CHAP, ii.] RESPIRATION. 313 



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 from the skin, and, as we have seen, 

 occasionally from the alimentary canal also ; and carbonic acid cer- 

 tainly passes away from the skin, and through the various secretions, 

 as well as by the lungs. Still the lungs are so eminently the chan- 

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

 in dealing at the present with respiration, we shall confine ourselves 

 entirely to pulmonary respiration, leaving the consideration of the 

 subsidiary respiratory processes till we come to study the secretions 

 of which they respectively form part. 



