154 OF THE BLOOD. 



and preventing the results of the destructive from exerting an injurious influence 

 on the system, the Circulation of the Blood serves the important purpose of 

 introducing Oxygen from the atmosphere, the presence of which appears to 

 be an essential condition of the peculiar vital activity of the Nervous and Muscu- 

 lar tissues, whilst it is also required in various other metamorphoses which form 

 part both of the constructive and of the destructive operations; and just as the 

 circulating current takes up, and carries to their appropriate outlets, the various 

 excretory matters which are set free in the course of its nutrient operations, so 

 does it also imbibe the Carbonic acid, which is one of the chief products of the 

 action of oxygen upon the tissues and fluids of the body, and convey this to the 

 lungs and skin for elimination. This product is continually being formed in such 

 large amount that its presence in the blood can always be readily demonstrated ; 

 and if its elimination be checked for even a few minutes, it accumulates to such 

 an extent as to occasion the immediate destruction of life. But besides 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 the combustive process ; and the union of their elements with oxygen 

 introduced from the atmosphere, which is continually going on, becomes an ad- 

 ditional source of the production of carbonic acid, and of its injurious accu- 

 mulation if its elimination be checked. 



135. From the variety of 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 con- 

 verse 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 characters between the blood which is 

 transmitted to the Liver by the vena portae, and that which is carried off 7 from 

 it by the hepatic vein, yet chemical analysis 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 containing 

 an appreciable quantity of sugar ( 45). In many other cases, we know that an 

 important difference must exist, although chemical analysis has not yet detected 

 it ; thus, the blood of the Renal vein must be more free from the components of 

 the urinary excretion than that of the renal artery which conveys them to the 

 kidney ; whilst the blood of the systemic veins in general must contain them 

 in greater amount than their corresponding arteries, since they are discharged 

 into the current during its passage through the tissues, of whose disintegration 

 they are among the products. In the account to be presently given of the Blood, 

 those most general characters and properties will be first described which it pre- 

 sents 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, some account will be given of the most import- 

 ant of those pathological alterations which it exhibits in disease. 



136. The precise determination of the quantity of Blood contained in the 

 body is more difficult than might have been at first 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 



