SECTION VI. 

 THE BLOOD. 



THE blood may be regarded as the main organ of nutrition, since, on 

 the one hand, the assimilated food-stuffs enter into it at the point of 

 their absorption to be carried to the points where they may be needed 

 by the economy ; and, on the other hand, the results of tissue waste are 

 given up to it to be removed from the economy. 



The blood may be regarded as a cellular tissue with a fluid inter- 

 cellular substance in perpetual motion within a system of branching 

 tubes. The blood is not a homogeneous solution, but is composed of 

 an immense number of minute-formed elements, the blood-corpuscles or 

 cells, suspended in a colorless, transparent fluid, the blood-plasma. Of 

 the blood-corpuscles, the so-called red cells are greatly in excess, and it 

 is to the haemoglobin, or red coloring-matter, which they contain that 

 the red color of the blood is due. 



The red hue of the blood drawn from a living animal will vary ac- 

 cording to the locality from which it is taken ; the blood taken from the 

 arteries, the left side of the heart, or the pulmonic veins being of a bright, 

 scarlet-red color, while that drawn from the veins, the right side of the 

 heart, or the pulmonic artery, is dark, brownish red. When exposed to 

 air or to oxygen gas, the dark venous blood becomes arterial in hue, 

 and this change occurs most rapidly when the blood and gas are shaken 

 up together. 



The specific gravity of defibrinated human blood varies from 1045 

 to 1062, the average being 1055, though greater variations than the above 

 are not inconsistent with health. Again, the specimens of blood first 

 drawn will have a higher specific gravity than that examined after con- 

 siderable hemorrhage, as the water in the blood will have then increased 

 from the abstraction of fluid from the tissues. The cells are specifically 

 heavier than the plasma, and of the former the red cells are heavier than 

 the white. Thus, if blood is prevented by cold from coagulation and 

 this is best accomplished with horse's blood the blood will separate 

 into three distinct strata, the lowest being composed of red corpuscles, 

 the middle of white cells, and the upper layer of the clear blood- 

 plasma. The corpuscles have a density of 1088-1105 ; the plasma, of 

 1027-1028. 



The reaction of the blood is always distinctly alkaline, due to sodium 



(469) 



