CHAP. iv. CONSTITUTION OF THE BLOOD. 277 



gland is collected into the abdominal subcutaneous vein, commonly 

 known as the " milk vein." In cows this vessel is particularly large ; 

 it extends along the under surface of the abdomen to near the end of 

 the sternum, or breast-bone, where it turns inwards to join the internal 

 thoracic, or internal mammary vein, the openings in the abdominal wall 

 through which these vessels pass being known as the milk fountains or 

 doors. The internal mammary conveys its blood to the vein of the arm, 

 and this joins the anterior vena cava which empties into the right side 

 of the heart. By this route, then, the blood which has been submitted 

 to the action of the mammary gland is returned to the heart. 



Although the nutrients of the food stuffs have been shown to enter 

 the blood, it is not implied that they there preserve their individuality. 

 Indeed, it is easy to show that the contrary is the case. Blood consists 

 of a liquid plasma in which are suspended enormous numbers of 

 microscopic solid bodies called corpuscles, and the red colour of the 

 great majority of these imparts the characteristic tint to blood. 

 Physically, niilk resembles blood, in that it also consists of a watery 

 fluid in which are suspended immens^ numbers of minute solid bodies, 

 the fat globules, which, being white, make the whole milk appear to be 

 this colour ; the different colour of skim-milk is partly due to the fact 

 that most of the white fat globules have been removed. Blood is slightly 

 heavier than milk, the specific gravity of the former being 1'055 and of 

 the latter 1'03. Blood placed in contact with non-living matter speedily 

 coagulates, milk in similar circumstances does not. The coagulation 

 of the blood is due to the separation of a material called fibrin from the 

 plasma, and the entanglement of the corpuscles in the meshes of the 

 fibrin. Thus is formed the clot, and the clear pale liquid which 

 remains after separation of the fibrin from the plasma is called the 

 serum. 1 Hence the blood consists of serum, fibrin, and corpuscles, 

 though the fibrin does not exist as such in the living blood. In round 

 numbers, the percentage composition of the serum is, of water 90 ; of 

 nitrogenous substances, 8 to 9 ; of fat, extractive, and saline matters, 

 2 to 1. Of the corpuscles there are two kinds, the red and the colour- 

 less, but the former are nearly a thousand times as numerous as the 

 latter^ and contain 56'5 per cent, of water, and 43*5 per cent, of solids, 

 the latter being almost entirely nitrogenous organic matter. The 

 fibrin which separates from the plasma is also made up of nitrogenous 

 organic matter. When the corpuscles on the one hand and the serum 

 on the other are dried and ignited, and their ashes analysed, the lead- 

 ing mineral constituents of the corpuscles are found to be the chloride 

 and phosphate of potassium, and of the plasma soda and chloride of 

 sodium. " The corpuscles differ chemically from the plasma, in con- 

 taining a large proportion of the fats and phosphates, all the iron, and 

 almost all the potash, of the blood ; while the plasma, on the other 

 hand, contains by far the greater part of the chlorine and of the soda." 5 

 The extractives of the blood, though not abundant in quantity, are 

 numerous and variable, the chief ones being urea, kreatin, sugar and 

 lactic acid. These few details may serve to show what a very complex 



1 Lat. serum, the watery part ; in particular, the watery part of curdled milk, whey. 

 s Huxley, " Physiology, " p. 72. 



