IIRATION IN AIK. 179 



membrane of the lungs into the blood : this oxygenated blood 

 is then carried to all the organs, of which the living portions 

 take up the oxygen from it, precisely as the corresponding parts 

 of insects take it up directly from the air by means of the 

 trachere. In all animals that breathe thus through the lungs, 

 there is a strongly marked contrast d in the blood contained in 

 different parts of the vascular system. That which is carried 

 back from the lungs to the heart is rich in oxygen and known 

 as arterial blood, and that which circulates in the vessels which 

 convey it from the organs to the heart, or from the heart to the 

 lungs, is poor in oxygen, and is called venous blood. 



We need not, however, in this place, investigate more closely 

 the relations of the vascular system to the respiratory organs, 



FIG. 51. Section of the Inng of the embryo of the Pig, showing the spongy texture. 



nor the physiological distinctions which are based on the dif- 

 ferent organs of respiration and their structure. On the other 

 hand, it is essential that we should in the first instance deter- 

 mine which of the constituents of the air are advantageous or 

 injurious to animal life. 



Air contains, when it is pure, almost 21 per cent, of oxygen, 

 with about 79 per cent, of nitrogen, and a variable trace of 

 carbonic acid, besides water which it holds in solution in the 

 form of vapour in a quantity varying according to the tempera- 

 ture. All the other kinds of gas which are occasionally present 

 in the atmosphere are of no importance. They are either irre- 

 spirable or actually injurious, while the above-mentioned mix- 

 ture is the normal one, and thus is the most favourable for 

 animal life. Certainly we must make this statement with 



