276 THE BODY AT WORK 



must stand on a different level to other reflexes. The tone- 

 impulses which cause them are incessantly patrolling to and 

 fro from sense-organs to nerve-endings. The paths they follow 

 must be the most open in the nervous system. Receptors and 

 effectors must, in an electrician's phrase, be incessantly 

 switched on ; or, to express the analogy more accurately, the 

 flexor and extensor tone mechanisms are incessantly and 

 reciprocally switching each other on and off. It must be con- 

 fessed that it is very difficult to explain the knee-jerk if it be 

 not a reflex action, but, as has been supposed, a direct response 

 of the thigh muscles to their own stretching. The latter 

 hypothesis does not appear to be reconcilable with its depen- 

 dence upon the maintenance of the nervous connection of 

 the muscles with the spinal cord. It cannot be elicited unless 

 the "spinal arc" is intact. It ceases after the severance of either 

 sensory or motor roots. Nor will it occur if the supply of 

 blood to the lower end of the spinal cord has been cut off. 

 Still more difficult is it to explain its extraordinary sympathy 

 with everything that happens in the whole nervous system, 

 if the impulses which cause it do not pass through the spinal 

 cord. By a very simple mechanical arrangement it is possible 

 to record the amplitude of the knee-jerk. The foot moves a 

 lever which writes on a travelling surface. The jerk is elicited 

 by the hammer of a clock strapped to the shin. In this way 

 it is possible to extend the period of observation over several 

 consecutive hours, the subject becoming completely oblivious 

 of the movement his foot is making once a second, if it be 

 screened from his view. In deep sleep the jerks stop ; but 

 the subject may doze, and still jerk follows tap. And the 

 record made by his foot mirrors all the changes in his nervous 

 system. If he clench his fist, the movement is reinforced, as 

 it is when a child cries, a lamp is lighted, his ear itches. There 

 is music in an adjoining room. His foot is the baton which 

 beats fortissimo to Wagner, and is lulled to piano by the 

 " Lieder ohne Worte." On a bright day this spinal pulse throbs 

 gaily. It is indolent in dull, depressing weather. The knee-jerk 

 is the physician's guide to the condition of the nervous system. 

 Elasticity of Muscles. Muscles are very extensible, and after 

 stretching return to their original length. Their elasticity is 

 a quality of great practical importance. It enables them to 



