CHEMICAL PHYSIOLOGY. 57 



titles after exertion or under the influence of a 

 heated atmosphere, contains much surface epithelium. 

 Of chemical ingredients there are observed lactic and 

 sudoric acid, the latter peculiar to sweat and not found 

 anywhere else, and urea. There is much sodium-chloride, 

 little or no phosphate. Sometimes volatile fatty acids, e.g. 

 valerianic, are found in small quantity, but it is possible 

 that they are formed after the sweat has been secreted. 



In respiration oxygen of the air inhaled together with Breath 

 its nitrogen is absorbed by the blood, and in exchange 

 carbonic acid and water are given out. The expired 

 air therefore contains less oxygen than the inspired, and 

 a quantity of carbonic acid instead. But the whole of 

 the oxygen inhaled does not return as carbonic acid 

 and water ; a portion is otherwise combined and leaves 

 the body in the urinary products of oxydation, parti- 

 cularly urea. A healthy strong man exhales in twenty- 

 four hours upwards of 400 litres of carbonic acid, and 

 inhales upwards of 500 litres of oxygen. The expired 

 air during rest contains about four volumes per cent, of 

 carbonic acid. During activity the expiration of car- 

 bonic acid becomes much more rapid, and in extreme 

 cases may for a short time rise to tenfold its normal 

 quantity in the same time. Activity or muscular work 

 on the other hand hardly increases the quantity of urea 

 excreted by a man. 



The breath in diseases may contain carburetted hydro- 

 gen (of which a trace is also excreted in health) and 

 volatile matters at present unknown, ammonia being 

 perhaps amongst them. These investigations have only 

 just become possible by the invention of an apparatus 



