THE TISSUES 37 



take food and breath, and to get rid of the waste of muscle 

 the organs of excretion act. Hence it is in connection with 

 muscle that all the problems of nutrition digestion, respira- 

 tion, circulation, and excretion have to be studied. 



I. MUSCLE 



The two great functions of muscle are 



To perform mechanical work. 

 To liberate heat. 



1. MUSCLE AT REST 

 Structure : Chemistry and Physical Characters 



The first trace of the evolution of muscle is found among 

 the infusoria, where, in certain cells, in parts of the proto- 

 plasm, the network or cytomitoma is arranged in long parallel 

 threads in the direction of which the cell contracts and 

 expands. Such a development has been termed a myoid. 



1. Structure of Muscle 



Even a cursory examination of mammalian muscle shows 

 that those of the trunk and limbs, skeletal muscles, are 

 different from those of such internal organs, as the bladder, 

 uterus and alimentary canal, visceral muscles. 



The visceral muscles appear to be formed from cells similar 

 to ordinary connective tissue cells. These elongate, acquire a 

 covering, and their protoplasm becomes definitely longitudinally 

 fibrillated by the arrangement of the cytomitoma. They thus 

 become spindle-shaped cells, varying in length from about 50 to 

 200 micro-millimetres. A covering membrane, the sarcolemma, 

 develops. This is thin, but tough and elastic, and it adapts 

 itself to the surface of the cell, unless when this is excessively 

 shortened, in which case the sarcolemma may be thrown into 

 folds, which give the cell the appearance of cross-striping. 

 The nucleus is usually long, almost rod-shaped, and is 

 independent of the cytomitoma (fig. 15, a). 



The skeletal muscles develop from a special set of cells, 

 early differentiated as the muscle- plates in the mesoblast 



