory nerve\ 

 which leaves the skull, and ramifies in the 

 lining membrane of the nose. 



