ACTION OF DIAPHRAGM . 121 



As the location determines the function of a muscle, so it often 

 suggests the name, as the pectoralis major and minor, the inter- 

 costals, etc. Sometimes the shape is named, as the orbicularis of 

 the mouth or of the eyelids (orbicular muscles, or sphincters, 

 surround and control openings) . Shape and location may together 

 suggest a name sometimes, as the latissimus dorsi, the rectus 

 abdominis (broadest of the back and straight of the ab- 

 domen) and others, expressing or implying the function of the 

 muscle. 



One of the most useful and interesting muscles in the body is the 

 diaphragm. Although a voluntary muscle in structure, it is asso- 

 ciated with visceral action. (For general description see page 98.) 



The special interest attending this muscle arises from its 

 location as well as its structure. Situated between the great 

 cavities of the trunk it acts upon the organs belonging to both. 

 In contraction, it encroaches upon the cavity of the abdomen 

 pressing upon abdominal organs, and thus aids in expelling the 

 contents of abdominal and pelvic viscera. In this act (expulsion 

 from abdomen or pelvis) it is fixed in contraction (holding the 

 breath) so that other muscles can act efficiently. Examples: 

 defecation, parturition. Ceasing to contract it returns to its inac- 

 tive or dome-shape; and as this is accompanied by slight ab- 

 dominal pressure upward, the effect upon the thorax is to shorten 

 it, causing gentle pressure upon the lungs. 



In contraction, therefore, it compresses the abdomen and 

 enlarges the thorax; in relaxation, it enlarges the abdomen and 

 compresses the thorax. This alternate enlargement and com- 

 pression of the thorax explains its most important function that 

 of a breathing muscle, especially a muscle of inspiration. 



Special points. The lateral portions of the diaphragm are the 

 most movable portions, being mostly muscular. Here the lungs 

 rest upon the falling and rising floor, themselves alternately ex- 

 panding and contracting. The heart lies upon the least movable 

 portion consequently the diaphragm supports the heart but does 

 not press against it unless pushed up from below. 



Similar functions pertain to another muscle constituting the 

 floor of the pelvis (the levator ani and coccygeus taken together), 

 which rises and falls with the displacement and functionating of 

 abdominal organs. With the combined contraction of these two, 



