ITS PHYSICAL, CHEMICAL, AND STRUCTURAL CHARACTERS. 179 



unoxidized state, united with the fat of the corpuscles, is converted by the respi- 

 ratory process into phosphoric acid, which passes into the serum and unites with 

 alkaline bases; and this view seems borne out by the more recent analyses of 

 Reich. 1 The most remarkable difference between Arterial and Venous blood, 

 however, lies in the amount of free gases which they respectively contain. It 

 may now be considered as unquestionably proved by the researches of Stevens, 

 Bischoff, J. Davy, Magnus, and others (but more especially by those of the last- 

 named experimenter), that both venous and arterial blood contain Oxygen, Nitro- 

 gen, and Carbonic acid in a state of solution ; these gases being yielded up by 

 the blood when it is placed in a perfect vacuum; 3 and carbonic acid being also 

 disengaged when the fluid is shaken with common air or with oxygen, hydrogen, 

 or nitrogen ; whilst oxygen is in like manner expelled by hydrogen or nitrogen, 

 which takes its place. The experiments of Magnus 3 show that from 10 to 12 1 

 per cent, of Oxygen (by volume) exists in arterial blood ; but that this is re- 

 duced in venous blood to half its amount. On the other hand, the quantity of 

 Carbonic acid which is thus removable amounts to about 25 per cent, (by volume) 

 in venous blood, and to only 20 in arterial. The per-centage of Nitrogen was 

 found to vary from 1.7 to 3.3; but no constant difference presented itself be- 

 tween the quantities contained in arterial and in venous blood respectively. . The 

 differences in the relative proportions of Oxygen and Carbonic acid in arterial 

 and venous blood respectively,' confirm the indications afforded by other facts 

 (CHAP, x.), that an exchange of oxygen for carbonic acid takes place in the 

 systemic circulation, and an exchange of carbonic acid for oxygen in the general 

 circulation. How far the gases thus introduced into the blood enter into chemi- 

 cal combination with any of its constituents, or are merely dissolved in the liquid, 

 has not been positively determined; there is reason to think, however, that if 

 combination thus takes place, the proportion so employed is extremely small. 4 

 The remarkable power of absorbing carbonic acid, which is possessed by the 

 Serum, and still more by the Red Corpuscles, has been already mentioned 

 ( 84, 142); and there would be no difficulty in accounting for the presence 

 of many times the amount of that gas which is actually found in the blood, 

 without supposing it to lose its freedom by combination. 



164. The increase of the Fibrin, however, which seems to be effected during 

 the aeration of the Blood, must be taken as an indication that a certain part of 

 the oxygen absorbed from the air is made directly subservient to changes in the 

 composition of the circulating fluid; and from what has been already stated 

 ( 25), it appears that the fibrin of arterial blood is in a state of higher oxida- 

 tion than that of venous. Now although, for the reasons formerly given 

 ( 25-29), we must regard the conversion of albumen into fibrin as rather a 

 vital than a chemical change, yet the existence of the difference in question 

 obviously points to the presence of oxygen as a condition essential to its per- 

 formance; and this inference is fully confirmed by the recent experiments of 

 Dr. Grairdner, 5 on the influence of the respiration of pure oxygen on the pro- 

 duction of fibrin. As the Rabbit was on many accounts the most convenient 

 warm-blooded animal for such a trial, he first set himself to determine the nor- 



1 "Archiv. der Pharmacie," and "Liebig and Kopp's Report," for 1849, p. 366. 



2 It has been found by Magnus, that carbonic acid is not given off under the receiver of 

 an air-pump, until the air has been so far exhausted that it only supports one inch of mer- 

 cury. This fact explains the negative result obtained by many experimenters ; since an 

 extremely good air-pump is required to produce such a degree of exhaustion. 



3 See "Ann. der Physik und Chemie," band Ixvi. p. 177 ; and an abstract in the "Philo- 

 sophical Magazine," Dec. 1845. 



4 See Lehmann, Op. cit., Bd.- ii. p. 181. 



5 Treatise "On Gout," 2d edit., pp. 153,4. 



