THE NUTRITION OF NERVOUS TISSUE 840 



affected by the lesion, it is possible to learn by this method just how 

 far the axons from any group of nerve cells extend. The atrophic 

 changes which occur in the nerve cell bodies after section of their 

 nerve fiber may also be used to show from what nucleus any tract of 

 fibers originate. The fact that regeneration does not occur in the brain 

 or cord after degeneration forms the basis of a method known as suc- 

 cessive degeneration by which the anatomical paths followed by reflexes 

 may be made out. For example the scratch reflex, by means of which the 

 dog relieves itself of irritation set up by parasites on the skin may be 

 initiated through afferent impulses entering the cord in the thoracic 

 region. The problem is to discover what neurons conduct these impulses 

 to the motor neurons of the leg which lie in the lower segments of 

 the cord. Section of the lateral column of the cord abolishes the 

 reflex, but we know that in this column lie also fibers descending from 

 centers in the cerebrum, midbrain, and medulla, and it is desirable 

 to differentiate these from the propriospinal fibers whose cell bodies 

 lie within the cord itself. By making a transection of the cord, above 

 the afferent path of the reflex, all these fibers will be caused to de- 

 generate. After a year the degenerated fibers will have disappeared 

 and their paths will be occupied by a neuroglial scar. A section is now 

 made of the lateral column just below the entrance into the cord of the 

 afferent path of the reflex. Degeneration will now set in in those de- 

 scending fibers whose cell bodies lie between the primary and secondary 

 lesion, and the course of these fibers may be traced to their junction with 

 the motor neurons. By methods such as these the paths followed by 

 the nerve impulses involved in the various functions of the nervous 

 system, with which we are to become familiar, have been made out. 



The nutrition of the nervous system is a subject of great importance 

 because of the fact that many manifestations of nervous disease are the 

 result of nervous lesions, secondary to some primary disturbance in 

 the circulation of the spinal cord or brain. Thrombosis or hemorrhage 

 may produce a mechanical block in the course of vessels supplying the 

 nervous tissue, or pathological modifications of the blood vessel walls 

 may interfere with the metabolic exchange between blood and tissue. In 

 myelitis, disseminated sclerosis, and poliomyelitis the nervous lesions 

 may have a close relation to the distribution of the blood vessels. In 

 this connection the section on the circulation of the brain may be re- 

 viewed with profit (page 254). Certain regions of the central nervous 

 system are supplied with blood from two sets of arteries so that occlu- 

 sion of one does not interfere with the nutrition of the part. We will 

 see that this is a characteristic of the foveal part of the visual center, 

 which consequently is rarely affected by the vascular lesions of civil 

 life (page 882). The importance of the nervous system to the existence 



