CHAPTER XXIX 



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, the majority of 

 which are coloured, and it is to their presence that the red colour of 

 the blood is due. In addition to the red corpuscles, there are a 

 smaller number of colourless corpuscles, and some extremely small 

 particles called blood-platelets. 



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

 account of the different refractive powers possessed by its two 

 constituents, 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 Roy. 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 to 

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

 being 37'8 C. (100 F.). The blood-stream is warmed by passing 

 through the muscles, and glands, but is somewhat cooled on 

 traversing the capillaries of the skin. Recently 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 an animal 

 is usually estimated in the following manner: A small quantity 



* The ampliioxus and the leptocoplialus. 



