34 LABORATORY STUDIES IN ZOOLOGY 



lime peiinits proceed to identify the cavities mentioned in tlie 

 following lines. The cavities of the forebrain hemispheres are 

 called lateral ventricles. The cavity of the interbrain is the third 

 ventricle, that of the oblongata region the fourth ventricle. The 

 narrow canal between the third and fourth ventricles is called 

 the aqueduct of sylvius. 



The Spinal Nerves. — In order to study these nerves, make a 

 midventral incision through the body wall from the pelvic girdle 

 to the cephalic margin of the pectoral girdle and lateral incisions 

 through the body wall just cephalad of the pelvic girdle. 

 Remove all of the viscera in a mass by cutting across the organs 

 at the cloaca and loosening them from the vertebral column with 

 a scalpel, lifting them as the necessary cuts are made. When 

 they are completely freed from the vertebral column, sever the 

 mass in the region of the oesophagus. The spinal nerves will 

 now be seen as white threads lying on the dorsal body wall. 



Note in the arm and leg regions the grouping of the spinal 

 nerves into plexuses. The brachial plexus is formed by the 

 mingling of the fibers of the second and third nerves. It gives 

 rise on either side to the brachial nerve, which is the important 

 supply to the arm. The sciatic plexus is similarly formed from 

 the seventh, eighth, and ninth nerves and gives rise on each side 

 to the sciatic nerve, the main nerve of the leg. The first spinal 

 nerve is small and arises just cephalad of the brachial plexus to 

 supply the muscles of the back and shoulder. 



The fourth, fifth, and sixth nerves are also small and supply the 

 muscles and skin of the body wall. The small tenth nerve issues 

 from the urostyle and sends out branches to the bladder and 

 cloacal region. 



Select one of the large nerves making up the sciatic plexus and 

 trace it to the point where it leaves the vertebral column, care- 

 fully cutting away the vertebrse on the dorsal side. Each of the 

 ten pairs of spinal nerves arises from the cord and leaves the 

 neural canal by way of openings between the vertebrse. Each 

 spinal nerve has two roots, a dorsal and a ventral. The dorsal 

 root is known as a sensory root, since it is composed of sensory 

 fibers and conducts impulses toward the cord (afferent impulses). 

 The ventral root is termed the motor root, because it is composed 

 of motor fibers which transmit nerve impulses away from the 

 cord (efferent impulses). The two roots unite to form a common 

 trunk outside the neural canal. Trace the roots further toward 



