202 CENTRAL NERVOUS SYSTEM 



along motor nerves are cut off. These impulses are, perhaps, 

 largely reflex in nature, for removal of the skin of the leg, or 

 section of the posterior roots, leaving the anterior roots intact, 

 produces the same result. There is abundant evidence that 

 in the case of many special centres of the brain and cord, such 

 a tonic activity exists. The degeneration of muscles after sec- 

 tion of their motor nerves favors the idea that a tonic activity 

 of the cord favors their nutrition, since there is no evidence for 

 the existence of special trophic fibers. The rigidity of the 

 muscles, often observed in paralysis from lesions of the central 

 nervous system, is also, at least partly, due to reflex impulses. 

 In such cases myotatic irritability is increased; the knee-jerk 

 and the ankle clonus, etc., are exaggerated. Myotatic irri- 

 tability depends upon a reflex muscular tone. 



Knee-jerk. If the leg is placed in an easy position, as when 

 it rests on the other leg, and a sharp blow is directed against 

 the patellar tendon, the foot will be brought forward with a 

 sudden jerk. This is known as the knee-jerk, and is caused 

 by the sudden contraction of the quadriceps femoris. Physiolo- 

 gists are divided in the explanation of this phenomenon, some 

 regarding it as a true reflex, while others maintain that the con- 

 traction of the muscle is due to a direct stimulation of the muscle 

 by vibrations set up in the tense tendon. 



Considered as a reflex, the afferent path of the impulses is 

 over fibers starting in the patellar tendon, running in the ante- 

 rior crural nerve, and reaching the cord by the posterior root 

 of the fourth lumbar nerve in man. The centre for the knee- 

 jerk lies in the third and fourth lumbar segments of the cord. 

 The efferent path is over fibers in the anterior crural nerve 

 which leave the cord by way of the anterior roots of the third 

 and fourth lumbar nerves. If any portion of this reflex arc is 

 destroyed, the knee-jerk is no longer to be obtained. It may 

 be augmented or depressed by nervous impulses from many 

 parts of the central nervous system. It is reenforced by cutting 

 the nerves of the antagonistic muscles, and depressed by stim- 

 ulating the central ends of these nerves. This is explained by 

 the fact that flexor muscles send impulses to the spinal centre, 

 and according to their condition influence the contractions of 

 the extensors. 



It is found that a deficiency of the knee-jerk exists usually, 



