33 3 Four Periods in Refpiration. [Book IX. 



pafles obliquely downwards to the loins, and feparates 

 the thoracic from the abdonainal vifcera. By the 

 contraction of the diaphragm, the abdominal vifcera 

 are prefled downwards and forwards, by which the 

 lungs are permitted to expand themfelves in the fame 

 direction j when the diaphragm is relaxed, and the 

 abdominal mufcles are thrown into action, a directly 

 oppofite motion takes p]?e ; the vifcera of the abdo- 

 men are prefied upwards and backwards againft the 

 lungs, from which part of the air is confequently ex- 

 pelled. 



The air, which is to be con fide red as poflefllng 

 many properties in common with other fluids, poffefFes 

 this, that by its weight it enters where it is lead re- 

 filled. Part of the refiftance to the entrance of the air 

 into the lungs being taken off by the action of the 

 mufcles dilating the thorax, it rufhes in through the 

 windpipe, in the fame manner as it rufhes into the 

 cavity of bellows, when the boards are feparated 

 from each other. Infpiration and expiration therefore 

 are not performed by the lungs themfelves, fince air 

 would be equally drawn into and expelled from the 

 cavity of the thorax when deprived of lungs, fup- 

 pofing that the parts of the thorax could be made to 

 perform their motions perfectly well after death. The 

 lungs may therefore be compared to the cavity of bel- 

 lows filled with any downy fubftance, the bones of the 

 thorax to the boards of the bellows, and the mufcles 

 of the thorax to the hands by which the bellows are 

 moved. 



Refpiration may be divided into four ftages or pe- 

 riods j rirft, infpiration ; fecondly, a paufe when the 

 lungs are filled j thirdly, expiration ; and laftly, a paufe 

 when the lungs are emptied. We are equally ftimu- 

 lated to infpiration and expiration by a fenfadon of 



uneafmefs A 



