8 PHYSIOLOGY OF FARM ANIMALS [CH. 



the windpipe or trachea, which opens uito the back of the mouth 

 at the root of the tongue. This tube at its lower end divides 

 into the two bronchi, and these subdivide agam and again withm 

 the lungs like the branches of a tree. Their finest divisions 

 widen out into air sacs which are in close relation to a mesh- 

 work of capillaries, the air in the air sacs and the blood in the 

 vessels being separated from one another by only a very thin layer 

 of protoplasm. These air sacs and capillaries form an essential 

 part of the structure of the lungs. Here the gaseous exchange 

 is effected, the blood taking up oxygen from the air sacs and 

 giving off carbon dioxide. When the blood passes to the tissues 

 this process is reversed, the carbon dioxide being taken up from 

 the tissue cells, while oxygen is given off to these cells. The 

 arterial blood, or blood which is being circulated to the tissues 

 after having been to the lungs and heart, is bright-red in colour 

 owing to the presence of a compound called oxyhaemoglobin, 

 which contains oxygen in loose combination : whereas venous 

 blood which is in process of being returned to the heart and lungs 

 is purple, the oxyhaemoglobin having been reduced, in part at 

 least, to haemoglobin, thereby causing a change in the colour of 

 the corpuscles. The complete course of the circulation in a 

 mammal is as follows : Right auricle of heart, right ventricle, 

 pulmonary arteries, lungs, pulmonary veins, left auricle, left 

 ventricle, arteries (excepting pulmonary arteries), capillaries, 

 veins (excepting pulmonary veins), and so back to the heart. 



The interchange of gases which takes place between the lungs 

 and the external air is efifected by the alternate expansion and 

 contraction of the chest wall, the air passing through the wind- 

 pipe in opposite directions in the acts of inspiration and expira- 

 tion. In this way the excess of carbon dioxide in the lungs is 

 got rid of, and a supply of oxygen from the external air is able 

 to take its place 



Excretory System. We have seen that the protoplasm of 

 which an animal is composed consists principally of protein. 

 This consists of carbon, hydrogen, oxygen, nitrogen, and sulphur. 

 As a result of katabolism these elements must be got rid of. 

 A great part of the waste carbon and oxygen is disposed of by 

 the lungs in the form of carbon dioxide as just described. 

 The hydrogen and a further proj^ortion of oxygen are excreted 

 in the form of water vapour both through the lungs and the 



