120 SKELETON. 



consequently, diminished. Whatever may be said of muscular 

 influence in producing this change, it is much exaggerated. It 

 is true, that there are certain muscles which may be applied to 

 this end, as the abdominal, and also some on the back, as the 

 longissimi dorsi and sacro lumbales; but that they are actually 

 so engaged, under ordinary circumstances, is rather questiona- 

 ble. In observing the phenomena of natural respiration, when, 

 by position, all these muscles are put into a state of relaxation, 

 it does not appear that the process is at all impaired by their 

 being thrown out of action. The only muscles, therefore, that 

 seem to be especially appropriated to produce expiration, are 

 few and small: they are the serrati inferiores postici, one on 

 either side of the spine. But, when the lower ribs are fixed by 

 the several muscles inserted into them, they become points of 

 support to the upper ones ; and then the intercostal muscles may 

 officiate in expiration, by drawing the ribs successively down- 

 wards, as they do, in inspiration, by drawing the ribs succes- 

 sively upwards. 



The elasticity of the cartilages, by which these bodies are en- 

 abled to return from the constrained state in which they were 

 placed by inspiration, has also been supposed important to ex- 

 piration, by Haller, and others. The power thus derived is 

 certainly of some value; but has much less than has been at- 

 tached to it. It unquestionably exists in early and middle life," 

 but is lost in old age, when the cartilages ossify, and, therefore, 

 are deprived of elasticity. The true and efficient cause of expi- 

 ration appears to be atmospheric pressure, upon the external 

 parietes of the thorax, acting along with the natural elasticity 

 of the lungs. The lungs, it is well known, when in a state of 

 repose, and removed from the thorax, are much smaller than 

 the cavities which they fill during life. They have, therefore, a 

 continual disposition, in the living state, to return to the size 

 which is most easy to them; and, when they are dilated by 

 inspiration, they subsequently contra<*t. These positions are 

 proved conclusively, by the condition of the inferior surface of 

 the diaphragm, in a healthy and entire thorax: where this mus- 

 cle, in consequence of atmospheric pressure from without, is 

 driven high up into its cavity. Its contraction in inspiration 

 draws it down, and the instant that the contraction ceases, it is 

 impelled upwards again. Now, the same power is applied to 



