212 Basic Structure of Vertebrates 



dividing into fine fibrils which are in close relation to cells of the 

 tissue — free nerve-termination like that in the skin. Such termina- 

 tions in an internal epithelium are doubtless interoceptive in function. 

 But the free nerve-terminations commonly found within masses of 

 muscle-tissue and in tendons and connective tissue are propriocep- 

 tive. Even the marrow in hollow bones contains proprioceptive nerve- 

 terminations. Some of these deep nerve-terminations are in relation 

 to specialized proprioceptors such as the "spindles" of muscles and 

 tendons. 



Muscle-spindles are found deeply embedded in striated muscles, 

 usually near the junction of a muscle with its tendon. The organ has 

 the form of a very long but thin spindle. In man the length may be 

 1.0 to 4.0 mm. and the diameter 0.1 to 0.2 mm. The spindle has a 

 relatively thick sheath of connective tissue. Within the sheath is a 

 bundle of several (up to 20 in man) muscle-fibers. They are shorter 

 and thinner than ordinary fibers of the muscle and their striations are 

 weakly developed. Each spindle is entered by one or more nerve-fibers 

 which are related in various ways to the muscle-fibers of the spindle. 

 The nerves may form rings or spirals around the muscle-cells or may 

 terminate in systems of branching fibrils (Fig. 183). The sheath is 

 usually attached by one end to the tendon, the other end continuing 

 into the intramuscular connective tissue. These spindles presumably 

 register the degree of contraction of a muscle and the varying positions 

 of the part of the body. 



Tendon-spindles resemble those of muscle, but the fibers con- 

 tained within the sheath are, of course, similar to those of the tendon 

 by which the spindle is enclosed. They presumably register the degree 

 of strain in the tendon. 



MUSCLE FIBERS pENDRITES RINGS SPIRALS SHEATH 





SHEATH 



Fig. 183. A portion of a muscle-spindle from a gold chloride preparation of a 

 cat muscle. The sensory nerve-terminations take the form of spirals, or rings, or 

 branched dendrites. A connective-tissue sheath surrounds the spindle. (Redrawn 

 from Jordan and Ferguson, after Ruffini. Courtesy, Neal and Rand: "Choi date 

 Anatomy," Philadelphia, The Blakiston Company.) 



