568 PHYSIOLOGY OF THE NERVE CELL AND THE SPINAL CORD 



first cervical ganglion, the cells in the ganglion belonging to them for the most 

 part perish. 



The structural changes appearing in the spinal ganglion cells after section 

 of a spinal nerve lead to complete loss of their integrity within about ninety 

 days (v. Gehuchten). In other words, to continue in a normal state, the spinal 

 ganglion cell must receive its impulses from the periphery. 



On the other hand, section of the posterior root central to the ganglion in 

 young animals does not stop the development of the ganglion or of the peripheral 

 nerve fibers connected with it (Anderson). 



Section of afferent nerves produces changes even in certain nerve cells of 

 the spinal cord, with which they are connected only secondarily. In young ani- 

 mals development of the cells of Clark's column is stopped by section of the 

 sciatic nerve (Anderson). In fact, even the motor cells of the anterior horn as 

 well as the anterior root fibers appear to be affected by section of the posterior 

 roots, and especially if the homolateral half of the spinal cord also is cut 

 through. 



In view of these results we can readily understand how after amputation 

 of a limb atrophy will gradually extend to those conducting pathways and gray 

 masses of the nervous system which were formerly in functional connection with 

 that particular limb. Such changes spread more rapidly in young than in older 

 or adult individuals. 



On the basis of these facts, Gudden has worked out an experimental method 

 of tracing the conducting pathways belonging to a given organ. He merely 

 extirpates the organ from a young animal, keeps the animal alive for a time, 

 then works out the extent and localization of the resulting atrophy. 



The most probable explanation of the atrophy resulting from such operations 

 is, that individual nerve elements connected together exercise a nutritive influ- 

 ence on one another in virtue of the excitation processes which they mediate, 

 and that failure of these excitation processes cuts off the nutritive influence and 

 the result is atrophy. Thus when a sufficient number of posterior root fibers are 

 sectioned, the normal excitation conveyed to the cells of Clark's column by these 

 fibers is prevented and those cells atrophy. The anterior horn cells robbed of 

 their peripheral impulses by section of the posterior roots, and robbed of most 

 of their central impulses by hemisection of the cord, on the same side, are thence- 

 forth devoid of the proper nutritive influence , and they atrophy. When a limb 

 is amputated the individual no longer has any occasion for sending impulses to 

 the motor cells of the lost member, and not being used, nutritive control over 

 them is withdrawn. 



The nutritive influence of the nerve cells extends also to the peripheral 

 tissues supplied by their fibers, for it has long been known, that the nutritive 

 state of many an organ depends upon its connection with the central nervous 

 system. We have already seen that a skeletal muscle degenerates when its 

 motor nerve is cut. The submaxillary gland decreases in size after section 

 of its cerebral secretory nerves (page 256), and undergoes degeneration of 

 the true glandular substance. 



So far as we know yet muscle substance receives only a single kind of 

 efferent nerve fibers. Consequently the very nerves which evoke the dissimila- 

 tory processes of the muscle serve at the same time in some way not yet under- 

 stood to maintain the muscle in its normal condition (cf. page 449), and 

 the same may be said of other organs. 



Even afferent nerves have a nutritive influence of this kind on their periph- 



