122 MANUAL OF HISTOLOGY. 



quantity of silicic acid, and probably also traces of fluoride of calcium. 

 Salts of ammonia are also present, in all probability, in healthy living 

 blood, though in very small amount. Iron, as was already mentioned, is 

 missing in the plasma. 



In conclusion, the plasma, like all animal fluids, contains absorbed 

 gases, small quantities of and N, and a larger amount of C0 2 ; 

 besides this, carbonic acid appears in double chemical combination; 

 loosely combined, it represents the second acid atom of bicarbonate of 

 sodium, and is besides united in a subordinate manner with the phosphate 

 of the latter ( 43). In fixed combination it is supposed to constitute the 

 first acid atom of carbonate of sodium. 



REMARKS. Volatile fatty acids belonging to the higher members of the series 

 appear to be not entirely absent ; witness the peculiar odour of fresh blood. This 

 odour may be due to the presence of butyric acid, although the latter has not yet 

 been proved to exist in the blood. 



76. 



In the foregoing section we have had an example of the mean com- 

 position of the blood. But it stands to reason that the latter must be 

 subject to great variation in the proportions of its constituents, according 

 to age, sex, and other circumstances according to the species of food, 

 and state of the secretions, even in our healthiest days. However, these 

 considerations belong more to physiology than to histochemistry. The 

 blood of men is generally supposed to be richer in cells than that of 

 women. The amount of corpuscles decreases also with increasing age, 

 and is in the earlier periods of life smaller than in the adult body. The 

 proportion of cells, further, sinks with bad nourishment, and also in conse- 

 quence of great loss of blood. Of the solid constituents of the intercellular 

 fluid, we know that the fibrin is subject to greater variation as to quantity 

 than the albumen. The latter, however> occurs in far greater proportion 

 than fibrin, and must, in fact, be looked upon as the most important con- 

 stituent of the plasma for the support and formation of the tissues. 



But the difference in the various kinds of blood of one and the same 

 body is a subject of much more importance. Blood being the general 

 nutritive fluid, enters everywhere into an interchange of matter with the 

 tissues ; it gives up certain substances, and receives others back again. 

 And in that the chemical constitution of the several tissues and organs is 

 different, and also their series of transmutations, the composition of the 

 blood must be considerably modified in the various regions of the circu- 

 latory system. For instance, we will find blood flowing from the secreting 

 breast of a woman of a different nature from that which returns from 

 supplying the substance of the brain. But these deviations are still more 

 remarkable in the glands and lungs. The blood which enters the kidney 

 must be richer in urea, uric and hippuric acids, and certain mineral con- 

 stituents, than that which leaves it by the renal vein. Blood which flows 

 from the lung has given off carbonic acid and water, and, on the other 

 hand, received oxygen ; and so on. 



Owing to the crude state of blood-analysis, this productive field of 

 inquiry has up to the present day yielded but little. We are even now 

 hardly able to ascertain anything accurately ; thus the difference between 

 arterial and venous blood, and that also between the blood of the vena 

 porta and hepatic vein : again, in what respect the fluid contents of the 

 splenic artery differ from that of the corresponding vein. 



