586 OF RESPIRATION. 



the small proportion of it that has passed through the lungs, very imperfectly 

 arterialized. By the experiments of Bischoff it was ascertained that, even 

 after the lungs of a Frog had been removed, a quarter of a cubic inch of car- 

 bonic acid was exhaled from the skin, during eight hours. Experiments which 

 have been made on the Human subject leave no room for doubt, that a similar 

 process is effected through the medium of his general surface ; for, when a 

 limb has been inclosed for some hours in an air-tight vessel containing atmo- 

 spheric air freed from carbonic acid, a sensible amount of this gas has been 

 found to 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 the blood in the cutaneous 

 capillaries by the action of the atmosphere upon them. We have no means 

 of ascertaining the usual amount of carbonic acid excreted through the Skin, 

 except by determining the whole quantity disengaged from the body, and sub- 

 tracting the portion exhaled from the lungs ; and no sufficiently precise experi- 

 ments upon this subject have yet been made. The only way to separate the 

 results of the pulmonary and cutaneous exhalation of carbonic acid, would be 

 to confine the body in a close chamber, into which the product of the cutaneous 

 respiration might freely pass ; whilst the pulmonary respiration during the 

 same period should be measured by a distinct apparatus. It is not improbable 

 that, in cases of obstruction to the due action of the lungs, the exhalation of 

 carbonic acid through the skin may undergo a considerable increase; for we 

 find a similar disposition to vicarious action in other parts of the excreting 

 apparatus. Moreover, there is evidence, that the interchange of gases between 

 the air and the blood, through the skin, has an important share in keeping up 

 the temperature of the body (Chap, xvi., Sect. 2) ; and we find the tempera- 

 ture of the surface much elevated in many cases of pneumonia, phthisis, &c., 

 in which the lungs seem to perform their function very insufficiently. 



3. Effects of Respiration on the Blood. 



769. That an important change is effected in the character of the Blood, by 

 exposure to Atmospheric air in the lungs, has been known, from the time 

 when it was first ascertained that it is regularly transmitted to those organs. 

 The most obvious part of this change is the alteration in its colour, from the 

 dark purple of the venous fluid, to the rich crimson of the arterial. But this 

 alteration is only the index of changes far more important, which occur in its 

 chemical constitution. Respecting the nature of these changes, there has 

 been, as formerly stated, much difference of opinion ; some maintaining that 

 the carbonic acid exhaled is formed in the lungs ; and others, that it is con- 

 tained in the venous blood, and is truly excreted from it. The latter opinion, 

 which was long since brought forward by La Grange and Hassenfratz, has 

 recently obtained such full confirmation, from the experiments of Spallanzani, 

 Edwards, Miiller, Bischoff, Magnus, and others, as to have a full claim for 

 adoption as a physiological truth. These experiments are of two kinds ; first, 

 those which show that an exhalation of carbonic acid may continue for a long 

 time, when the animal is breathing an atmosphere in which no oxygen exists ; 

 and, secondly, those which prove that much more carbonic acid exists in an 

 uncombined state in venous blood than in arterial, whilst more oxygen exists 

 in a similar condition in arterial blood than in venous. The results of these 

 will now be briefly stated. It was shown by Spallanzani, that Snails might 

 be kept for a long period in Hydrogen, without apparent injury to them; and 

 that during this period they disengaged a considerable amount of Carbonic 

 acid. Dr. Edwards subsequently ascertained that, when Frogs were kept in 



