XIII.] THE FEOG. 233 



7. On each side of the two last-mentioned struc- 

 tures and arched over in front by the optic 

 tracts, are the crura cerebri, 

 h. Divide the cerebral hemispheres horizontally with 

 a sharp scalpel : in each will be found a cavity — 

 the lateral ventricle. Each lateral ventricle com- 

 municates with the third ventricle. The optic 

 lobes, divided in the same way, will be seen to 

 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. 



/S. Its form: subcylindrical ; wider from side to 

 side (especially opposite the second pair of 

 nerves) than dorso-ventrally. 



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

 tion : mount in glycerine, and examine with 1 

 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 on to its back, lay open its abdomi- 

 nal 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 com- 

 munications between the different cords of the 

 same side. 



