35 2 TEXT-BOOK OF PHYSIOLOGY. 



direction downward and forward, the slips blend to form a single 

 muscle, which is inserted mainly into the outer lip of the anterior 

 half of the crest of the ilium and into the central abdominal tendon 

 or aponeurosis. 



The internus abdominis arises mainly from the anterior two- thirds 

 of the inner crest of the ilium and the lumbar fascia. Its fibers pass 

 upward and forward to be inserted into the cartilages of the last three 

 ribs and into the central abdominal tendon. 



The rectus abdominis arises from the crest of the pubes and is 

 inserted above into the cartilages of the fifth, sixth, and seventh ribs, 

 and occasionally into the ensiform cartilage. 



The transversalis arises from the cartilages of the last six ribs, 

 the lumbar fascia, and the anterior half of the crest of the ilium. 

 After passing transversely across the abdomen, the fibers are inserted 

 mainly into the linea alba. 



The conjoint action of these muscles is to diminish the convexity 

 of the abdominal walls and to exert a pressure on the ^abdominal 

 organs. These, taking the line of least resistance, are forced upward 

 against the inferior surface of the diaphragm, which in consequence 

 becomes more strongly curved and ascends higher into the thorax. 

 The vertical diameter of the thorax is thus diminished. Acting from 

 the pelvis as a fixed point, these muscles will also draw downward 

 and inward the lower end of the sternum and the lower ribs and 

 diminish the antero-posterior and transverse diameters. 



The triangularis sterni arises from the side of the posterior surface 

 of the lower third of the sternum and is inserted by fleshy slips into 

 the cartilages from the second to the sixth ribs. From their anatomic 

 relations it is asserted that these muscles assist in the depression of 

 the anterior extremities of the ribs during forced expiration. 



The intercostales interni, eleven in number, occupy the spaces 

 between and are attached to the ribs from the tubercle to the anterior 

 extremity of the cartilages. Their fibers, which are also arranged 

 in parallel bundles, are directed from above downward and back- 

 ward (Fig. 170). 



The action of the internal intercostals is less clearly understood 

 than that of the externals, and for the same reasons. If, however, 

 Bernoulli's model discloses the action of the latter, it equally well 

 discloses the action of the former. Thus, if the parallel bars be joined 

 by an elastic band having the direction of and representing the inter- 

 nal intercostals, and then forcibly elevated, the band is elongated and 

 made tense. On releasing the bars, the elastic band at once recoils 

 and depresses them. Here, again, though the elastic force is the same 

 in both directions, the bars are depressed, for the reason that the 

 component acting downward on the long arm of the lever pre- 

 ponderates over that acting upward on the short arm of the lever. 



