THE NERVOUS SYSTEM 151 



The synapses in the cord are not only capable of acting 

 reflexly to set up definite contractions in muscles, but they 

 seem to exercise a constant tonic action upon them, and 

 when this tonic action is interfered with, the effect of directly 

 stimulating a muscle is diminished. This is very well seen 

 in the contraction of the quadriceps extensor femoris which 

 occurs when the ligamentum patellae is struck sharply causing 

 a kick at the knee joint the knee jerk (Fig. 80). When 

 the reflex arc in the cord in the lower lumbar region 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 not a reflex is shown by the fact that the latent 

 period is very much shorter than that of a reflex action 

 (Fig. 80). The reflex arc, however, is necessary for the tonus. 

 This tonus is increased by tension of the muscle and also by 

 fatigue of the nervous system which may lead to cramp. 



The degeneration of the cells which follow amputation of 

 the leg at different levels seems to indicate that the various 

 groups of cells in the anterior horn of grey matter have 

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



B. CONDUCTING PATHS. 



Outgoing and ingoing fibres chiefly pass down the side of 

 the cord upon which they act or from which they come. 

 Section of one side of the cord leads to loss of the so-called 



Spinal Nerve 



FIG. 81. To show the course of upgoing (dotted line) and downgoing 

 fibres (continuous line) in the spinal cord. 



voluntary movements and loss of sensation below the point 

 of section on the same side. Since there is, at least some- 

 times, a slight loss of voluntary power and of sensation on 



