THE SPINAL CORD AS A REFLEX CENTRE 335 



tion of the reflex arc was to keep the muscle in a state of wakefulness, 

 ready to respond to the slightest local stimulation. No one has however 

 succeeded in imitating, by a slight continuous stimulation of the motor 

 nerve or otherwise, this reputed action of the reflex arc, and recent re- 

 searches by Snyder and by Jolly indicate that, in spite of the rapidity of 

 the response, the knee-jerk may nevertheless be a true reflex action, and 

 in fact the most rapid reflex known. 



Jolly, using the string galvanometer, has taken the current of action in the vastus 

 internus muscle as an index of the commencing contraction of this muscle in the knee- 

 jerk. He has also by the same method, by leading off the afferent and efferent nerves 

 respectively, measured the lost time in the sense-organs and in the motor end-plates 

 of the muscle. In the spinal cord he obtained the following electrical latencies in one 

 case : 



Latency of knee-jerk ..... 



Afferent endings . 



Nerve conduction ..... 



Motor endings and action current . 



Synapse time .... 2-2cr 



In this case the shortest latency determined for nerve-endings has been deducted 

 from the shortest latent period obtained from the knee-jerk in the spinal cat. On 

 the other hand, some decapitated preparations have been found to present considerably 

 longer latent periods, e.g. 11 and 12cr. This variation in the latent period supports 

 the view that the knee-jerk is a reflex of which the synapse time is very short, about 2<r, 

 and that in certain cases there may be increased delay in the spinal cord. When the 

 latencies of the knee-jerk and the homonymous flexor reflex are compared by the 

 electrical method, it is found that the latter is roughly double the former, the average 

 latency of the knee-jerk in the spinal cat being 6-60-, and of the homonymous flexor 

 reflex 13-2rr. Jolly suggests that this difference may be due to the fact that the knee- 

 jerk mechanism involves only one spinal synapse or set of synapses, while the flexor 

 reflex may involve two. In these estimates the rate of conduction in mammalian nerve 

 has been taken at 120 metres per second. 



Especially interesting is the relation shown by Sherrington to exist 

 between the tonic condition of antagonistic muscles, e.g. between the ham- 

 strings and the vastus internus of the quadriceps extensor muscle. Section 

 of the hamstring muscles (so as to relax them) or even section of their nerve 

 causes at once great increase in the jerk elicited by tapping the patellar 

 tendon . On the other hand, the knee-jerk is abolished by stretching the ham- 

 string muscles, or by weak stimulation of the central end of the cut nerve to 

 the hamstrings (Fig. 168). 



In this way a voluntary flexion of the knee by contraction of the ham- 

 strings automatically abolishes the resistance which would be offered by 

 the tonic contraction of the extensor muscles. In the absence of such an 

 arrangement every movement of a joint, by stretching the antagonistic 

 muscles, would automatically increase their tone, and thus set up a resistance 

 to itself. The subject would thus be muscle-bound. 



* a- = -001 sec. 



