GENERAL CONSIDERATIONS. 115 



As it is almost exclusively through the blood that the tissues and organs are supplied 

 with oxygen, and as the blood receives and exhales most of the carbonic acid, the respira- 

 tory process may be said to consist chiefly in the change of venous into arterial blood. 

 But experiments have demonstrated that the tissues themselves, detached from the body 

 and placed in an atmosphere of oxygen, will absorb this gas and exhale carbonic acid. 

 Under these circumstances, they certainly respire ; and it is evident, therefore, that, in 

 this process, the intervention of the blood is not an absolute necessity. 



The tide of air in the lungs does not constitute respiration, as we now understand it. 

 These organs merely serve to facilitate the introduction of oxygen into the blood and the 

 exhalation of carbonic acid* If the system be drained of blood, or if the blood be 

 rendered incapable of interchanging its gases with the air, respiration ceases, and all the 

 phenomena of asphyxia are presented, although air be introduced into the lungs with 

 perfect regularity. It must be remembered that the essential processes of respiration 

 take place in all the tissues and organs of the system and not in the lungs. Respiration 

 is a process similar to what are known as the processes of nutrition ; and, although it is 

 much more active and uniform than the ordinary nutritive acts, it is inseparably con- 

 nected with, and strictly a part of the general process. As, in the nutrition of the sub- 

 stance of tissues, the nitrogenized principles of the blood united with inorganic matters 

 are used up, transformed into the tissue itself, finally changed into excrementitious prod- 

 ucts, such as urea or cholesterine, and discharged from the body, so the oxygen of the 

 blood is appropriated, and carbonic acid, which is an excrementitious product, is produced, 

 whenever tissues are worn out and regenerated. There is a necessary and inseparable 

 connection between all these processes ; and they must be considered, not as distinct 

 functions, but as different parts of the one great function of nutrition. As we are as 

 yet unable to follow out all the intermediate changes which take place between the 

 appropriation of nutritive materials from the blood and the production of effete or ex- 

 crementitious substances, it is impossible to say precisely how oxygen is used by the 

 tissues and how carbonic acid is produced. "We only know that more or less oxy- 

 gen is necessary for the nutrition of all tissues, in all animals, high or low in the scale, 

 and that the tissues produce a certain quantity of carbonic acid. The fact that oxygen is 

 consumed with much greater rapidity than any other nutritive principle and that the 

 production of carbonic acid is correspondingly active, as compared with other effete 

 products, points pretty conclusively to a connection between the absorption of the one 

 principle and the production of the other. 



In some of the lowest of the inferior animals, there is no special respiratory organ, 

 the interchange of gases being effected through the general surface. Higher in the ani- 

 mal scale, special organs are found, which are called gills when the animals live under 

 water and respire the air which is in solution in the water, and lungs when the air is 

 introduced in a gaseous form. Animals possessed of lungs have a tolerably-perfect cir- 

 culatory apparatus, so that the blood is made to pass continually through the respiratory 

 organs. In the human subject and the warm-blooded animals generally, the lungs are 

 very complex and present an immense surface by which the blood is exposed to the air, 

 separated from it simply by a delicate and permeable membrane. These animals are like- 

 wise provided with a special heart, which has the function of carrying on the pulmonary 

 circulation. Although respiration is carried on to some extent by the general surface, the 

 lungs are the important and essential onrans in which the interchange of gases takes place. 



The essential conditions for respiration in animals which have a circulating nutritive 

 fluid are: air and blood, separated by a membrane which will allow the passage of gases. 

 The effete products of respiration in the blood pass out and vitiate the air. The air is 

 deprived of a certain portion of its oxygen, which passes into the blood, to be conveyed 

 to the tissues. Thus the air must be changed to supply fresh oxygen and get rid of the 

 carbonic acid. The rapidity of this change is in proportion to the nutritive activity of 

 the animal and the rapidity of the circulation of the blood. 



