NERVE-ENDINGS. 315 



The muscle spindle consists of the following structures: 

 capsule, periaxial space, axial sheath, intrafusal muscle fibres, 

 and spindle nerves (Fig. 238). 



The capsule (perimysial sheath) consists of white fibrous 

 connective tissue arranged in six to eight consecutive layers. 

 Practically no elastic fibres are present. In mammals the cap- 

 sule is usually thicker than in lower vertebrates. The capsule 

 is continuous with a connective-tissue sheath surrounding the 



Cross-section of muscle spindle tVom plantar muscle of cat. ( After Huber.) c, capsule ; 

 a. s., axial sheath: ?../., intrafusal fibre: p. a. s., periaxial space; s. n., medullated spindle 

 nerve ; s. m., ordinary muscle fibres. 



muscle fibres which enter the muscle spindle at its proximal 

 end. The long axis of the spindle lies parallel to that of 

 the muscle fibre. The distal end is continuous with the peri- 

 mysium internum. 



T\\Q periaxial space is a lymph space described by Golgi 

 and Sherringtoh lying immediately under the capsule. It 

 is widest at the centre of the spindle. 



The axial sheath consists of a layer of fine white fibrous 

 connective tissue enclosing the intrafusal muscle fibres. The 

 latter vary in number (one to twenty), and are smaller than 

 ordinary muscle fibres. They are rich in protoplasm, and are 

 formed by the division of ordinary muscle fibres entering the 

 spindle. They run more or less parallel to the long axis of the 

 spindle. A sarcolemma is not always present (Sherrington). 

 At the periphery of the fibre there is, according to Sherrington, 

 a series of muscle nuclei. These are regarded by Huber as be- 



