SPINAL COED 161 



arc in the lower lumbar region of the cord is interfered with, 

 the knee jerk is diminished or is absent, and when the activity 

 of the arc is increased, by the removal of the influence of the 

 brain, the jerk is increased. That the jerk is probably not a 

 true reflex is shown by the fact that the latent period is very 

 much shorter than that of most reflex actions (fig. 80). The 

 reflex arc, however, is necessary for the tonus. This tonus is 

 increased by tension of the muscles and also by fatigue of the 

 nervous system, and this condition may lead to cramp. 



The degeneration of special groups of cells the anterior horn 

 of grey matter which follows amputation of the leg at different 

 levels seems to indicate that the various groups of cells have 

 definite connections with individual muscles (see fig. 79). 



B. CONDUCTING PATHS 



The study of the course of the ingoing and outgoing fibres in 

 the cord has proved to be one of great difficulty. It is possible 

 in animals to divide the cord completely or to divide one half, 

 or to divide any one of the white columns, and to observe any 

 loss of muscular action which may ensue and to trace the course 

 of degenerated fibres; but to determine what changes in the 

 sensibility have resulted on animals, unable to give any expres- 

 sion to their sensations, is practically impossible. 



On the other hand, the clinical method of carefully studying 

 the changes in sensibility during life and determining post- 

 mortem the exact site of the lesion which has produced these 

 symptoms has generally proved somewhat unsatisfactory on 

 account of the want of precision in the lesions produced by 

 injury or disease of the cord. 



A. Ingoing Fibres. In the cord there seems to be a sorting of 

 fibres into those the stimulation of which can effect consciousness 

 and give rise to sensations and those which simply produce reflex 

 responses. Thus, those kinsesthetic fibres which give rise to the 

 muscle and joint sense run up in the posterior columns and cross 

 above the cord, while those which are concerned with the 

 unconscious adjustment of muscular action through the agency of 

 the cerebellum are shunted off through the synapses in Clarke's 

 column to the direct cerebellar tract of the same side and the 

 spino-ventral cerebellar tract of the opposite side. In all 



II 



