268 THE SPECIAL SENSES. 



the body, or, if this also is involved, it may be referred to the area 

 next above or below in the spinal order. The above law, according 

 to which projection is made to the area of high sensibility most 

 closely connected with the area of low sensibility, seems to hold 

 in this case also. 



The Muscle Sense. The existence of a special set of sensory 

 nerve fibers distributed to the muscles was clearly recognized by 

 some of the older physiologists. Charles Bell,* for example, says: 

 "Between the brain and the muscles there is a circle of nerves; one 

 nerve conveys the influence from the brain to the muscle; another 

 gives the sense of the condition of the muscle to the brain." The 

 conclusive proof of the existence of such fibers, however, has only 

 been furnished within recent years. It has been demonstrated 

 that there are special sensory endings in the muscles, the so-called 

 muscle spindles, and in the attached tendons, the tendon spindles 

 or tendon organs of Golgi. The muscle spindles are found most 

 frequently in the neighborhood of the tendons, at tendinous inter- 

 sections or under aponeuroses. Sherringtonf has shown that the 

 nerve fibers in them do not degenerate after section of the anterior 

 roots of the corresponding spinal nerves and are therefore derived 

 from the posterior roots. In the muscles of the limbs he estimates 

 that from one-half to one-third of the fibers in the muscular nerve 

 branches are sensory, and that most of these sensory fibers end in 

 the muscle spindles. On the physiological and clinical side facts of 

 various kinds have accumulated that make clear the existence of 

 this group of sensory fibers and emphasize their essential importance 

 in the co-ordination of our muscular movements. It has been shown 

 that stimulation of the nerves distributed to the muscles or mechani- 

 cal stimulation of the muscles themselves causes a depressor effect 

 upon blood-pressure, thus demonstrating the presence of afferent 

 fibers in the muscles. As described in the section upon the central 

 nervous system, the numerous experiments upon the effect of section 

 of the posterior and lateral columns of the cord and observations 

 upon the results of pathological lesions of the posterior columns 

 (tabes dorsalis) give results which are interpreted to mean that fibers 

 of muscular sensibility form the most important group in the 

 posterior columns and constitute, as well, perhaps, the long, ascend- 

 ing fibers in the tracts of Flechsig and Gowers in the lateral col- 

 umns. It is believed, therefore, that our so-called voluntary muscles 

 are richly supplied with afferent fibers and that the impulses carried 

 by these fibers to the brain are necessary for the proper contraction 

 of the muscles and particularly for the adequate combination of 

 the contractions of groups of muscles in the co-ordinated movements 



* Bell, "The Nervous System of the Human Body," third edition, Lon- 

 don, 1844, p. 200. 



t Sherrington, " Journal of Physiology," 17, 237, 1894. 



