MUSCLES OF THE ABDOMINAL CAVITY. 345 



intercostals, which elevate the ribs, and the cavity of the thorax 

 is more enlarged. In time of violent exercise, or whatever 

 cause drives the blood with unusual celerity towards the lungs, 

 the pectoral muscles, the serrati antici majores, the serrati pos- 

 tici superiores, and scaleni muscles, are brought into action. 

 These effect the lateral dilatation of the thorax. And in labo- 

 rious inspiration, the muscles which arise from the upper part 

 of the thorax, when the parts into which they are inserted are 

 fixed, likewise assist. In expiration, the diaphragm is relaxed 

 and pushed up by the pressure of the abdominal muscles upon 

 the viscera of the abdomen ; and at the same time that they 

 press it upwards, they also, together with the sterno-costales 

 and serrati postici inferiores, pull down the ribs, and are assist- 

 ed, in a powerful manner, by the elasticity of the cartilages 

 that join the ribs to the sternum ; by which the cavity of the 

 thorax is diminished, and the air suddenly pushed out of the 

 lungs : and, in laborious expiration, the quadrat! lumborum, 

 sacro-lumbales, and longissimi dorsi, concur in pulling down 

 the ribs. The diaphragm, contributes the principal share to 

 the dilatation of the chest during inspiration. When relaxed, 

 the diaphragm is arched, and the top of the arch is nearly on 

 a horizontal level with the anterior portion of the fourth rib, as 

 seen in fig. 88, page 342. When contracted, the arch is flat- 

 tened, (though the cordiform tendon itself, is but little depress- 

 ed,) and the capacity of the thorax is increased, at the same 

 time that the abdominal viscera are pressed downwards, so as 

 to produce the protrusion of the abdomen observed during in- 

 spiration. The abdominal muscles and the diaphragm, usually 

 antagonize each other, by contracting alternately. Occasionally 

 they contract in unison, as in straining during defecation, parturi- 

 tion, &c., and compress the viscera and their contents, between 

 the two planes which they form, with such force as to give rise 

 at' times to hernial protrusions. 



In natural tranquil inspiration, the dilatation of the chest is 

 effected almost wholly by the diaphragm. 

 The four pair are, 



