214 MANUAL OF ANIMAL BIOLOGY 



spinal cord near the center of the abdomen, are distributed to the 

 muscles of the body wall in the abdominal region. (W. f. 142, Sp. 4.) 

 7, 8, and 9. These three pairs of large spinal nerves run poste- 

 riorly and laterally and soon anastomose on each side of the body 

 to form a large and important sciatic plexus, from which arises 

 the sciatic nerve that innervates each hind leg. The seventh 

 spinal nerve, anterior to the plexus, gives off a small nerve which 

 is distributed to the abdominal muscles. The sciatic nerves can 

 be traced into the legs. Numerous branches are given off which 

 innervate the various leg muscles. (W. f. 142, Js.) 



10. This pair arises near the posterior end of the spinal cord. 

 The tenth nerve usually receives a small branch from the ninth, 

 and together they form a small plexus which chiefly innervates 

 parts of the urogenital system. An eleventh pair of spinal nerves 

 is occasionally found which, when present, joins this plexus. 



C. Histology of Nerve Tissue. The microscopic study of nerve 

 tissue shows that it is composed of highly specialized nerve cells, 

 or neurons. The neurons are structurally distinctive in that each 

 cell body possesses a long process, termed the axon, and one or 

 more shorter irregular processes, termed the dendrites. The den- 

 drites convey impulses to the cell, while the axon conveys impulses 

 away from the cell. A nerve consists of a large number of axons 

 bound together by connective tissue elements. Each of the axons 

 in a nerve, if traced to its source, will be found to have its origin 

 in the cell body of a single neuron which may be located a con- 

 siderable distance from the peripheral end of the nerve. The cell 

 bodies are not found indiscriminately placed along the nerves, but ? 

 in general, they are grouped in the central nervous system and in 

 ganglia such as those present on the dorsal roots of the spinal 

 nerves. (W. fs. 7 ; 32, F; 138.) 



A microscopic examination of a section through a nerve shows 

 that its method of construction may be compared in a general way 

 to that found in a telephone cable. Covering the nerve is a con- 

 nective tissue sheath, the perineurium, from which strands of 

 connective tissue are given off. These strands divide the nerve 

 into a number of compartments, the funiculi, each of which 

 encloses a great many microscopic nerve fibers, or axons. Carrying 

 the analysis further, each of the axons is found to have a thin con- 

 nective tissue sheath, the neurilemma. Lying between the outer 

 neurilemma and the enclosed axon is a comparatively heavy sheath 



