AN INTRODUCTION TO ZOOLOGY 



a faint longitudinal striation. A sarcostyle appears striated from 

 the presence in it of two substances, one of which is clearer and 

 singly refracting, isotropic, and hence appears light, and the other is 

 dense, doubly refractive, i.e. anistropic, and so appears dark. The 

 whole fibre appears striped, because the dark and light bands of the 

 various fibrillae coincide. When examined under a still higher power 

 of the microscope the light band is seen to be divided into two by a 

 very thin dark line, usually taken to be partly membranous, and 



so called Krause's membrane. 

 Between one membrane of 

 Krause and the next, therefore, 

 is included half a light band 

 on each side of a complete dark 

 band, and this constitutes a 

 unit of striate muscle, a sar- 

 comere, and a row of such end 

 to end forms a sarcostyle. The 

 dark band is similarly divided 

 into two parts by a narrow 

 white line, Henson's line. The 

 fibrillae may or may not be uni- 

 formly distributed throughout 

 the fibre, and often in a cross- 

 section we find them grouped 



FIG. 13. Striate muscle. A, Muscle 

 fibre partly teased ; B, part of single 

 fibrilla or sarcostyle in resting con- 

 dition, highly magnified ; C, part of 

 single fibrilla in state of contraction, 



highly magnified. 



a., ainisotropic element ; H., Heuson's line 

 '., isotropic element ; K. t Krause's membrane 

 s., sarcostyle ; sm., sarcomere. 



together in polygonal areas, the 

 areas of cohesion or muscle 

 columns. Striate muscle is not 

 composed of simple uninucleate 



cells, but, as has been pointed out, beneath the sarcolemma are 

 numbers of nuclei. It is found that in development all the structures 

 in one fibre are formed from and by a long multinucleate piece of 

 protoplasm, which arises by the union of many cells. Such a cell 

 fusion is termed a syncytium. 



This striped muscle is well adapted for strong quick movements, 

 such as are required in locomotion, and the various constituent fibres 

 contract in unison, a wave of contraction passing along them from 

 the origin and affecting the sarcomeres successively. The muscle is 

 under the control of the will, and the nerve supplying it branches on 

 entering its substance. The small terminal branches are joined to 

 the muscle fibres by characteristic structures known as end plates, 

 and so all the fibres can be called into activity simultaneously. 

 As a result of a change in the disposition of its constituent sub- 

 stances each sarcomere shortens in length and increases in diameter. 

 It appears as if the material of the light bands passes into that of the 



