370 



TEE HUMAN BODY. 



an inspiration. The elasticity of the lungs, however, 

 causes them to resist this distension and oppose the cardiac 

 systole. The matter may be made clear by an arrangement 

 like that in Fig. 113. A is an air-tight vessel with a tube, 

 e, provided with a stop-cock, leading from it; b is a highly 

 distensible elastic bag in free communication through d' 

 with the exterior; and c, representing 

 the heart, is a less extensible sac, from 

 which a tube leads and dips under 

 water in the vessel B. If air be- 

 pumped out through e both bags will 

 dilate, 1) filling with air, and c with 

 water driven up by atmospheric pres- 

 sure. Ultimately, if sufficiently ex- 

 tensible, they would fill the whole- 

 space, the thinner walled, #, occupy- 

 ing most of it. If then the stop-cock 

 be closed, things will remain in equi- 

 librium, each bag striving to collapse 

 and so exerting a pull on the other, 

 for if b shrinks c must expand and vice 

 versa. If c suddenly shrink, as the 

 heart does in its systole, ~b will dilate; but as soon as the- 

 systole of c ceases, b will shrink again and pull c out to 

 its previous size. In the same way, after the cardiac sys- 

 tole, when the heart-walls relax, the lungs pull them out 

 again and dilate the organ. The contracting heart thus 

 expends some of its work in overcoming the elasticity of the- 

 lungs, which opposes their expansion to fill the space left 

 by the smaller heart; but during the diastole of the heart 

 this work is utilized to pull out its walls again, and draw 

 blood into it. Since the normal heart has muscular power,, 

 and to spare, for its systole, this arrangement, by which 

 some of the work then spent is stored away to assist the 

 diastole, which cannot be directly performed by cardiac 

 muscles, is of service to it on the whole. It is a physio- 

 logical though not a mechanical advantage; no work power 

 is gained, but what there is, is better distributed. 



FIG. 1 13. Diagram illus- 

 trating he influence of as- 

 piration of the thorax on 

 the circulation of the 

 blood. 



