226 Agricultural Chemistry. 



and blood regulation are wonderfully and delicately balanced and 

 are by no means completely understood. 



Feces. The portion of the food which has escaped solution 

 and absorption, together with certain substances already absorbed 

 but re-excreted by way of the intestines, constitute the feces. 

 Epithelial cells from the intestinal walls, parts of the digestive 

 juices, bile, bacterial cells, etc., will make up a large portion of 

 the fecal matter. 



Respiration. The nutrients, prepared by the various process- 

 es of solution and reconstruction in the intestines and intestinal 

 wall, enter the blood on its return to the heart, coming into the 

 venous circulation by way of the thoracic duct and liver (hep- 

 atic vein), as already described. By this route, the blood, laden 

 with nutrients, passes to the right side of the heart. It is then 

 carried to the lungs, by way of the right ventricle, to be returned 

 to the left side of the heart, and from which it is pumped to all 

 parts of the body. In the lungs the blood is supplied with oxy- 

 gen. The purple of venous blood is changed to a scarlet, due 

 to the absorption of oxygen by the haemoglobin, with the forma- 

 tion of oxy-haemoglobin, the important oxygen carrier of the 

 blood. At the same time, a considerable quantity of carbon 

 dioxide, most of which was in solution in the blood plasma, pos- 

 sibly as a bi-carbonate, is given up to the air within the lungs. 



Inspired air contains about 21.0 per cent of oxygen and .03 

 per cent of carbon-dioxide, while expired air carries approx- 

 imately 16.5 per cent of oxygen and 4.4 per cent of carbon-diox- 

 ide< Though the absorption of oxygen takes place in the lungs, 

 it is not there that the processes of combining the oxygen with 

 the carbon and hydrogen of the body tissues takes place. The 

 blood, through the haemoglobin of the red-blood corpuscles, acts 

 as a carrier of oxygen and the actual combustion of the products 

 derived from the food occurs in the tissues themselves. The rate 

 of combustion in the tissues is a variable one, dependent upon 

 the amount of work the animal is doing and the temperature to 



