THE MET.VBULIC MACHINERY OF ANIMALS 311 



to vaporize the sweat on the body surface, and body heat is lost. 

 Conversely, by performing muscular work, heat may be produced in 

 greater quantity through the increase of oxidations in the body. 



Clotting is another very important function of the blood. We are 

 all familiar with the fact that while blood is fluid when drawn from 

 the body it soon becomes viscous and later gelatinous. Finally a 

 clot is formed, which may be seen floating in the blood serum. It 

 was initiated in part by the dissolution products of the blood 

 platelets. In the gelatinous stage, both red and white corpuscles 

 are caught in the fibrin network, and as the clot shrinks the red 

 cells are held more tightly by needlelike fibers of fibrin. There are 

 too many theories of clotting to present here, but when blood is 

 exposed to air chemical changes finally transform the soluble fibrino- 

 gen, which occurs normally in the blood stream, into insoluble fibrin. 

 The blood of a normal person ordinarily clots in about five minutes. 

 The blood of a few persons, however, forms clots very slow^ly or 

 refuses to clot at all. Such a condition is known as hertiophilia, and 

 the person affected as a hemophiliac. 



Finally, the blood plays an important part in health and disease 

 both through the distribution of antibodies and the defense mechan- 

 ism of the white corpuscles against bacterial invasion. 



SECTION C. RESPIRATORY DEVICES 



Respiration 



Every living organism requires oxygen for its metabolic processes, 

 which demands that every cell shall take in oxygen and give off 

 wastes, largely carbon dioxide and water. This exchange of free 

 oxygen and carbon dioxide is necessary for combustion. In all ver- 

 tebrates respiration may be divided into two types, external and inter- 

 nal respiration. The former involves the exchange of gases between 

 the atmosphere and the blood through some specialized device such 

 as gills or lungs, while internal respiration is an interchange between 

 the blood and the cells of the body. 



In looking into the story of respiration, one finds the first relevant 

 suggestions coming from John Mayo who in 1668 suggested that res- 

 piration and combustion were analogous processes. His work was 

 antedated by another early worker, Robert Hooke, the same man 

 who described the dead cells in cork, and who demonstrated by the 

 use of experiments that air is necessary for the maintenance of life 

 H. w. H. — 21 



