THE BLOOD. [CH. xxvi. 



research has taught us that they are either proteid in nature or 

 are substances closely allied to the proteids. 



The distinction between organised ferments and enzymes is 

 more apparent than real ; for the micro-organisms exert their 

 action by enzymes that they secrete. By crushing yeast cells 

 Buchner has succeeded in obtaining from them an enzyme that 

 produces alcoholic fermentation. 



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 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 

 coloiir. 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 colour. At first sight the red colour 

 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 colourless 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 

 in the fluid 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 

 constituents, viz., the plasma and the corpuscles. On treatment 

 with ether, water, and other reagents, however, it becomes trans- 

 parent 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 



* The amphioxus and the leptocephalus. 



