167 



CHAPTER IV. 



OF RESPIRATION. 



LXIX. OF the different changes which the blood undergoes in the 

 different organs, none are more essential or more remarkable than those 

 it receives from the air, which, during respiration, is alternately received 

 into the lungs, and expelled from them. The blood which the veins 

 convey to the heart, and which the right ventricle transmits to the lungs, 

 is of a dark colour, and heavy: its temperature is only thirty degrees 

 (Reaumur's thermometer;) if laid by, it coagulates slowly, and there is 

 separated from it a considerable quantity of serum. The blood which is 

 brought by the pulmonary veins to the left side of the heart, and which 

 is conveyed to all parts of the body, by means of the arteries, is, on the 

 contrary, of a florid red colour; it is spumous, lighter, and warmer by 

 two degrees. It likewise coagulates more readily, and contains a smaller 

 quantity of serum. All these differences, which are so easily distin- 

 guished, depend on the changes which it has undergone, by being in 

 contact with the atmospherical air. 



LXX. Of the Atmosphere. The mass of air which surrounds the globe, 

 and to which we give the name of atmosphere, bears on all bodies with 

 a pressure proportioned to their surface. That of man* bears a weight 

 of air amounting to about thirty-six thousand pounds. Moreover, one of 

 its constituent principles is absolutely necessary to the keeping up of 

 life, of which it is a principal agent. 



The variations in the weight of the atmosphere have, in general, but 

 little influence on the exercise of the functions; nevertheless, when by 

 ascending the tops of very high mountains, man rises several thousand 

 fathoms above the level of the sea, the very remarkable diminution of 

 the weight of the air, produces a very sensible effect. Respiration be- 

 comes laborious and panting, the pulse is quickened, and there is felt an 

 universal uneasiness, joined to excessive weakness, and haemorrhages 

 come on: these symptoms are occasioned both by the diminished pres- 

 sure of the air, and by the smaller quantity of oxygen contained in a 

 rarer atmospheref. 



* The surface of the body is estimated at fifteen or sixteen square feet, in a man of 

 middle size. 



f Several travellers, whose reports on the subject I have consulted, agree in repre- 

 senting the corporeal, as well as some of the mental functions, to be very strangely in- 

 fluenced by a rarified condition of the atmosphere. But the celebrated be Saussure, a 

 writer, who unites to the profundity of philosophical research, the polish of literary re- 

 finement, has from personal experience described these affections with the most pre- 

 cision. To his description, I shall, therefore, principally adhere in the ensuing 1 inquiry. 



He states, that at a certain height above the level of the sea, there uniformly takes 

 place a sudden and uncommon exhaustion of the muscular power. The natives of the 

 Alps, who can climb for hours at the foot of the mountains without being at all wea- 

 ried, are forced TO stop, and take breath every few minutes, when they ascend the 



