3. Respiration is performed by receiving the^air 

 info the lun*s, am! breathing it out alternately. In 

 the former, the cavity of the breast is enlarged by the 

 sinking of the diaphragm, and the erection of the ribs, 

 through the force of the muscles placid between them. 

 In the latter it is contracted, the diaphragm rising and 

 the ribs failing again. Whenever the cavity of the 

 breast is enlarged, the air by its weight naturally pre?* 

 ses into it, and mixing with the blood in the vesicle of 

 t-clurigs, makes it more fluid, globular, and fit for 

 motion. Air is likewise absolutely necessary in the 

 body, to counteract the pressure of the outward air. 



But if the blood in the lungs of a foetus has not the 

 advantage of respiration, it receives a portion of air, 

 transmitted with its mother's blood by the umbilical 

 vessels, to be diffused through the body. This is quite 

 iroassary, as appears hence; tie the navel string very 

 tight, and the child dies like a man strangled. 



One use of respiration is, to push the blood from the 

 r'ght to the left ventricle of the heart : hence it is, that 

 persons strangled so suddenly die, because with respi- 

 ration the circulation of the blood ceases. And this 

 is the true cause of the diastole of the heart ; the 

 weight of the incumbent atmosphere being the true an- 

 tagonist to all the muscles that serve both for inspira, 

 tion and the contraction of the heart. As in the ele- 

 vation of the ribs a passage is opened for the blood in. 

 to the lungs, so in the depression thereof, by the sub 

 sfciing of the lungs and compression of the blood-vessels 

 thereby, the blood is driven through the pulmonary 

 vein into the left ventricle of the heart. And this, to- 

 gether with the general compression of the body, by 

 the weight of the atmosphere, is that power which 

 causes the blood to mount in the veins, when the 

 force impressed on it by the heart is nearly spent, and 

 jwhich forces the heart ttself from its natural state of 

 contraction to that of dilatation. 



- W;*en in an ordinary expiration, tl^e pressure on 

 the larynx is two ounces, the pressure on the whole 



