408 ON RESPIRATION. 



be generated. Moreover, it has been observed not unfrequently, that the livid 

 tint of the skin which supervenes in Asphyxia, owing to the non-arterialization 

 of the blood in the lungs, has given place after death to the fresh hue of health, 

 owing to the reddening of blood in the cutaneous capillaries by the action of 

 the atmosphere upon them. (See also 726.) 



544. We have no means of ascertaining the usual amount of carbonic acid 

 excreted through the skin, except by determining the whole quantity disen- 

 gaged from the body, and subtracting the portion exhaled from the lungs. 

 This determination has been attempted in various ways. By Liebig the total 

 quantity of carbon in the food consumed by a certain number of soldiers, was 

 compared with that excreted in the faeces and urine ; and an excess of 13-9 

 oz. daily for each man was found in the former ; which excess is regarded by 

 him as the amount disengaged in the form of carbonic acid, by the lungs and 

 skin. The experiment, however, was far from being exactly conducted ; as 

 many items among the ingesta are set down by guess merely ; and no exact 

 estimate was made of the quantity of carbon in the urine. The amount con- 

 tained in the solid matter excreted from the skin, too, was altogether neglected. 

 The estimate is in all probability much too high. Another mode of deter- 

 mining the total amount of carbon thrown off in the form of carbonic acid, in 

 the twenty-four hours, has been recently tried by Professor Scharling. He 

 constructed an air-tight chamber, of dimensions sufficient to allow an indi- 

 vidual to remain in it for some time without inconvenience ; and so arranged 

 that he could eat and drink, read, or sleep, within it. This was connected 

 with an apparatus, by which the air was continually renewed ; and the air 

 drawn off was carefully analyzed, In order to determine the quantity of car- 

 bonic acid contained in it. The following are the principal results deduced 

 from his experiments. The amount given off during sleep and when fasting, 

 was the least ; and after a meal it was always the highest. (It may be re- 

 marked, however, that the construction of the apparatus did not admit of active 

 exercise, which would probably increase the quantity of carbonic acid gene- 

 rated, to a still higher proportion.) Children exhaled more carbonic acid, in 

 proportion to their weight, than adults. The total quantity of carbon thus 

 extricated in the twenty-four hours, allowing seven hours for sleep in the 

 adult, and nine hours for the child, appears to be as follows : In the adult 

 male, from 7 oz. to 7 ; in the adult female, about 5d oz. ; and in the child of 

 ten years old, from 4 oz. to 4 oz.* These comparative results accord well 

 with those of Dr. Prout, and of MM. Andral and Gavarret, upon the exhalation 

 of carbonic acid by the Lungs alone ( 534, note, 535) : and also with that 

 already given of the actual quantity of carbon thrown off by them ; for if this 

 be estimated at 5<| oz., the remaining 2 oz. may very fairly be supposed to be 

 exhaled from the Skin in a similar form. 



545. From the facts which have been stated, and from many others of the 

 same kind, the conclusion seems indisputable, that the changes produced by 

 Respiration are of the following nature. The Arterial blood propelled from 

 the heart to the System contains a large proportion of oxygen, either free or 

 in loose combination with it ; and also a certain amount of carbonic acid. 

 During its passage through the systemic capillaries, it loses a part of its 

 oxygen, and acquires a great increase in its amount of carbonic acid, together 

 with some addition to its water; and it returns to the heart in the state of 

 Venous blood, its colour having been darkened by the loss of its oxygen, and 

 by the influence of the acid. In the Lungs, to which it is then transmitted, it 

 undergoes, by exposure to the atmosphere, the converse change to that which 

 took place in the systemic capillaries ; a large part of its carbonic acid and 



* Annalen der Cheinie und Pharmacie, xlv. p. 214. 



