I.] 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 central canal. 



p. The nerve 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 anatomy 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 hemispheres (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. 



M. 7 



