206 HAND-BOOK OF PHYSIOLOGY. 



into the chest through the trachea is expended in overcoming the elas- 

 ticity of the lungs themselves. The amount thus used up increases as 

 the lungs become more and more expanded, so that the pressure inside 

 the thorax during inspiration as far as the heart and great vessels are con- 

 cerned, never quite equals that outside, and at the conclusion of inspira- 

 tion is considerably less than the atmospheric pressure. It has been ascer- 

 tained that the amount of the pressure used up in the way above described, 

 varies from 5 or 7 mm. of mercury during the pause, and to 30 mm. of 

 mercury when the lungs are expanded at the end of a deep inspiration, 

 so that it will be understood that the pressure to which the heart and 

 great vessels are subjected diminishes as inspiration progresses. It will 

 be understood from the accompanying diagram how, if there w r ere no 

 lungs in the chest, but if its capacity were increased, the effect of the 

 increase would be expended in pumping blood into the heart from the 

 veins, but even with the lungs placed as they are, during inspiration the 

 pressure outside the heart and great vessels is diminished, and they have 

 therefore a tendency to expand and to diminish the intra-vascular pres- 

 sure. The diminution of pressure within the veins passing to the right 

 auricle and within the right auricle itself, will draw the blood into the 

 thorax, and so assist the circulation: this suction action aiding, though 

 independently, the suction power of the diastole of the auricle about 

 which we have previously spoken (p. 124). The effect of sucking more 

 blood into the right auricle will, cceteris paribus, increase the amount 

 passing through the right ventricle, which also exerts a similar suction 

 action, and througji the lungs into the left auricle and ventricle and thus 

 into the aorta, and this tends to increase the arterial tension. The effect 

 of the diminished pressure upon the pulmonary vessels will also help 

 toward the same end, i.e., an increased flow through the lungs, so that as 

 far as the heart and its veins are concerned inspiration increases the blood 

 pressure in the arteries. The effect of inspiration upon the aorta and its 

 branches within the thorax would be, however, contrary; for as the 

 pressure outside is diminished the vessels would tend to expand, and thus 

 to diminish the tension of the blood within them, but inasmuch as the 

 large arteries are capable of little expansion beyond their natural calibre, 

 the diminution of the arterial tension caused by this means would be in- 

 sufficient to counteract the increase of arterial tension produced by the 

 effect of inspiration upon the veins of the chest, and the balance of the 

 whole action would be in favor of an increase of arterial tension during 

 the inspiratory period. But if a tracing of the variation be taken at the 

 same time that the respiratory movements are recorded, it will be found 

 that, although speaking generally, the arterial tension is increased during 

 inspiration, the maximum of arterial tension does not correspond with 

 the acme of inspiration (Fig. 163). 



As regards the effect of expiration, the capacity of the chest is dimin- 



