92 TEXT-BOOK OF PHYSIOLOGY. 



themselves undergo division, so that a single fiber may give origin to a num- 

 ber of branches, each of which contains a portion of the parent axis-cylinder 

 and myelin. 



Sympathetic Ganglia. A sympathetic ganglion consists essentially of 

 a connective-tissue capsule with an interior framework. The meshes of 

 this framework contain nerve-cells possessing dendrites and branching 

 axons. The majority of the axons are devoid of myelin and are therefore 

 known as non-myelinated nerve fibers. Owing to the absence of the myelin 

 they present a rather pale or grayish appearance. In all instances, with 

 the exception of the ganglion cells of the heart, the axons are distributed to 

 non-striated muscle tissue and to the epithelium of glands. 



The nerve-cells of the ganglia are also in histologic connection with the 

 terminal branches of certain fine medullated nerve-fibers which leave the 

 spinal cord by way of the anterior roots of the spinal nerves. These 

 nerve-fibers are designated pre-ganglionic fibers, while those emerging 

 from the cells are designated post-ganglionic fibers. (See Sympathetic 

 System.) 



Blood-supply. Nerves being parts of living cells require for the main- 

 tenance of their nutrition a certain amount of blood. This is furnished by 

 the blood-vessels ramifying in and supported by the connective-tissue frame- 

 work. Here as elsewhere there is a constant exchange, through the capillary 

 wall and the neurilemma, of nutritive material to the nerve proper and of 

 waste materials to the blood. 



The Chemic Composition and Metabolism. Chemic analysis of 

 nerve-tissue has shown the presence of water, proteins (two globulins, a 

 nucleo-protein and neurokeratin) , certain lipoids, e.g., (a) cholesterin (a 

 monotomic alcohol free from both nitrogen and phosphorus), (b) several 

 cerebrosides or galactosides (nitrogen-holding bodies, free from phos- 

 phorous, compounds of a glucoside character, as shown by their yielding on 

 hydrolysis the reducing carbohydrate galactose), (c) phosphatids (com- 

 pounds containing both nitrogen and phosphorus, e.g., lecithin, kephalin, 

 sphingo-myelin) , inorganic salts, and a series of nitrogen-holding bodies 

 such as creatin, xanthin, urea, leucin, etc. As to the metabolism that is 

 taking place in nerve-cells and fibers, practically nothing definite is known. 

 That such changes, however, are taking place would be indicated first by the 

 blood-supply, and second by the fact that withdrawal of the blood-supply is 

 followed by a loss of irritability. The metabolism of the central organs of 

 the nerve system is more active and extensive. In this situation any with- 

 drawal of blood from compression or occlusion of blood-vessels is followed 

 by impairment of nutrition and loss of function. 



THE RELATION OF THE PERIPHERAL ORGANS OF THE NERVE 

 SYSTEM TO THE CENTRAL ORGANS. 



Spinal Nerves. The nerves in connection with the spinal cord are 

 thirty-one in number on each side. If traced toward the spinal column, it 

 will be found that the nerve-trunk passes through an intervertebral foramen. 

 Near the outer limits of the foramina each nerve-trunk divides into two 

 branches, generally termed roots, one of which, curving slightly forward and 

 upward, enters the spinal cord on its anterior or ventral surface, while the 



