MUSCLES OF THE TRUNK AND CHEST. 215 



extensive visceral sympathy, as well as an important agent 

 in respiration. It is indeed a muscle of the chest, there 

 being but two occasions when its action favours the abdo- 

 men, and then the other muscles of the chest do the 

 same ; that is, daring straining, to expel the foeces or 

 foetus. In cattle, during the propulsion of the morsel into 

 the mouth, which takes place in rumination, a very gentle 

 effort is perceptible when respiration is suspended, and the 

 diaphragm is inactive. The diaphragm is only partially a 

 muscle of voluntary motion. 



Action of the intercostals and diaphragm. The inter- 

 costals, internal and external, cross one another, their fibres 

 running in opposite directions, and being mingled with 

 much tendinous structure, whereby they gain endurance. 

 Each would draw the ribs more closely together, conse- 

 quently the whole acting simultaneously could but perform 

 that action more energetically. The contraction of the 

 intercostals enlarges the cavity of the thorax ; and nature, 

 to prevent a vacuum, sends the air through the nostrils 

 into the lungs. But the air thus received is not sufficient 

 for the perfect expansion of the lungs ; wherefore the ribs 

 stop when they have gained their full extent, become fixed 

 points, and the diaphragm begins to operate. This last 

 muscle, which was relaxed, and bulging into the thorax, 

 now contracts ; becomes more level, or pushes the abdominal 

 viscera back, and bulges less into the chest. By the 

 retraction of the diaphragm the thorax is still further 

 enlarged, and the quantity of atmosphere now received 

 perfectly expands the lungs. When the lungs are thus 

 inflated, the air within them renders the pressure of the 

 atmosphere without the chest inoperative ; but in a 

 short space the breath taken in becomes unequal to this 

 purpose, and the pressure from without is severely felt. 

 The abdominal muscles contract, draw downward the ribs, 

 push forward the viscera, driving back the diaphragm, and 

 thus lessen the cavity of the thorax ; they expel from the 

 lungs the inspired air, which by this time is highly car- 

 bonized. 



Action of the iviuscles of the back. These are spoken 

 of in connexion with the muscles of respiration, because 

 the two are found to be connected. It is impossible to 



