336 PHYSIOLOGY 



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 contraction, 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 phenomena, 

 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 charac- 

 teristic 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 appropriate 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 

 Rufnni. 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 : 



Latency of knee-jerk ..... 5-3o"* 



Afferent endings . . . . . 0-4o- 



Nerve conduction . . . . .1-4 

 Motor endings and action current . . 1-3 



-3-1 



Synapse time . . . 2-2o- 



In this case the shortest latency determined for nerve -endings has been deducted 

 from the shortest latent period obtained from the knee-jerk in the spinal cat. On 

 the other hand, some decapitated preparations have been found to present considerably 

 longer latent periods, e.g. 11 and 12<r. This variation in the latent period supports 

 the view that the knee-jerk is a reflex of which the synapse time is very short, about 2<r, 



* o- = -001 sec. 



