SENSORY ENDINGS. 103 



meniscus, may thereby be modified so that it is larger and clearer, having 

 a more vesicular nucleus, than those around it. Cells thus differentiated 

 are called tactile cells. 



The sensory nerves to muscles similarly may end freely, or may be 

 in special relation with modified muscle fibers. In the former case (Fig. 

 131, sensory fibers] the nerves become non-medulla ted and their fibers 

 arborize extensively, terminating in long slender filaments between the 

 muscle cells. The specially modified muscle fibers in contact with which 

 sensory nerves end, constitute the muscle spindles (Fig. 105, p. 88). 

 These are bundles of from 3 to 20 muscle fibers, i to 4 mm. long, varying 

 in width from 80 to 200 . They are surrounded by a thick connective 

 tissue sheath or capsule, continuous with the perimysium and described 

 as divided into an inner and an outer portion by a considerable tissue 

 space filled with fluid. The muscle fibers of the spindle are distinctly 

 striated toward their tapering and very slender ends. In their middle 



Epidermis. 

 Tactile cell.-'*" 



FIG. 121. FROM A VERTICAL SECTION OF THE SKIX OF THE GREAT TOE OF A MAX TWENTY-FIVE YEARS 



OLD. X24o. 



The outlines of the cells and the nuclei of the epidermis can only be indistinctly seen, x, Tactile cells 

 in the corium, resting upon the ramifications of a delicate nerve fiber. 



portions the striations are obscure; there the sarcoplasm is abundant and 

 the muscle nuclei are numerous. Three or four nerves terminate in each 

 muscle spindle. Their connective tissue sheaths blend with the perimysial 

 capsule, and they branch and lose their myelin as they pass though this 

 capsule to the muscle cells. They may encircle the muscle fibers of the 

 spindle, forming spirals or rings (as in the upper part of Fig. 122) or they 

 may form a panicle of branches with enlarged club-shaped ends. Muscle 

 spindles are not found in the muscles of the eye, pharynx, larynx, and 

 oesophagus, the muscles of expression, the diaphragm and the ischio- and 

 bulbo-cavernosus muscles. They are especially numerous in the muscles 

 of the hand and foot. The nerves of the spindles are stimulated by pres- 

 sure caused by the contraction of adjoining muscle fibers. 



In tendons there are said to be free nerve endings, but the sensory 

 fibers which terminate in tendon spindles are better known. These 

 are small portions of the tendon, from i to 3 mm. long, 170 to 250 , wide, 



