76 



THE HUMAN BODY. 



walls of various hollow organs of the body, as the stomach 

 (Fig. 34), the intestines, and the arteries ; by their con- 

 tractions they move things contained in those cavities. 

 Like the voluntary muscles, the involuntary consist of 

 contractile elements, with accessory con- 

 nective tissue, blood-vessels, and nerves ; 

 but their fibres have a very different appear- 

 ance under the microscope. They are not 

 cross-striped, but are made up of elongated 

 cells united by a small amount of cement- 

 ing material. Each cell (Fig. 35), is flat- 

 tish, and tapers off toward its ends ; in its 

 centre is a nucleus with one or two nucle- 

 oli. The cells have the power of shortening 

 in the direction of their long axes. 



Heart muscle. The muscular tissue of 

 the heart is not under the control of th& 

 will ; it, however, is cross-striped, and more 

 like the voluntary than the ordinary in- 

 voluntary muscle, though it differs in some 

 FIG. 35. Unstriped respects from both. 



muscle-cells. 



Speaking generally, we may say that 

 the movements necessary for the nutrition of the body are 

 not left for us to look after ourselves, but are carried on 

 by muscles which work involuntarily ; the blood is 

 pumped round by the heart, and food churned up in the 



What is their function? What are they composed of ? What is 

 seen when a cell from an involuntary muscle is examined with the 

 microscope? 



Is the heart muscle voluntary? In what respect does it resemble 

 voluntary muscle? 



What movements of the body does nature not leave to our own 

 control? Give examples. 



