406 COMPARATIVE PHYSIOLOGY. 



cent less oxygen and 4 per cent more carbonic anhydride, be- 

 sides certain indifferently known bodies, the result of tissue 

 metabolism, excreted by the lungs. 



The quantity of air actually moved by a respiratory act, as 

 compared with the total capacity of the respiratory organs, is 

 small ; hence a great part must be played by diffusion. The 

 portion of air that can not be removed fromi, the lungs by any 

 respiratory effort is relatively large. 



It is customary to distinguish tidal, complementary, supple- 

 mentary, and residual air. 



The vital capacity is estimated by the quantity of air the 

 respiratory organs can move, and is very variable. 



The blood is the respiratory tissue, through the mediation 

 of its red cells, by the haemoglobin they contain. This sub- 

 stance is a ferruginous proteid, capable of crystallization, and 

 assuming under chemical treatment many modifications. When 

 it contains all the oxygen it can retain, it is said to be saturated 

 and is called oxy-hsemoglobin, in which form it exists (with 

 some reduced haemoglobin) in arterial blood, and to a lesser 

 extent in venous blood, which differs from arterial in the rela- 

 tive proportions of hsemoglobin (reduced) it contains, as viewed 

 from the respiratory standpoint. 



Oxy-haemoglobin does not assume or part with its oxygen, 

 according to the Henry-Dalton law of pressures, nor is this gas 

 in a state of ordinary chemical combination. It is found that 

 the oxygen tension of the blood is lower and that of carbonic 

 anhydride higher than in the air of the alveoli of the lungs, 

 while the same may be said of the tissues and the blood re- 

 spectively. This has been, however, recently again denied. 



Respiration is a vital process, though certain physical con- 

 ditions (temperature and pressure) must be rigidly maintained 

 in order that the gaseous interchanges shall take place. Res- 

 piration is always fundamentally bound up with the metabo- 

 lism of the tissues themselves. All animal cells, whether they 

 exist as unicellular animals (Amoeba) or as the components of 

 complex organs, use up oxygen and produce carbonic dioxide. 

 Respiratory organs, usually so called, and the respiratory tissue 

 par excellence (the blood) are only supplementary mechanisms 

 to facilitate tissue respiration. Carbonic anhydride exists in 

 blood probably in combination with sodium salts, though the 

 whole matter is very obscure. 



Respiration, like all the other functions of the body, is con- 



