336 PHYSIOLOGY. 



complex compound to a simpler one leads to (1) liberation of energy 

 upon which depend the numerous activities of life and (2) to a new 

 combination of the simpler molecules with oxygen. Thus, oxygen 

 is the cause of combustion, and the complement of catabolism. 



Respiration is the general term that includes all of those activ- 

 ities that are involved in the furnishing of oxygen to the tissues and 

 the removal of C0 2 from the tissues of a living organism. 



The respiratory phenomena do not exist in man and the aerial 

 vertebrates only. They are found, of the most varied kinds, in all of 

 the animal species, even in the lowest; these last, lacking true blood 

 as well as a digestive tube, have particular juices introduced by ab- 

 sorption, the nutritive quality of which can develop only under the 

 vivifying influence of atmospheric oxygen. It may here be added 

 that the intervention of this gas is as indispensable to the plant as to 

 the animal in all periods of life. The sap, analogous to the blood, 

 cannot be sufficiently elaborated and become a really nourishing 

 fluid except by the oxygen. 



When a function is found in all living beings, it is logical to 

 conclude that it represents one of the fundamental conditions of 

 their existence. Respiration incontestably offers that character. 

 Not only do all living species breathe at their different ages, but they 

 cannot develop, or persist in their development, except by the accom- 

 plishment of that function. The most positive experiments have 

 demonstrated that the cell of the plant and the cell, of the animal 

 breathe, one in the seed and the other in the egg in which it is organ- 

 ized, and that all development is arrested as soon as communication 

 with the atmospheric air is prohibited. The seed absorbs oxygen 

 from the air for the benefit of the young plant that it contains, fixes 

 some traces of nitrogen, and at the same time exhales a considerable 

 quantity of carbonic acid. 



It was in a chicken's egg that respiration of the embryo was first 

 recognized; when the surface of the egg was covered with an im- 

 pervious coating of oil or varnish, the embryo failed to develop. 

 Later it was proved that the egg containing a chick in the process 

 of development also absorbs oxygen and exhales carbonic acid. 



The life of mammals shows another form of the phenomenon: 

 in them the foetus, by reason of a certain union of its vascular appa- 

 ratuses, draws from the blood of the mother the necessary oxygen 

 which its pulmonary surface cannot yet supply. The villi of the 

 placenta, plunged into the vascular sinuses of the uterus, effect 

 a kind of respiration there. 



