926 KNEE-JERK. [BOOK in. 



impulses seems analogous to that of a supply of arterial blood in 

 maintaining an adequate irritability of the nervous substance, in 

 the latter the afferent impulses lead directly to a discharge of 

 energy. And it is convenient to distinguish the two things by 

 different names. 



598. The close connection between tone and reflex action is 

 illustrated by the so-called ' tendon-phenomena,' which, on the one 

 hand, are considered as cases of ordinary reflex action, and, on the 

 other hand, have been regarded as exemplifying a special influence 

 of the spinal cord on the irritability of the muscles. It is well 

 known that when the leg is placed in an easy position^ resting for 

 instance on the other leg, a sharp blow on the patellar tendon will 

 cause a sudden jerk forward of the leg, brought about by a 

 contraction of the quadriceps femoris ; it is necessary or at least 

 desirable for a good development of the jerk that the tendon (and 

 muscle) should be somewhat on the stretch. Similarly the muscles 

 of the calf may be thrown into action by tapping the tendo 

 Achillis put somewhat on the stretch by flexion of the foot ; and 

 in some cases the same muscles may be made to execute a series 

 of regular rhythmic contractions, called 'clonic' contractions, by 

 suddenly pressing back the sole of the foot so as to put them on 

 the stretch. These, and other instances of a like kind, at first 

 sight appear to be, and indeed are by many observers maintained 

 to be, cases of reflex action, due to afferent impulses started in the 

 tendon; hence they have been frequently spoken of as 'tendon- 

 reflex.' Other observers maintain that they are not reflex, but 

 due to direct stimulation of the muscles, the vibrations set up in 

 the more or less tense tendon being transmitted to the muscles 

 and so throwing the latter into contractions. The chief arguments 

 against their being reflex are that the interval between the tap 

 and the contraction is very short *03 or '04 sec., shorter than the 

 ordinary interval of a reflex action ( 594) and that the movement 

 persists after section of the nerves of the tendon. The first 

 argument is perhaps not a very strong one, and the second may be 

 met by supposing that, in such a case at least, if not always, the 

 reflex act really begins in the muscle being started in it by the 

 vibrations transmitted to it along the tendon. 



But even if we admit that the movements are purely muscular, 

 started and carried out in the muscle without the help of the usual 

 reflex chain of afferent impulses, spinal centre and efferent im- 

 pulses, we must at the same time admit that they are closely 

 dependent on the integrity of the spinal cord and of the connec- 

 tions between the cord and the muscle. In the case of animals 

 they disappear when the spinal cord is destroyed, or the nerves 

 going to the muscles are severed, or even when the posterior roots 

 only are divided. The measure of their development both in 

 animals and in man is also closely dependent on the condition of the 

 spinal cord and of the central nervous system generally. They may 



