316 MICROSCOPIC AS ATOMY OF THE ORGANS. 



longing to the connective-tissue sheath of the fibre. Fibril 

 bundles are often absent in the centre of the fibre. Here 

 there is a core of sarcoplasm containing a number of nuclei. 

 This is more noticeable in the equatorial region of the spin- 

 dle than at either pole. In comparing this structure with 

 that of developing muscle fibres, as described in a previous 

 section, it will be noted that there are many points of resem- 

 blance. 



Spindle Nerves. These are large medullated nerve fibres 

 which break up in the muscle spindle. They are spinal gan- 

 glion fibres which do not degenerate on the destruction of motor 

 fibres supplying the muscle (Sherrington). Two to eight nerve 

 fibres enter each spindle near the proximal end. The capsule 

 of the spindle becomes continuous with Henle's sheath or with 

 the connective tissue covering a bundle of fibres. Medullated 

 fibres pass through the axial sheath with which the remains of 

 the connective-tissue sheath become continuous. Within the 

 axial sheath the fibres may become non-medullated. The ulti- 

 mate nerve-endings are non-medullated branches which occur 

 between the sarcolemma and the connective-tissue sheath sur- 

 rounding the intrafusal fibres. According to Ruffini, the ter- 

 minations of these branches may be spiral, annular, or flower- 

 like. In the first kind the nerve fibre flattens out and winds 

 spirally around the intrafusal fibre. This is the most charac- 

 teristic ending, and that described as annular is only a modi- 

 fication of this. The flower-like endings are formed by a 

 branching of the spiral fibres. 



The blood supply of the muscle spindle is well developed. 

 Vessels enter the capsule and give off branches which form a 

 network surrounding the spindle. Other branches proceed 

 to the intrafusal fibres. 



(4) Nerve-endings in Nervous Tissue. 



Under this section the general relation of the neurones to 

 one another in the central nervous system must be considered. 

 Fig. 239 shows the relation of the sensory neurones to the 

 motor. This may involve only two neurones, or there may be 



