288 THE SPECIAL SENSES. 



Muscular or Deep Sensibility. 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 fur- 

 nished 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. Sherrington* 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 funiculi of the cord, and observations 

 upon the results of pathological lesions of the posterior funiculi 

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

 fibers of muscular sensibility form the most important group in 

 the posterior funiculi and constitute, as well, perhaps, the long, 

 ascending fibers in the cerebellospinal fasciculus in the lateral 

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

 muscles are richly supplied with afferent fibers and that the im- 

 pulses carried by these fibers to the brain are necessary for the 

 proper contraction of the muscles, and particularly for the ade- 

 quate combination of the contractions of groups of muscles in 

 the co-ordinated movements of equilibrium. Indeed, section of 

 the posterior roots of the spinal nerves supplying a given region 

 is followed by a loss of control of the muscles in this region 

 hardly less complete than the paralysis produced by direct 



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

 don, 1844, p. 200. 



t Sherrington, " Journal of Physiology," 17, 237, 1874. 



