THE MUSCULAR SYSTEM 



24; 



being attached to the bones, they usually form a distinct 

 layer in the walls of small cavities or of tubes (Fig. in). 

 Since they are controlled by the 

 part of the nervous system which 

 acts independently of the will, 

 they are said to be involuntary. 

 They contract and relax slowly. 

 Work of the Non-striated Mus- 

 cles. The work of the non- 

 striated muscles, both in purpose 

 and in method, is radically dif- 

 ferent from that of the striated. 

 ,, , . . FIG. in. Non-striated 



They do not change the position muscle cells. A. Cross section 

 of parts of the body, as do the of small artery magnified, show- 

 striated muscles, but they alter in s CO the Ia 7 er of non-striated 

 the sise and shape of the parts cells ' B ' Three non - striated 



f cells highly magnified. 



which they surround. Their pur- 

 pose, as a rule, is to move, or control the movement of, 

 materials within cavities and tubes, and they do this by 

 means of the pressure which they exert. Examples of their 

 action have already been studied in the propulsion of the 

 food through the alimentary canal and in the regulation of 

 the flow of blood through the arteries (pages 159 and 49). 

 While they do not contract so quickly, nor with such great 

 force as the striated muscles, their work is more closely 

 related to the vital processes. 



Structure of the Heart Muscle. The cells of the heart 

 combine the structure and properties of the striated and the 

 non-striated muscle cells, and form an intermediate type be- 

 tween the two. They are cross-striped like the striated cells, 

 and are nearly as wide, but are rather short (Fig. 112). 

 Each cell has a well-defined nucleus, but the sarcolemma is 

 absent. They are placed end to end to form fibers, and 



