CHAPTER XXVI 



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 resulting from the metabolism of the tissues 

 which are of no further use in the economy are carried to the excre- 

 tory organs. It is a somewhat viscid fluid, and in man and in all 

 other vertebrate animals, with the exception of two,* is red in colour. 

 It consists of a yellowish fluid, called plasma or liquor sanguinis, 

 in which are suspended numerous blood corpuscles, which are, for 

 the most part, coloured, and it is to their presence that the red colour 

 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 con- 

 stituents, viz., the plasma and the corpuscles. On treatment with 

 ether, water, and other reagents, however, it becomes transparent and 

 assumes a lake colour, in consequence of the colouring matter of the 

 corpuscles having been discharged into the plasma. The average 

 specific gravity of blood at 15 C. (60 F.) varies from 1055 to 1062. 

 A rapid and useful method of estimating the specific gravity of blood 

 was invented by Eoy. Drops of blood are taken and allowed to fall 

 into fluids of known specific gravity. When the drop neither rises 

 nor sinks in the fluid it is taken to be of the same specific gravity as 

 that of the standard fluid. The reaction of blood is faintly alkaline 

 and the taste saltish. Its temperature varies slightly, the average 

 baing 3?-8 C. (100 F.). The blood-stream is warmed by passing 

 through the muscles, nerve centres, and glands, but is somewhat 

 cooled on traversing the capillaries of the skin. Eecently drawn 

 blood has a distinct odour, which in many cases is characteristic of 

 the animal from which it has been taken; it may be further 

 developed by adding to blood a mixture of equal parts of sulphuric 

 acid and water. 



Quantity of the Blood. The quantity of blood in any animal 



* The amphioxus and the leptocephalus. 



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