CENTRAL NERVOUS SYSTEM OF THE FROG x (2) 



1. Continue the dissection of the central nervous system by very care- 

 fully removing, with the tips of the scissors, the bony, dorsal arches of all 

 the vertebrae, thus exposing the spinal cord throughout its entire length. 



2. The brain and spinal cord should now be removed from the animal. 

 In order to do this it will be necessary to cut the cranial and spinal nerves 

 which run from the central nervous system to the various regions of the 

 body. Begin at the anterior end of the brain and cut the olfactory nerves 

 first. Then carefully raise the anterior end of the brain, note the large 

 optic nerves on the ventral surface anterior to the optic lobes, and cut 

 them. Continue posteriorly in this manner, and when the entire central 

 nervous system is free, remove it and place in a small dish of water for 

 further study. 



3. Study the ventral surface of the brain under the dissecting micro- 

 scope and locate the regions of the fore-brain, mid-brain, and hind-brain. 

 Note: (a) the origin of the olfactory nerves; (b) the origin of the 

 optic nerves and the crossing of the fibers of each in the mid-ventral 

 line under the optic lobes, to form (c) the optic chiasma ; (d) the heart- 

 shaped infundibulum which lies underneath the optic chiasma, and 

 (e) the hypophysis lying posterior to the infundibulum. The latter is fre- 

 quently detached from the brain when removing it from the brain case. 

 The infundibulum and hypophysis constitute the pituitary body — an 

 important endocrine organ. 



4. Study the spinal cord and note : (a) the enlargement in the brachial 

 region where the nerve supply of the fore limbs arises ; (b) the enlarge- 

 ment in the pelvic region where the nerve supply of the hind limbs arises ; 

 and (c) the small terminal portion (filum terminale). 



5. Make a drawing of the ventral surface of the brain and spinal cord 

 at least twice natural size, showing all the structures observed. 



i B. pp. 206-213. 



345 



