ii SENSIBILITY OF THE INTERNAL ORGANS 89 



them lie in the tendons or tendinous sheaths, the fascia, and the 

 surfaces of the joints. So that the term " muscular sensations " 

 is justifiable only if we admit, with Sherrington, that it covers 

 the sum of the sensations which originate in the motor apparatus, 

 that is, in the muscles and accessory organs of movement. 



Various theories of the nature and origin of these sensations 

 have been put forward, which we will next examine one by one, 

 this being the best way to discuss the whole subject and its 

 significance. 



It is evident that the ' sensations that originate in the active 

 state of the muscles are intimately mingled with the cutaneous 

 sensations of contact or pressure. When a muscle contracts, the 

 soft parts are displaced, so that the skin relaxes and forms folds 

 in certain regions, while in others it becomes tense. The relaxa- 

 tion or tension of the skin, and the rapidity with which it occurs, 

 are proportional to the extent, rapidity, and duration of the 

 movement or contraction of the muscle. So that the sensations 

 aroused by means of the tactile cutaneous nerves are able to 

 inform us of the energy, speed, and duration of the muscular 

 contraction. 



On the strength of this fact, and in view of the delicacy of 

 cutaneous sensibility, certain physiologists, including M. Schiff 

 (1855), Lotze, Hansen, and Auber, assumed that in explaining the 

 origin of the so-called muscular sensations it was unnecessary to 

 recognise the existence of a specific peripheral sensory apparatus, 

 with special afferent nerves, located in the organs of movement, 

 inasmuch as they were adequately aroused by excitation of the 

 tactile cutaneous nerves which inevitably takes place whenever 

 the muscles become active. 



But even if we admit the more or less appreciable intervention 

 of cutaneous sensation during the activity of the muscles, it can 

 easily be demonstrated that this is not sufficient to explain the 

 whole of the phenomena included under the term " muscular 

 sensations." 



In the frog it is possible to suppress the whole of the sensations 

 that are cutaneous in origin by removing the skin from the four 

 legs without perceptibly affecting the regularity of the customary 

 movements of walking, jumping, and swimming. This is Cl. 

 Bernard's experiment, whicli proves that the regularity of the 

 frog's movements is independent of any possible controlling action 

 by the cutaneous sensations, making it highly probable that the 

 controlling action depends fundamentally upon sensations trans- 

 mitted directly from the muscles or indirectly from the passive 

 organs of the motor system. 



For the true solution of the question it is necessary to take 

 into consideration the phenomena observed on man in cases of 

 pathological alterations of cutaneous and muscular sensibility. 



