456 A MANUAL OF VETERINARY PHYSIOLOGY 



citatory character, while the other efferent conveys inhibitory 

 impulses. These work reflexly under the control of an afferent 

 service, and the nature of the impulses in the afferent determine 

 which of the efferent impulses are to be placed in operation. 

 Muscles, as we have seen, are provided with an effective 

 afferent system of nerves, more than half the fibres belonging to a 

 muscle being of this nature. Sherrington has shown that where 

 the muscle passes into the tendon, a ' nest of afferents ' exists, 

 and he has also proved that the Golgi organ found in tendon is 

 afferent in nature. In spite of this liberal afferent system, there 

 is only a limited efferent outflow ; muscles are well supplied with 

 nerves conveying excitatory (motor) impulses, but there are no 

 nerves conveying inhibitory impulses. Notwithstanding, in- 

 hibitory functions in skeletal muscle are of a most important 

 and widespread character, and they are effected in a purely 

 reflex manner bjr inhibiting the neurone which conveys the 

 excitatory impulses. 



Under the title of reciprocal innervation Sherrington de- 

 scribes what occurs in all normal movement — i.e., a relaxation 

 of antagonistic muscles. For instance, while the flexors of a 

 limb are contracting, their antagonists, the extensors, are re- 

 laxing. This effect is brought about by the tonus (p. 402) of 

 the antagonistic muscles being reduced, owing to an inhibition 

 of the motor cells in the cord which supply the extensor muscles. 

 Reciprocal innervation also saves a waste of nervous energy in 

 overcoming the contraction of antagonists, and it ensures 

 muscular co-ordination. In certain muscular movements antago- 

 nistic muscles may contract concurrently. This is evident in 

 those cases where a muscle is connected with two or more joints, 

 being a flexor of one and an extensor of another. In this case 

 antagonistic muscles are capable by their contraction of inhibiting 

 the extensor movement of one joint in order to permit the muscle 

 to act as flexor of another. The innervation of the antagonistic 

 muscles so employed is no longer reciprocal, but identical, as they 

 must contract and relax at the same time, and not alternately. 



The purposes served by reflex inhibition in skeletal muscular 

 actions are classified by Sherrington as follows :* 



1 . Reflex inhibition cuts short the contraction of one set of muscles 

 when another set is called into play ; it also guards and maintains 

 a reflex already in operation, by preventing its being interrupted by 

 other stimuli. 



2. It grades the degree of intensity of the discharge from nerve 

 centres, by diminishing it to any required extent, so that the needful 

 discharge from a motor centre can be adjusted with the greatest 

 precision. 



* ' The Role of Reflex Inhibition,' British Medical Journal, March 25, 

 191 1, and Science Progress, No. 20, April, 191 1. 



