CHAPTER V. 



THE BLOOD. 



THE blood is the fluid medium by means of which all the tissues of 

 the body are directly or indirectly nourished; by means of it also such 

 of the materials which result from the metabolism of the tissues as are 

 of no further use in the economy, are carried to the excretory organs to 

 be removed from the body. It is a somewhat viscid fluid, and in man 

 and in all other vertebrate animals with the exception of two,* is red 

 in color. The exact shade of red is variable; that taken from the 

 arteries, from the left side of the heart and from the pulmonary veins 

 is of a bright scarlet hue, that obtained from the systemic veins, from 

 the right side of the heart, and from the pulmonary artery, is of a much 

 darker color, and varies from bluish-red to re'ddish-black. At first 

 sight, the red color appears to belong to the whole mass of blood, but 

 on further examination this is found not to be the case. In reality blood 

 consists of an almost colorless fluid, called plasma or liquor sanguinis, 

 in which are suspended numerous minute rounded masses of proto- 

 plasm, called blood corpuscles, which are, for the most part, colored, 

 and it is to their presence in the fluid that the red color of the blood is 

 due. 



Even when examined in very thin layers, blood is opaque, on account 

 of the different refractive powers possessed by its two constituents, viz., 

 lihe plasma and the corpuscles. On treatment with chloroform and 

 other reagents, however, it becomes transparent and assumes a lake 

 color, in consequence of the coloring matter of the corpuscles having 

 been discharged into the plasma. The specific gravity of the blood 

 varies slightly at different times in accordance with the changing pro- 

 portions of its component parts. Of these, water is the most variable 

 constituent, and salts the next, while albumens are the most constant 

 and the last affected by disease. Under normal conditions the average 

 specific gravity in adults is about 1.059, the normal limits of variation 

 being given by Becquerel and Rodier as 1.054-1.060, by Hammerschlag as 

 1.056-1.063, and by Jones as 1.045-1.066. The physiological variations 

 may be considerable, depending on the age, sex, time of day, amount of 

 exercise or sleep, etc. The specific gravity is increased by residence in 



* The amphioxus and the leptocephalus. 

 n 129 



