OF THE BLOOD GENERAL CONSIDER A T10NS. 231 



activity its presence appears to be an essential condition, the same clement 

 being also required in various other metamorphoses which form part both of 

 the constructive and of the destructive operations: whilst conversely it im- 

 bibes the Carbonic acid, which is one of the chief products of the action of 

 oxygen upon the tissues and fluids of the body, and conveys this to the lungs 

 and skin for elimination. But in addition to the Histogenetic materials and 

 Oxygen, on the one hand, and the various products of the disintegration of 

 the tissues on the other, the blood contains those non-azotized substances, 

 which are received into it for the purpose of supplying the pabulum of those 

 combustive processes that are requisite for the production of heat and the 

 generation of nervo-muscular energy ; and the union of their carbon with 

 oxygen introduced from the atmosphere which is continually going on, be- 

 comes an additional source of the production of carbonic acid, and of its in- 

 jurious accumulation if its elimination be checked. 



169. From this variety in the operations to which the Blood is subservient, 

 it naturally follows that the changes which it undergoes in different parts of 

 its circulation are of a very diversified nature, and that the composition of 

 the fluid in the several parts of its course will be far from uniform. Between 

 the blood which is being distributed by the systemic Arteries to the body at 

 large, and that which is being collected from it again by the systemic Veins, 

 after having percolated the tissues, there is not only an obvious difference in 

 hue, which indicates an important change, but there is also a considerable 

 difference in composition, which is revealed by chemical analysis ; and a 

 difference of a converse nature presents itself, between the blood that is on 

 its way to be distributed to the Lungs, and that which is returning from 

 them. So again, although there is no obvious dissimilarity in physical char- 

 acters between the blood which is transmitted to the Liver by the vena portse, 

 and that which is carried off from it by the hepatic vein, yet chemical analy- 

 sis reveals a very remarkable difference in their composition, and shows that 

 the blood of the ascending vena cava (above the entrance of the hepatic 

 vein), that of the right cavities of the heart, and that of the pulmonary 

 artery, differs from all other blood in the body, in constantly containing an 

 appreciable quantity of a peculiar substance readily convertible into sugar, 

 which is formed in its passage through the Liver. In like manner the 

 researches of M. Picard have shown that the blood of the renal vein contains 

 a smaller proportion of the chief component of the urinary secretion, urea, 1 

 than that of the renal artery; and in many other cases of blood returning 

 from particular organs we know that important differences must exist, al- 

 though they have not yet been detected by chemical analysis. In the 

 account to be presently given of the Blood, those most general characters 

 and properties will be first described, which it presents in all parts of its 

 circulation ; the principal differences which have been substantiated in the 

 composition of the blood in the several portions of its circuit will then be 

 noticed ; and lastly, a summary will be given of the most important of those 

 pathological alterations which it exhibits in disease. 



170. The quantity of Blood contained in the Human body is probably, 

 for a man of average height, from 12 to 15 Ibs. ; but its precise determina- 

 tion is more difficult than might at first be supposed, and the estimates 

 which have been made of it have been most strangely discrepant. The 

 entire amount which flows from a large arterial trunk freely opened, can by 

 no means be taken as a measure; since, however readily it may be per- 

 mitted to escape, a considerable quantity still remains within the bloodves- 

 sels, especially if the heart's action fail before the loss of blood has proceeded 



1 0. Bernard, Le9ons, 1859, vol. ii, p. 31 ; Picard, These sur-l'llree, 1856. 



