MUSCLE 275 



and crureus, have contracted. This phenomenon is easy to 

 account for. When we are standing upright, the trunk is 

 supported on three joints, of which one the hip is a perfect 

 ball and socket, and the other two knee and ankle are of 

 the same order so far as the absence of any provision for lock- 

 ing them is concerned. If the muscles on the front and the 

 back of the leg did not constantly adjust our balance, by 

 swaying the trunk forward when it falls back, and pulling 

 it back when it sways forward, the joints of the leg would 

 double up beneath us. A photographer knows how little 

 confidence is to be placed in a man's assertion that he is able 

 to stand still. This see-saw of alternate contraction and re- 

 laxation is kept up by means of nerve-impulses which ascend 

 from the nerve-endings surrounding the separate bundles of 

 tendnos, or from the Pacinian bodies which are found in 

 abundance in the neighbourhood of tendons and ligaments, or 

 from the elaborately twisted nerve-fibres found in muscle- 

 spindles, or possibly from all three classes. Muscle and tendon 

 are richly supplied with sense-organs susceptible to pressure 

 and stretching. There is an abundance of nerve-endings to 

 choose from. The slightest change in their tension, whether 

 due to the muscle's own contraction or to the action upon it 

 of other muscles or weights, is recorded not only in the spinal 

 cord, but also in the cortex of the cerebellum, and, if the con- 

 traction is an act of volition, in the cortex of the great brain. 

 Although it was skin which was tapped, skin-nerves have 

 nothing to do with the jerk. It was the result of the slight 

 sudden stretching. In short, the tone-mechanism has been 

 fooled. Notice the position of the leg. The knee is semiflexed ; 

 the foot is hanging free. There is nothing for the extensor 

 muscles of the thigh to do. Now, if ever, they are justified in 

 dozing. It is not to be wondered at that the sudden stretching 

 of the ligament takes them off their guard, or that on waking 

 they give a quite unreasonable start. The phenomenon is, as 

 we asserted, easy to account for. It would also be easy to ex- 

 plain, if it were not for the extreme rapidity with which the 

 jerk follows the tap. The interval is about one-hundredth of a 

 second. This is thought to be tpo short to allow an impulse 

 to ascend a sensory nerve, pass through the cord, and descend 

 a motor nerve. It is true that these reflexes of adjustment 



182 



