380 PHYSIOLOGY 



The value of the tendon phenomena as a means of diagnosis has 

 tended to obscure their great importance in the normal individual. 

 Every joint is protected by inextensible ligaments and by muscles. 

 A sudden strain on a ligament either will have no effect, or will 

 rupture some of its fibres and perhaps injure the adjacent joint 

 surfaces. An ordinary reflex contraction would be powerless to 

 prevent this, since the mischief would be done before the reaction 

 could take place. But the central nervous system confines itself to 

 keeping the muscles awake, so that they themselves may react to 

 any sudden increase in their tension by an equally sudden contrac- 

 tion, which saves the joint before the central nervous system has 

 even become aware of the strain. 



The tone of the muscles, as well as the consequent tendon pheno- 

 mena, is dependent on the integrity of the reflex arc governing the 

 muscles in question. It has been shown by Sherrington that the 

 afferent part of the arc is represented by the afferent nerves from 

 the muscle itself, and that these nerves receive their sense impressions 

 from the special nerve-endings characteristic of muscle the ' muscle- 

 spindles.' Even in the purely muscular nerves a large proportion 

 of the fibres are afferent in function, and, after section of the appro- 

 priate posterior roots distal to the ganglia, as many as 40 per cent, 

 of the fibres going to a muscle may be found degenerated. Though 

 most of these have the muscle-spindles as their destination; a certain 

 number pass to the tendon and aponeuroses connected with the 

 muscle, where they end in the end organs known as the organs of 

 Golgi and the organs of Ruffini. After section of the motor nerves 

 the muscle fibres degenerate, with the exception of the modified 

 fibres, which, enclosed in a connected tissue sheath, are concerned 

 in the formation of the muscle-spindles. Muscle tone and tendon 

 phenomena may therefore be abolished by lesions of afferent nerves, 

 which leave a considerable part of the cutaneous sensibility of the 

 limb intact. In man the spinal reflex mechanism connected with 

 the knee-jerk is situated in the third and fourth lumbar segments. 

 The jerk may be abolished by section of the third and fourth posterior 

 nerve-roots, although to render, the whole hind limb anaesthetic it 

 would be necessary to divide all the roots from the second lumbar to 

 the fourth sacral inclusive. 



Recent researches by Snyder and by Jolly indicate that the reflex 

 nature of the knee-jerk cannot be entirely excluded. 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: 



