9 o 



AGRICULTURE. 



and woody fibre, which pass away in the excrement, 

 after the useful portions have been separated. This 

 separation of valuable food materials, is carried out 

 by a series of absorbent vessels, and being passed into 

 the blood is conveyed to the lungs where the oxidation 

 by the atmospheric air is commenced. 



190. We have now arrived at an intermediate stage, 

 and having introduced the nutriment of food into the 

 blood, we may indicate its course onwards until made 

 use of by the animal. Its duty commences in the lungs 

 and bloodvessels, for in the oxidation which there takes 

 place, we have carbonic acid largely formed, the tem- 

 perature of the blood thereby maintained (166), and 

 the warmth of the animal is provided for. From the 

 lungs, the blood is pumped by the heart through 

 arteries, which terminate in beautiful hair-like tubes, 

 called capillaries. These form a complete network 

 throughout the body, and they are so generally dis- 

 tributed that even the prick of a pin reveals their 

 existence. These capillaries allow the serum of 

 the blood, which contains the fibrin in solution, 

 to pass through and exude from them, and when that 

 matter is brought into contact with living tissue, it is 

 made use of for its growth. In the same way, the 

 oily matter in the blood is rendered available for 

 the growth of fatty tissues. 



191. The growth of the body is therefore dependent 

 upon the food which the animal receives, and upon that 

 general condition of health, which enables the processes 

 of digestion and absorption to proceed in a proper 

 manner. A considerable portion of the food is neces- 

 sary for the maintenance of the body in a healthy con- 

 dition, but this does not contribute to its growth, 

 The warmth of the body must be kept up if the animal 

 is to be preserved in health, and this necessitates the 

 use of a certain amount of non-nitrogenous matter, 

 without any increase of growth. In the same manner 

 the ordinary waste of the tissues of the body must be 



