* ELEMENTARY BIOLOGY. [CHAP. 



roof over a cavity which communicates with the 

 third ventricle in front, and with the fourth 

 ventricle behind. 



c. The spinal cord. 



a. The anterior fissure running along its ventral 

 surface. 



P. Its form: subcylindrical ; wider from side to 

 side (especially opposite the second pair of 

 nerves) than dorso-ventrally. 



y. Imbed it in paraffin and cut a transverse sec- 

 tion : mount in glycerine, and examine with i 

 inch obj.; note its peripheral portion (white 

 matter) different in appearance from the cen- 

 tral parts (grey matter) : the canal (canalis 

 centralis) running up its centre. 



5. Turn the frog over en to its back, lay open its abdo- 

 minal cavity and remove all the alimentary canal 

 from the gullet to the rectum, along with the liver, 

 kidneys and generative organs. 



6. The sciatic plexus. 



a. This is now seen as a number of large nerve- 

 cords on each side of the dorsal aorta; note 

 the communications between the different cords 

 of the same side. 



b. Follow down the plexus on one side : it ends 

 below in a large trunk which is continuous with 

 the sciatic nerve. 



c. Trace the nerve-trunks forming the plexus up to 

 the spinal column. They are continuous with 

 the 7th, 8th, and 9th spinal nerves. 



