256 VERTEBRATE ZOOLOGY 



Back of the optic thalami are the corpora quadrigemina, two 

 pairs of elevations which belong to the midbrain. The anterior are 

 somewhat smaller than the posterior. 



Remove the cerebellum. The roof of the fourth ventricle will 

 now be seen, the presence of which is marked by a large triangular 

 depression. The prominent ridge on each side of the ventricle is 

 the area ovalis. 



Exercise 43. Draw a sketch showing these structures. 



The Spinal Cord. Remove the skin and muscles from the ver- 

 tebrae of the neck and trunk. With bone forceps remove the roof 

 of the neural arches of all the vertebrae and expose the spinal 

 cord. Note the spinal nerves issuing from the invertebral fo- 

 ramina, and their dorsal and ventral roots. Observe also the gan- 

 glion in the dorsal root, and the two rami of the nerve. Note the 

 enlargement of the cord in the cervical and the lumbar region. 

 Posteriorly the cord diminishes in size until it becomes a small 

 strand called the filum terminale, which goes into the tail. In the 

 sacral and caudal regions the spinal nerves are small and pass 

 almost directly backward. They lie thus close together and form, 

 with the filum terminale, a bunch of fibers called the cauda equina. 

 On both the dorsal and ventral sides of the spinal cord a median 

 longitudinal fissure is present. 



Exercise 44. Draw an outline of the dorsal surface of the cord. 



Make a cross section of the cord and note the inner gray and 

 the outer white matter, the small central canal, and the dorsal 

 and ventral fissures. 



Exercise 45. Draw the cross section. 



The Muscular System. The superficial muscles alone will be 

 studied. Kill a cat as directed on page 205 and skin it. The cu- 

 taneous muscles will be removed with the skin. These are two in 

 number, the great cutaneous and the platysma. The former is a 

 thin, transparent muscle which extends beneath the skin over 

 almost the entire side of the body. The latter covers the sides 

 and back of the neck and the sides of the head. 



