CHAP, ii.] RESPIRATION. 527 



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 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. We may turn at once to the structure 

 of the lungs and bronchial passages, including in the latter the 

 trachea but leaving the larynx until we come to study the voice. 



315. The lung takes origin as a diverticulum from the 

 alimentary canal, and we may consider it as a large branched 

 specially-modified gland lined with mucous membrane and con- 

 sisting of a conducting portion and a secreting portion; the 

 trachea, the two bronchi into which this divides, and the numerous 

 bronchia, or smaller passages branching out from these, represent 

 ducts, and the secreting alveoli of an ordinary gland are repre- 

 sented by what we shall presently describe as air-cells or pulmonary 

 alveoli ; but it must be borne in mind that, as we have just said, 

 active secretion by the epithelium lining these pulmonary alveoli 

 is reduced to a minimum or possibly absent altogether. 



The complex structure of the mammalian lung will be rendered 

 easier of comprehension if we first say a few words on the structure 

 of a much simpler lung, such as that of the newt or the frog. 



The lung of the newt is a long oval sac opening by a short 

 single bronchus into a very short trachea. It may, by inflation, 

 be largely distended, and when the pressure is removed collapses 

 and shrinks to a very small bulk. Its walls are therefore highly 

 elastic, in the sense in which we have so often used that word. 

 They consist, like mucous membrane elsewhere, of an epithelium 

 resting on a connective tissue basis. This connective tissue basis, 

 which is very thin when the lung is distended, contains a very 

 large number of elastic fibres of various sizes but mostly small ; 

 these give the wall the elasticity just spoken of. The pulmonary 

 artery, carrying venous blood, divides near the neck of the sac 

 into branches which, running in the connective-tissue of the wall, 

 break up into an exceedingly close-set network of capillaries 

 immediately underneath the epithelium. The capillaries are 

 themselves relatively wide but the meshes are very narrow, being 

 in many cases less than the diameter of a capillary. The epithe- 

 lium over the whole of the sac consists of a single layer of cells, 

 which, except at the neck of the sac, are modified into thin plates 

 in a somewhat peculiar manner. Three or more cells converge 

 together towards the middle of each of the islands or meshes of 

 the capillary network. The nucleus of each -cell is placed within 

 the area of the mesh or island near the convergence of the cell 

 with its neighbours, but a large part of the cell stretches over the 



