622 THE FUNCTION OF THE SPINAL CORD 



root toward the cord. The deduction immediately to be derived 

 from these facts, is that the trophic center (cell-bodies) of the fibers 

 composing the former, is situated in the spinal cord, while that of the 

 posterior root fibers lies in the ganglion with which this root is associ- 

 ated. Having been told previously that the efferent fibers composing 

 the anterior root originate in the ganglion cells of the gray matter of the 

 anterior horn, and that the afferent fibers of the posterior root are derived 

 from the cells of the spinal ganglia, we are now able to localize the 

 degeneration in these neurons in the manner indicated by Fig. 309. 

 It might be mentioned that the descending type of degeneration, ob- 

 served in anterior poliomyelitis, is represented by neuron A of this 

 figure, because it is commonly accepted that the active agent of this 

 disease destroys the cells of the anterior horn, and thus produces a 

 functional uselessness of the corresponding nerve fibers and motor 

 organ. A degeneration very similar to that represented by neuron 

 C, occurs in the course of tabes dorsalis, or locomotor ataxia. In this 

 disease the lesion remains localized at first in the terminals of the pos- 

 terior root fibers with the result that the muscle and tendon sense is 

 rendered defective, thereby preventing the proper coordination of 

 muscular movements. 



These facts, however, do not justify us in recognizing the existence 

 of special neurons with an exclusive trophic function, because the 

 nutrition of a tissue is dependent primarily upon its activity, and the 

 latter in turn upon the excitatory and regulatory power of the nerve 

 cells. Thus, an injury to 'these nervous elements invariably leads to 

 trophic disturbances in the tissues even without their being equipped 

 with special trophic qualities. For this reason, we find that the skele- 

 tal muscles atrophy when separated from their ganglion cells. Ex- 

 ceptions to this rule are few in number and are referable to the fact that 

 some muscles, such as the sphincter ani, are not under the direct 

 control of the central nervous system. Upon this basis, we are 

 also able to explain the trophic disturbances which are frequently 

 observed in the course of degenerating afferent nerves (Fig. 309, B). 

 Thus it is found that the inflammation of the ganglia upon the posterior 

 roots leads to the condition of herpes zoster ("shingles") in the area 

 from which the corresponding fibers are derived. l The skin may also 

 become glossy and desquamate, which condition may eventually give 

 rise to a loss of the hair and nails, or to a formation of ulcers. In- 

 clusive of this trophic influence, the functions of the spinal cord may 

 therefore be summarized as follows: 



(a) It is an important seat of reflex action. 



(b) It forms one of the principal conducting paths. 



(c) Its centers are automatically active and give rise to the tonicity of the 

 musculature. 



(d) It regulates the activity and trophic condition of the tissues and plays an 

 important part in the heat production of our body. 



1 Head and Campbell, Pathology of Herpes Zoster, Brain, xxiii, 1901, 353. 



