ONENESS OP THE LUNGS AND CHEST. 87 



city and contractility of the lungs themselves. Thus the force of 

 the diaphragm and of the muscles between the ribs engenders inspi- 

 ration, and overcomes the elasticity of the lungs ; the elastic power 

 of the lungs produces expiration. The prolonged alternation of 

 these two forces is " a contest in which victory declaring on one 

 side, or the other, is [under ordinary circumstances] the instant 

 death of the fabric." 



In the act of breathing we notice four divisions, each of import- 

 ance to our sequel. First, inspiration ; secondly, a pause which 

 ensues when the inspiration is completed; thirdly, expiration; and 

 fourthly, a pause when the expiration ends ; after which inspiration 

 again occurs, and the same course is measured. Inspiration rises to 

 a certain level, and there rests for a time ; expiration descends also 

 to its level, and registers it by a pause. Further, the inspiration 

 may either take place continuously in one long breath, or by several 

 smaller inhalations and pauses; the expiration likewise may either 

 proceed without a stop, or it may be divided into several levels of 

 exhalation, each with its own proper pause. 



But let us come to the use of respiration, or the benefits of breath- 

 ing. These are twofold: 1. The use of the air drawn in, towards 

 the renovation of the blood, and of the air emitted, towards its puri- 

 fication. 2. The mechanical effect of the breathing upon the circu- 

 lation and the body generally. We speak first of the first of these 

 uses, because of the exclusive importance usually attached to it. 



The result of investigations on this subject need detain us but a 

 short time. It is in substance this — that when the air is breathed 

 in, the expanded network of capillaries besetting the air cells absorb 

 from it oxygen into the blood, and at the same time pour forth car- 

 bonic acid gas from the blood, which is carried out of the system by 

 the air leaving the lungs. In consequence of these two changes, the 

 reception of oxygen and the expulsion of carbonic acid, the altera- 

 tion of the blood from dark venous to florid arterial in the air cells, 

 is accounted for. 



These changes, authors tell us, are purely chemical, and the same 

 as happen to venous blood out of the body. We rejoin, however, 

 that this theory is out of place, being chemical and not organic. It 

 deals only with what is external to the body. The air of inspira- 



