208 RESPIRATION, 



pulmonary artery and aorta is under the same circumstances, but 

 the propelling force of the ventricles at one moment, and the ac- 

 tion of their valves during their relaxation, prevent its retrogres- 

 sion. The atmospheric pressure on the blood-vessels creates a 

 necessity for greater strength in the ventricles, as it impedes the 

 progress of blood from the heart; but it also facilitates the 

 return. Thus the smaller pressure on the heart acts, by the in- 

 tervention of the blood, as an antagonist to its contracting fibres., 

 assisting to dilate them when they become relaxed. 



That the blood is drawn towards the heart during inspiration 

 has been long acknowledged. " In my experiments," says Haller, 

 " if you open the chest, abdomen, neck, or fore-extremities of an 

 animal, and lay bare the great veins, the superior and inferior 

 cava, the jugular, subclavian, brachial, or mammary, you will see 

 the blood return to the heart whenever the animal inspires, and 

 these veins recede some lines from it, become empty and pale, 

 flat and bloodless : " depleri, palescere, explanari, exsangues 

 fieri." ! In the words of Dr. Magendie, sixty years afterwards m , 

 " when the chest dilates, it inspires the blood of the cavae, and 

 successively that of the veins ending in them, much in the 

 same way as it does the air into the trachea." Were Dr. 

 Carson's account of expiration correct, as a vacuum would be 



mentaneo quasi vacuo, continue in cor dextrum impellit sanguinis quantum 

 facile capit pondus atmosphserae." Quoted by Dr. M. Hall. 



1 1. c. lib. vi. sect. iv. p. 333. 1760. 



m Journal de Physiologic, t. i. p. 136. 1821. For the same reason, if a tube 

 is placed in the jugular vein, the air rushes into it during respiration with a noise, 

 and the ill effects of air in the heart occur. (Magendie, 1. c. p. 195.) And if a 

 large vein is opened in surgical operations, and any thing prevents the sides from 

 collapsing, the air may rush in and destroy life, as happened a few years ago at 

 Paris. (1. c. p. 1 92. sqq. ) This may be shown also, by inserting a tube, immersed 

 in a coloured fluid, into a large vein, when the liquid will rise during inspiration, 

 and stop or descend during expiration. (See Sir D. Barry, 1. c. who conceives 

 another source of vacuum to the pulmonary veins and venous sinuses, by the 

 distraction of their parietes during inspiration, p. 29. 1826. And Dissertation, 

 &c. p. 13. sq.) Still more recently, Sir D. Barry has applied the barometer to the 

 chest of a pigeon, a viper, a common snake, and a frog, and found the mercury 

 descend during inspiration. When connected with the exterior of the pericardium 

 of an eel, the mercury became concave each time that the heart retired from the 

 pericardium, so that its pulsations could be counted, and also at every effort of 

 the animal to open its gill covers. Sur replication du fiarometre, $c. Annnles. 

 des Sciences Naturelles. Avril, 1827. 



