396 



THE HUMAN BODY. 



ginning a little after the beginning of inspiration, the fall a 

 little later than the commencement of expiration. 



In still another way the aspiration of the thorax assists 

 the heart. The heart and lungs are both distensible, though 

 in different degrees, and each is stretched in the chest some- 

 what beyond its natural size; the one by the atmospheric 

 pressure directly, the other by that pressure indirectly ex- 

 erted through the blood exposed to it in the extra-thoracic 

 veiiis. Supposing, therefore, the heart suddenly to shrink, it 

 would leave more space in the chest to be filled by the lungs; 

 these must accordingly, at each cardiac systole, expand a lit- 

 tle to fill the extra room, just as they do when the space 

 around them is otherwise enlarged, as during an inspiration. 

 The elasticity of the lungs, however, causes them to resist 

 this distention and oppose the cardiac systole. The matter 

 may be made clear by an arrangement like that in Fig. 131. 

 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, b filling with air, 

 and c with water driven up by atmos- 

 pheric pressure. Ultimately, if suffi- 

 ciently extensible, they would fill the 

 whole space, the thinner- walled, b, occu- 

 pying most of it. If then the stop-cock 



be cl Sed > thin g S W J U . remain in ^quilib- 



rium, each baff striving to collapse and 



r 



FIG. 131.-Diagram il- 



lustrating the influence of 



aspiration of the thorax 



on the circulation of the so exerting a pull on the other, lor it o 



blood. , T 



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 systole, 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 elas- 

 ticity 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 



