SENSORY NERVE-ENDINGS. 



175 



ing capsule, within which one or several sensory nerves terminate. 

 They are spindle-shaped structures varying in length from 0.75 to 4 

 -mm., and in breadth, where widest, from 80 to 200 // (Sherrington, 

 94). In them there is recognized a proximal polar region, an 

 equatorial region, and a distal polar region. The muscle-fibers of 

 this nerve-ending, known as the intrafusal fibers, which may vary in 

 number from 3 or 4 to 20 or even more, are much smaller than the 

 ordinary voluntary muscle-fibers and differ from them structur- 

 ally, and result from a division of one or several muscle-fibers of 

 the red variety. In the proximal polar region the intrafusal fibers 

 present an appearance which is similar to that of voluntary muscle- 

 fibers of the red variety ; in the equatorial region they possess rela- 



Nucleus of lamellae. 



End-cell of core. 

 Lamellae. 



Axis-cylinder in core. 

 Cubic cells of core. 



Termination of medul- 

 lary sheath. 



Axis-cylinder of 

 nerve-fiber. 



Medullary sheath of 

 nerve-fiber. 



Neurilemma and sheath 

 of Henle. 



Fig. 142. Corpuscle of Herbst from bill of duck; X o. 



tively few muscle-fibrils and are rich in sarcoplasm and the muscle- 

 nuclei are numerous ; the striation is here indistinct. In the distal 

 polar region the intrafusal fibers are again more distinctly striated 

 and, a short distance beyond the end-organ, become greatly reduced 

 in size, and terminate as very small fibers, still showing, however, a 

 cross-striation. In figure 143 is shown a single intrafusal muscle- 

 fiber. Owing to the length of such a fiber it was necessary to rep- 

 resent it in several segments. 



The intrafusal muscle-fibers are surrounded by a capsule con- 

 sisting of from four to eight concentric layers of white fibrous tissue. 

 At the proximal end this capsule is continuous with the connective 



