CHAPTER II. 

 RESPIRATION. 



254. ONE particular item of the body's income, viz. 

 oxygen, is peculiarly associated with one particular item of the 

 body's waste, viz. carbonic acid, inasmuch as the means which 

 are applied for the introduction of the former are also used for 

 the getting rid of the latter. Both are gases, and the ingress 

 of the one as well as the egress of the other seems to be more 

 directly dependent on the simple physical process of diffusion 

 than on any active vital processes carried on by means of tissues. 

 Oxygen passes from the air into the blood mainly by diffusion, 

 and mainly by diffusion also from the blood into the tissues ; 

 in the same way carbonic acid passes mainly by diffusion from 

 the tissues into the blood, and from the blood into the air. 

 Whereas, as we have seen, in the secretion of the digestive 

 juices the epithelium-cell plays an all-important part, in respira- 

 tion the entrance of oxygen from the lungs into the blood, and 

 from the blood into the tissue, and the passage of carbonic acid 

 in the contrary direction, appear to be affected, if at all, in a 

 wholly subordinate manner, by the behaviour of the pulmonary, 

 or of the capillary epithelium. What we have to deal with in 

 respiration then is not so much the vital activities of any par- 

 ticular tissue, as the various mechanisms by which a rapid 

 interchange between the air -and the blood is effected, the 

 means by which the blood is enabled to carry oxygen and car- 

 bonic acid to and from the tissues, and the manner in which the 

 several tissues take oxygen from and give carbonic acid up to 

 the blood. We have reasons for thinking that oxygen can be 

 taken into the blood, not only from the lungs, but also to a cer- 

 tain small extent from the skin, and, as we have seen, from the 

 alimentary canal also ; and carbonic acid certainly passes away 

 from the skin, and through the various secretions, as well as 

 by the lungs. Still the lungs are so eminently the channel of 

 the interchange of gases between the body and the air, that in 

 dealing at the present with respiration, we shall confine our- 

 selves entirely to pulmonary respiration, leaving the considera- 

 tion of the subsidiary respiratory processes till we come to 

 study the secretions of which they respectively form part. 



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