io8 TEXT-BOOK OF PHYSIOLOGY. 



of the body, v FrojiLJiLLLC^ 



fibers retain their individuality, and never become blended with 



adjoining fibers. 



As nerves pass from their origin to their peripheral terminations, 

 they give off a number of branches, each of which becomes invested 

 with a lamellated sheath an offshoot from that investing the parent 

 trunk. \^This division of nerve-bundles and sheath continues through- 

 out all the branchings down to the ultimate nerve-fibers, each of 

 which is surrounded by a sheath of its own, consisting of a single 

 layer of endothelial cells. %This delicate transparent membrane, the 

 ^heath of Henle, is separated from the nerve-fiber by a considerable 

 spaceTliTwhich is contained lymph destined for the nutrition of the 

 fiber. Near their ultimate terminations the nerve-fibers themselves 

 undergo division, so that a single fiber may give origin to a number 

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

 cylinder and myelin. 



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

 the maintenance of their nutrition a certain amount of blood. This 

 is furnished by the blood-vessels ramifying in and supported by the 

 connective-tissue framework. 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, proteids (two glob- 

 ulins and a nucleo-proteid), neurokeratin and nuclein, two phos- 

 phorized bodies (protagon and lecithin), several cerebrosides (nitro- 

 gen-holding bodies of a glucoside character, as shown by their yielding 

 the reducing carbohydrate galactose), 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, how- 

 ever, 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.|iThe metabolism of the central nervousf 

 system is more active and extensive. ( \ In this situation any withdrawal 

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

 by impairment of nutrition and loss of function. ( \ 



The Relation of the Nerves to the Central N 

 The nerve-trunks in connection with the spinal cord, 

 in number, pass through the intervertebral foramina. 

 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 other, curving backward and upward, enters on the posterior 

 or dorsal surface. The former is termed the anterior or ventral 



