CENTRAL NERVOUS SYSTEM OF THE FROG ' (1) 



1. Secure your dissected Frog and carefully remove the skin from all 

 parts of the body. Fasten the specimen, dorsal surface up, in a dissecting 

 pan and then with a sharp, pointed scalpel, pick off, bit by bit, the bony 

 roof of the skull in the region between the eyes, taking great care not to 

 injure the brain underneath by inserting the scalpel too deeply. Small 

 pieces of the bone may also be snipped off with the tips of the scissors. 

 Continue the area of dissection anteriorly to the nasal region and poste- 

 riorly to the end of the skull, thus entirely exposing the dorsal surface 

 of the brain. Note that it is covered by a thin pigmented membrane 

 (dura mater) which should be carefully removed with fine forceps. 



2. Examine the brain from the dorsal surface and, beginning at the 

 anterior end, note the following parts : (a) the fused olfactory lores, 

 from which the olfactory nerves may be seen extending anteriorly to 

 the nasal region ; (6) the pair of cererral hemispheres which merge an- 

 teriorly into the olfactory lobes ; (c) an unpaired portion (diencephalon) 

 bearing (d) a small, median structure (pineal rody) ; (e) a pair of egg- 

 shaped optic lores, back of which is (/) a transverse elevation (cererel- 

 lum), and, posterior to the cerebellum, (g) the medulla orlongata, 

 which appears as the enlarged anterior end of the spinal cord and contains 

 (h) a triangular depression, the fourth ventricle. The fore-rrain 

 consists of the olfactory lobes, cerebral hemispheres, and diencephalon ; 

 the mid-rrain, as seen from the dorsal surface, of the optic lobes ; and 

 the hind-rrain, of the cerebellum and medulla oblongata. 



3. Make a drawing at least twice natural size to show the dorsal surface 

 of the brain in its normal position. 



» B. pp. 206-213. 



343 



