Chap, ix.] STRIPED MUSCULAR TISSUE. 7 1 



substance, is the material at the expense of which 

 new fibres are formed, or fibres already formed 

 become thickened, as is the case when muscle fibres 

 are kept at constant work. 



In the muscular fibres of man and most verte- 

 brates (except the fibres of the heart), the muscular 

 corpuscles are situated on the surface of the muscular 

 substance ; but in invertebrates (especially insects and 

 Crustacea) they are often found in the central part 

 of the fibres, and here they are occasionally seen 

 forming almost a continuous cylindrical mass of nu- 

 cleated protoplasmic cells. 



89. In the embryo the muscular fibres are de- 

 veloped from spindle-shaped nucleated cells (Remak, 

 Weissmann, Kolliker). One spindle-shaped cell with 

 an oval nucleus grows rapidly in length and thick- 

 ness, its nucleus divides repeatedly, and the offspring 



Fig. 42. A Striped Muscular Fibre of the Diaphragm of a 

 Guinea-pig. 



The muscle-corpuscles are much increased in size and numbers; they are 

 probably used here for the new formation of muscular substance. (Atlas.) 



become shifted from one another as the cell con- 

 tinues to grow in length. The protoplasmic sub- 

 stance all along one side of the cell gives origin to 

 the muscular substance sarc.ous elements and lateral 

 disc while a small rest of protqDlasm remains col- 

 lected around the nucleus as the muscle corpuscle. 

 This protoplasm continues to increase in amount, 



