Theories and Experiments 257 



of animal heat, at the very time when, on account of the altitude, 

 this production should be increased. 



In fact, our author says very fittingly: 



Prudent Nature at sea level has established laws which assist, 

 through the atmosphere, these variations in the production of human 

 heat. For in winter the cold air is denser, and contains a greater 

 part of the vivifying principle in a certain volume. The warmth of 

 summer, on the contrary, by expanding the atmosphere, gives the 

 lungs a proportion of oxygen in keeping with the small amount of 

 heat which the body must produce. Thus the source, from which we 

 draw the elements of our respiration, itself varies in a certain 

 measure, which, at sea level, is a kindness of Providence. 



This is not true at high elevations, where the density of the air, 

 lessened by the decrease of the barometric pressure, is no longer 

 proportionate to the temperature surrounding us, but to the altitude 

 which we have reached. And note particularly this extremely impor- 

 tant fact: whereas at sea level the exterior causes which chill us take 

 care to give us the means of combatting this drop in temperature, in 

 Mexico, on the contrary, the decrease of pressure which produces cold 

 in the air alters the source of heat for us by compelling us to breathe 

 a rarefied atmosphere. So that, on the one hand, the increased expan- 

 sibility of the air and the easier evaporation chill us constantly, while, 

 on the other hand, the increased rarity of oxygen refuses us the 

 normal means of calorification. 



Upon these data, so clear and so exact, the physiological pecu- 

 liarity of altitudes rests entirely. (P. 83.) 



It is, therefore, not surprising to see that: 



Persons in a state of repose chill very easily. Their lower limbs 

 are almost never warm. Muscular exercise would stimulate the 

 circulation and the respiratory movements; but the blood, deprived of 

 oxygen, produces apathy of the muscles and makes one prefer repose. 

 Here then appears the result of the experiment made by M. Becquerel 

 upon the muscle fiber which loses its contractility and becomes ener- 

 vated when it lacks contact with the arterial blood. (P. 86.) 



Here M. Jourdanet meets the phenomenon described by all 

 mountain travellers, of exaggerated fatigue, pain in the thighs, and 

 heaviness of the lower limbs; he protests energetically against the 

 explanation of the Weber brothers, which was accepted by von 

 Humboldt and almost all later authors, although this explanation, 

 to use his apt expression, "does not bear careful examination": 



In fact, if we estimate in square centimeters the surface on the 

 plane of opening of the cotyloid cavity, the diameter of which is 54 

 millimeters, we get a result of 22.89 square centimeters, which, multi- 

 plied by 1003 grams, a weight equivalent to a square centimeter of 

 surface, give us 23,645 grams, to represent the real weight in the 

 cavity of the joint. If we remember that many travellers have felt 



