EESPIRATION 169 



there is a marked increase of oxidizable material (possibly 

 the result of the cleavages described in Chapter IV), and 

 for this reason there is a corresponding increase of oxidation 

 in the cell. It follows that, in general, cell oxidation can 

 be increased only by increasing cell work ; it cannot be in- 

 creased by the mere act of deep breathing. " We may lead 

 a horse to water or fetch water to a horse, but we cannot 

 make him drink." The assertion, too frequently heard, that 

 some special form of breathing movement leads to more 

 efficient oxidation of wastes throughout 

 the body betrays lamentable ignorance of 

 this fundamental fact of physiology. This, 

 however, is not denying that one type of 

 breathing movement may still be prefer- 

 able to another, nor affirming that deep- 

 ened breathing may not sometimes be 

 desirable. Breathing movements accom- FlG 77 Two ad j a _ 

 plish other things than oxygenation of the cent alveoli of the 

 blood, and we may now proceed to study lun & 



their physiology. " Showing the air cells 



7. Structure of the lungs. In Chapter II the anatomical 

 relations of the air passages (trachea, bronchi, etc.) and lungs 

 have been described. The student at this point should con- 

 sult especially Fig. 5 (p. 13) in order to obtain a clear idea 

 of the structure of the lungs. The bronchi which enter the 

 lungs branch, much as the ducts of a gland, and their ulti- 

 mate branches end in the alveoli, which, like those of a gland, 

 consist of a single layer of cells, but in this case of very thin, 

 flattened cells. Fig. 77 shows two of these alveoli dissected, 

 and Fig. 78 a section taken lengthwise through the same. 

 Connective tissue binds together the alveoli and bronchial 

 tubes, thus forming the lobes of the lungs. In this connective 

 tissue and hence between the alveoli are the larger blood 

 vessels, branches of the pulmonary artery and pulmonary 

 veins. The arterioles supply an exceedingly close network 



