34 



THE FUNCTIONS OF ANIMALS 



capillaries in the walls, enclosed in an air-tight box, the 

 chest, the size of which constantly and rhythmically varies. 

 When we take in a breath, the size of the chest is increased, 

 the air pressure within is lowered, and the air from without 

 rushes down the windpipe until the pressure is equalised. 

 The oxygen of this air combines with the coloured iron- 

 containing protein called haemoglobin, contained in the red 

 corpuscles of the blood, and is thus carried to all parts of 

 the body. From the blood it passes to the tissues usually 

 through the medium of the lymph. It is used in the tissues 

 for oxidation. The carbon dioxide formed as a waste 

 product is temporarily combined with bases set free from 

 the blood proteins, especially haemoglobin, and so in time 

 reaches the lungs. But as the partial pressure of the 

 carbonic acid in the air is lower than it is in the serum, 

 the gas escapes from the latter into the air chambers of 

 the lungs. When the size of the chest is decreased, the 

 pressure is increased, and the gas escapes by the mouth or 

 nose until the pressure is equaUsed. 



Many very different types of respiratory organ are met 

 with in the animal kingdom. Fundamentally, a respiratory 

 organ is a region of the body-surface, usually either an 

 in-folding or an out-folding, at which the external air 

 or water is brought into close relation with either the 

 body-fluids or with the cells themselves, so that diffusion 

 of gases takes place readily. But different animals have 

 solved the problem in different ways, and within each 

 group many adaptations to the conditions of various 

 environments may be demonstrated. 



There may be deep differences in the physiology of 

 respiration between different phyla. In birds and 

 mammals, which maintain a constant body temperature, 

 there is no direct relation between the external temperature 

 and the amount of oxygen consumed ; but in most cold- 

 blooded animals metabolism increases with rising tempera- 

 ture until the heat becomes harmful. It is usually found 

 that the consumption of oxygen by the body is, within 

 wide limits, independent of the concentration of oxygen 

 in the external medium ; but in some Invertebrates, in 

 which increase of size has not been balanced by the de- 

 velopment of efficient respiratory and circulatory systems, 



