EFFECTS ON THE CIRCULATION 167 



be overcome by the ventricle ; and during the diastole, it 

 superadds itself to the elasticity of the arterial walls in 

 driving the blood onwards towards the capillaries, inas- 

 much as all progress in the opposite direction is stopped 

 by the semilunar valves. 



It is, therefore, clear, that "the inspiratory movement, 

 on the whole, helps the heart, inasmuch as its general 

 result is to drive the blood the way that the heart 

 propels it. 



In expiration, the difference between the pressure of 

 the atmosphere on the surface, and that which it exerts 

 on the contents of the thorax through the lungs, becomes 

 less and less in proportion to the completeness of the ex- 

 piration. Whenever, by the ascent of the diaphragm and 

 the descent of the ribs, the cavity of the thorax is so far 

 diminished that pressure is exerted on the great vessels, 

 the veins, owing to the thinness of their walls, are especi- 

 ally affected, and a check is given to the flow of blood 

 in them, which may become visible as a renons pulse in 

 the great vessels of the neck. In its effect on the arterial 

 trunks, expiration, like inspiration, is, on the whole, 

 favourable to the circulation ; the increased resistance to 

 the opening of the valves during the ventricular systole 

 being more than balanced by the advantage gained in the 

 addition of the expiratory pressure to the elastic reaction 

 of the arterial walls during the diastole. 



When the skull of a living animal is laid open and the 

 brain exposed, the cerebral substance is seen to rise and 

 fall sjTichronously with the respiratory movements ; the 

 rise corresponding with expiration, and being caused by 

 the obstruction thereby offered to the flow of the blood in 

 the veins of the head and neck. 



The eflfects of the respiratory movements on the flow of 

 blood towards the heart must be the same for any other 

 structure contained in the thorax and connected with 

 vessels lying outside the thorax. Now the thoracic duct 



