CHAPTER II. 

 RESPIRATION. 



SEC. 1. THE STRUCTURE OF THE LUNGS AND 

 BRONCHIAL PASSAGES. 



314. 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, in as much 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 far more de- 

 pendent on the simple physical process of diffusion than on any 

 active vital processes carried on by means of tissues. Oxygen 

 appears to pass 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 to pass 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 respiration 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, seem to be affected, if at all, in a wholly sub- 

 ordinate 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 particular 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 carbonic 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 certain small extent from the skin, and, as we have 



