286 ELEMENTS OF PHYSIOLOGY 



which wires used to conduct electricity are sometimes 

 coated. As a rule, a large nerve accompanies an artery 

 down the inside of each limb, and across the joint on the 

 side toward which the limb bends. Thus they are well 

 protected. One exception to this is a 

 nerve cord which crosses the elbow on 

 the side away from which the elbow 

 bends. Therefore it is sometimes hit, 

 and the end of the bone there is called 

 the funny bone because of the tingling 

 sometimes felt, as if it were in the hand 

 at the termination of the nerve. 



521. A Nerve Ganglion and a Nerve 

 Center. The cells are not scattered 

 singly throughout the nervous system 

 but are gathered into groups. This 

 seems to afford easier communication 

 from one cell to another through their 

 branched dendrons, which are unusually 



FlG. 195. Section of . _ r .. . ,, . 



Nerves, showing Many short. One SUCh group of Cells IS Called 



Nerve Fibers in Con- a ganglion. One or more ganglia, 



nective Tissue Sheaths. 6 r 6 . 



performing a definite function, such as 

 controlling the muscles of breathing, form what is called a 

 nerve center. The brain consists of a number of large nerve 

 centers with their connecting fibers. There are many 

 nerve centers in the spinal cord also. We usually speak 

 of the brain and spinal cord as the central nervous system. 

 Where ganglia and gray fibers are abundant, the nerve 

 substance is gray ; where medullated fibers with their hid- 

 den gray cores are abundant, the nerve substance is white 

 in appearance. This led anatomists and physiologists in 

 times past to classify nerve substance as gray matter and 

 white matter. 



522. Neuroglia. The fibers and cells of both the gray 

 and the white matter are held in place by a tissue called 



