THE FROG 37 



3. The Peripheral System. This is composed of the nerves 

 which come from the brain and spinal cord; the former are 

 called cranial nerves, the later spinal nerves. There are ten 

 pairs of cranial nerves, but on account of their close relation 

 to the bone of the skull their origin and distribution will not 

 be worked out. If possible examine a demonstration prep- 

 aration showing these nerves. 



The spinal nerves come out in pairs, and each single nerve 



arises from the cord by a dorsal and a ventral root. Since 



these roots pass through the bone of the vertebral column 



their origin is difficult to determine, but the spinal nerves 



themselves may easily be found along the dorsal wall of the 



body cavity. Determine how many of these nerves there 



are, and trace their course and distribution. Give especial 



attention to networks or fusions of nerves called plexuses: 



(a) The brachial, in the region of the fore limb. Of how 



many nerves is the plexus composed? Is the fusion 



of these nerves complete? Trace a large nerve, the 



brachial, into the arm. 



(6) The sciatic or lumbar plexus, is in the posterior part 

 of the body anterior to the place where the legs are 

 attached. Of how many nerves is the plexus com- 

 posed? What is the nature and the extent of the 

 union? Trace a large nerve, the sciatic, into the leg. 

 Follow it in its course through the leg and foot. 

 Construct a large drawing showing the spinal nerves, and 

 the sympathetic system so far as it has been studied. 



VI. Sections of the Body. 



Having studied the organs and the parts of the body 

 thoroughly, we should have a good idea of the relations of 

 one part to another. Make a drawing of an ideal cross- 



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