PHYSICAL SUPPORT AND MOVEMENT 



95 



the walls of the digestive tract, urinary bladder, gall bladder, arteries 

 and veins, and in certain glands and their ducts. 



Striated muscle differs greatly in its structure from smooth muscle. 

 For one thing, it has many nuclei in each cell. The cells of an embryo 

 from which striated muscle cells develop have only one nucleus apiece, 

 but after a time the nucleus divides a number of times without an 

 accompanying division of the cell body. Many nuclei are thus present 

 in the muscle cells of the adult. The striated muscle cell is roughly 

 cylindrical in form and usually very long. It is covered by a firm mem- 

 branous sheath, the sarcolemma. Within this is the rather liquid proto- 





"P-j- 



m-'^ 



A B C 



Fig. 85. — General appearance of striated muscle. A, part of a muscle fiber of a frog; 

 B, part of a fiber teased out to show myofibrils; dh, darli bands; lb, light bands;/, myofibril; 

 n, nucleus; s, sarcolemma; C, a myofibril, diagrammatic; dh, dark band; Ih, light band with 

 a thin band of dark material dividing it into two portions. (A and B from Parker and 

 Haswell, " Textbook of Zoology.") 



plasm called the sarco'plasm. Imbedded in the sarcoplasm, and forming 

 a large part of the bvilk of the cell, are numerous slender strands, the 

 contractile myofibrils (Fig. S5B,f). Each myofibril consists of alternate 

 segments of different substances, light and dim in appearance. In the 

 muscle cell these myofibrils extend parallel to each other and to the long 

 axis of the cell and are so aligned that the dim segments are side by side, 

 and light segments are side by side. Collectively they give the whole 

 cell the appearance of being marked by light and dark transverse bands 

 (Fig. 85A). These are the marks to which the term "striated" refers. 

 Little is known of the chemical or physical properties of the substances 

 in the light and dim bands, but when they are examined with polarized 

 light it is found that the dark substance is doubly refractive. 



