TISSUES OF SPECIAL FUNCTION. 99 



form a part of the " muscle-spindles," structures that contain nerve- 

 endings which have to do with the muscular sense. These muscle- 

 spindles have not been observed in the pharynx and oesophagus or 

 in the muscles of the eye, larynx, and penis. In other voluntary 

 muscles they appear to be present in the proportion of about one 

 spindle to one hundred muscle-fibres. The muscle-fibres connected 

 with these spindles are grouped in collections of from three to twenty 

 individual fibres. They are much smaller than the ordinary muscle- 

 fibre and their cross-striations much coarser. They possess a sarco- 

 lemma and nuclei situated immediately beneath it, but in addition 

 contain oval or round nuclei situated in the contractile substance- 

 itself. These fibres, sometimes called the fibres of Weismann, pass, 

 through the muscle-spindles, so that one may distinguish an intrafusal 



Fig. 85. 



- - - y >I 4i ■ ~ 



Cross-section of muscle-spindle from plantar muscle of cat : c, capsule ; a. s., axial sheath ; 

 i. /., intrafusal fibre with central nucleus ; p. a. s., periaxial space ; s. n., medullated 

 nerve ; s. m., ordinary muscle-fibres. (After Huber.) 



portion and extrafusal parts. Toward the centre of the intrafusal 

 part the nuclei within the contractile substance are more abundant 

 than elsewhere and may entirely displace the contractile substance, 

 so that the fibre in this region consists of a sarcolemma so crowded 

 with nuclei that they flatten each other where in contact. The 

 muscle-spindle consists of a capsule, continuous with the general 

 perimysium, which here becomes laminated and much thicker, resem- 

 bling in structure the outer layers of a Pacinian corpuscle. The 

 sheaths of the nerves which pass into the spindle blend with its 

 capsule, and the nerve-fibres then branch and form a spiral about 

 the muscle-fibres, often ending in flattened expansions in close con- 

 nection with the latter. Besides these sensory nerves there are motor 

 nerves ending in motor plates similar to those terminating the motor 

 nerves distributed to the ordinary muscle-fibres (Fig. 85). 



