40 HUMAN PHYSIOLOGY 



threads in the direction of which the cell contracts and 

 expands. Such a development has been termed a myoid. 



i. Structure of Muscle. 



Even a cursory examination of mammalian muscles 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 cytomi- 

 torna. They thus become spindle-shaped cells, varying 



FIG. 15. (a) Fibres of Visceral Muscle ; (b) Fibres of Skeletal Muscle 

 to show sarcolemma, muscle corpuscles, and sarcous substance 

 composed of fibrils showing transverse markings. 



in length from about 50 to 200 micro-millimetres. The 

 covering membrane, sarcolemma, is thin, but tough and 

 elastic, and 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 cytomi- 

 toma (Fig. 15, a). 



The skeletal muscles develop from a special set of cells, 

 early differentiated as the muscle plates in the mesoblast 

 down each side of the vertebral column of the embryo. Each 

 cell elongates. The nucleus divides across, but the cell, 

 instead of also dividing, lengthens and continues to elongate 

 as the two daughter nuclei again divide. The cytomitoma 

 becomes arranged longitudinally, and a series of transverse 



