THE STRUCTURE OF THE MOTOR ORGANS. 121 



sarcoplasm lying between them and the displaced portion ac- 

 cumulates nearer the ends of the sarcous elements, in the 

 space left by that portion of the hyaloplasm which has entered 

 the tubules: compare gg in A and B, Fig. 58. 



Arguing from the analogy of the amoeboid cell in which, 

 as we have seen (p. 27), parts consisting only of hyaloplasm 

 can exhibit movements, it would seem probable that in the 

 muscle-fibre the hyaloplasm is to be regarded as the active 

 contractile portion and the sarcosomes as a framework 

 directing the form which the contracted hyaloplasm shall 

 assume, and assuring that it shall be a precise and definite 

 shortening in the direction of the long axis of the fibre with 

 a widening in the transverse direction, instead of such irreg- 

 ular changes of form as are exhibited by the amoeboid cell 

 with its irregularly arranged or, sometimes, entirely absent 

 reticulum. That the hyaloplasm and not the sarcoplasm 

 form the contractile part of the fibre is proved by the fact 

 that in some insect-muscles in which they are unusually large, 

 it is possible to isolate them while alive and observe them still 

 contracting. 



The nuclei of the fibres lie in the sarcoplasm, which rep- 

 resents a part of the original protoplasm of the row of cells 

 from which each muscle-fibre develops, that has remained 

 but little changed while the rest was differentiated into 

 sarcous elements. 



The blood-vessels and nerve-fibres supplied to the skeletal 

 muscles are numerous. The larger blood-vessels run in the 

 coarser partitions of the connective tissue lying between the 

 fasciculi and give off fine branches which form a network be- 

 tween the individual fibres but never penetrate the sarcolemma. 



Connected with each muscle-fibre is a nerve-fibre of the 

 white variety (Chap. XII). The central core of the nerve-fibre 

 ends in an oval expansion (end plate) which contains many 

 nuclei and lies close under the sarcolemma, its deeper side 

 being in immediate contact and possibly continuous with 

 the striated contents. These nerve-fibres are motor or con- 

 cerned in exciting a contraction of the muscle-fibre. Other 

 white nerve-fibres are connected with very peculiar bodies 

 found scattered throughout the muscle, but especially numer- 

 ous near the tendons. They are usually of a size just visible 

 to the unaided eye and from their form have been named 

 muscle-spindles. They appear to be sensory in function. 



