92 



THE VERTEBRATE ANIMAL: METABOLSIM 



the environment of the animal. Water-dweUing forms have struc- 

 tures known as gills, which are richly supplied with blood, covered 

 by a thin layer of cells and exposed to a constantly changing cur- 

 rent of water. Air-dwelling forms, by the process of breathing, 

 draw air, by way of the air passages, into their lungs, which are 

 cavities separated from a rich capillary network by a cellular mem- 

 brane (Fig. 51). In both types, oxygen passes into the blood 

 stream; but in one case it is dissolved in water, while in the other 



it is free in the air. 

 This is known as 

 external respiration 

 and is analogous to 

 the absorption of 

 nutrients from the 

 digestive tract. As 

 is the case with ali- 

 mentary absorption, 

 there is no agree- 

 ment as to the phys- 

 ical explanation of 

 the proc3SS. Here, 

 also, simple diffusion 

 has been proposed 

 as the underlying 

 principle. The con- 

 centration of oxygen 

 in the water or air 

 being greater than 

 that in the blood, 

 from which the oxy- 

 gen is constantly removed by the cells of the body, the oxygen 

 would diffuse in the direction of the blood to estabhsh equi- 

 hbrium. However, the fact that the membrane is composed 

 of protoplasmic units cannot be ignored, and all the factors 

 governing the absorption of oxygen into the blood are not 

 understood. In the blood stream the oxygen enters into a 

 loose combination with hoemogJohin, an iron compound carried by 

 the red blood cells, and is distributed throughout the body in 

 that way. This combination with haemoglobin, which is respon- 

 sible for the red color of oxygenated blood, is unstable, and, in the 



Fig. 51. — Diagram of a section through a portion 

 of the lung. 



The capillaries, which have been cut across, are shown 

 in black. (From Hough and Sedgwick, "The Human 

 Mechanism," copyright, 1918, by Ginn and Co., reprinted 

 by permission.) 



