THE FROG 21 



side just dorsal to the aorta and the radices aortse. It is com- 

 posed of a number of enlargements, the sympathetic ganglia and 

 smaller nerves rami intergangliares connecting them. How 

 many sympathetic ganglia do you find? Do the rami communi- 

 cantes join the ganglia or the interganglionic nerves? Note 

 several (how many?) small nerves arising from the median 

 sympathetic ganglia to form a trunk (splanchnic nerve) which 

 follows the common intestinal artery to be distributed to the 

 viscera. 



Remove carefully the corresponding vertebral centra and 

 then follow the proximal portion of the eighth, ninth, and tenth 

 spinal nerves through the periganglionic glands to the spinal 

 cord, which lies just dorsal to the centra. At the gland each 

 nerve is divided into two portions or roots, one dorsal, the other 

 ventral in position. Which of these, the dorsal or ventral, bears 

 an enlargement (ganglion)? 



Draw the spinal nerves, their roots and ganglia, together 

 with the sympathetic system as far as made out, X2. 



Spinal Cord. Place the frog for several days in nitric alcohol 

 (equal parts of 95% alcohol and 10% nitric acid) so as to decalcify 

 the skeleton. When the bone is softened remove the neural 

 arches from the vertebrae, exposing the spinal cord. What parts 

 of it are larger than the rest? Does the distribution of the 

 spinal nerves suggest any reason for the enlargements? Notice 

 its small hinder portion, the filum terminale. Is there a groove 

 (dorsal fissure) running along the dorsal surface of the cord? Can 

 you see any of the dorsal roots of the spinal nerves and note 

 where they enter the cord? 



Make a drawing of the spinal cord, X4, leaving space in 

 front to insert the brain later. 



The Brain. Lay open the brain cavity by removing the skin 

 and fronto-parietal bones and underlying cartilage, beginning 

 a little in front of the eyes and proceeding backwards in the 

 most careful manner, taking great pains not to injure the underly- 

 ing structures. In this operation you will doubtless tear the 

 slender stalk (paraphysis) passing from the brain to the roof of 

 the cranium beneath the brow spot. Continue to develop until 

 the whole dorsal surface of the brain is exposed back to its con- 

 nection with the spinal cord. In this brain make out the follow- 

 ing regions. 



