Cells and Tissues. 



11 



3. In muscular tissue the cell -protoplasm is partially or 

 entirely modified into a peculiar contractile substance, which 

 does not move spontaneously, but only contracts in response to 

 some stimulus.* It differs in another respect also from other 

 protoplasm : the movement of the muscle cell always occurs in a 

 definite way : first, there is a shortening and thickening of the 

 cell followed by a lengthening and narrowing, so that it has assumed 

 its original condition by the end of the movement. The streaming 

 movements seen in the Amoeba, which are characteristic of protoplasm 

 in general, are never found in the contractile substance of muscle 

 cells. 



The simplest form of muscular tissue is composed of smooth 

 muscle cells, which are spindle-shaped, elongate or ribbon-like, 

 and pointed or occasionally forked at 

 both ends. A nucleus is present in 

 each, lying either in the middle of the 

 cell, surrounded by a small quantity 

 of protoplasm, or on one side of the 

 contractile substance, enveloped in a 

 varying amount. The contractile 

 substance is homogeneous and shin- 

 ing, and is destitute of granules or 

 transverse striation, though it may 

 sometimes be longitudinally striped. 

 The muscles of most of the Inverte- 

 brata, with the exception of the 

 Arthropoda, consist of this tissue, and 

 it is found also in Vertebrates, in the 

 walls of the digestive tract. To 

 the smooth are nearly allied the 

 striped muscle cells, the only 

 essential difference being that the 



contractile substance of the latter has a transversely striated 

 appearance, due to its division into regularly alternating discs of 

 ■different refrangibility : it occurs, for example, in the Vertebrate 

 heart. 



The t V an sversely striated muscle fibres differ from the 

 striped muscle cells in that they are multinucleate. A striped muscle 

 fibre originates as a simple cell with one nucleus, which divides repeat- 

 edly without corresponding division of the cell, and in the perfect fibre, 

 the nuclei lie either in or upon the surface of the contractile substance. 

 The fibre is not only transversely striped, but a more or less distinct 

 longitudinal striation, depending on the very delicate fibrillee of which 

 the fibre is composed, may also be noticed. The whole muscle fibre is 



Fig. 13. a — 6 Smooth muscle cella 

 (a with a lar^e residue of protoplasm), 

 c transversely striated muscle cell, 

 d transversely striated muscle fibre. — 

 Orig. 



* As to the source of the stimulus, see below. 



