RESPIRATION. 211 



1. In the foetus, and in animals which do not respire at all, or 

 not by a thoracic vacuum, the vacua arising from the dilatation of 

 the heart's cavities, and from its diminished bulk under contrac- 

 tion, only can occur. s 



2. If we suspend respiration and prevent the influence of both 

 sources of vacuum, the circulation continues till the want of 

 chemical changes arrests it ; and, if the vena cava, or any great 

 vein, is obstructed so as to cut off connection with the heart, it 

 becomes distended with blood 1 coming up towards the heart ; and, 

 if wounded between the ligature and the extremities, the blood 

 flows, whatever the position of the animal, till death ensues. u In 

 these cases no vacuum assists. If the pericardium is laid bare, so 

 that no vacuum can occur, except that from the dilatation of the 

 heart's cavities, and the trachea tied, the right ventricle swells 

 enormously with the arriving blood v , a fact not to be explained 

 by vacuum, not even by the heart's own vacuum. The influence 

 of the left ventricle upon the course of the blood in the veins 

 was also shown by Magendie, who firmly tied every part of a 

 dog's leg, except the great artery and vein, and then tied the 

 latter and wounded it below the ligature, when the blood was pro- 

 jected to some distance, and continued to be so, except when the 

 artery was compressed ; and, as long as the circulation continued, 

 the stream from the vein was regulated at pleasure by compress- 

 ing or liberating the artery . x If a turgid vein in the hand is com- 

 pressed, it will not become empty above, as it should if suction 

 from one or all of the three sources mentioned were considerable ; 

 and the jet of blood from an artery was found by Hales to be 

 greater during a deep inspiration y, (probably from the more 

 abundant supply to the left side through the lungs), showing the 

 action of the ventricle to be proportionably greater than the 

 power of the thoracic vacuum at the moment of inspiration to 

 oppose the discharge of blood from it. Still the effects of the 

 vacuum are such as we have seen. 



s On connecting the barometer v/ith the interior of the pericardium of an eel, 

 Sir D. Barry found the mercury move. 



s Hunter, On the Blood, p. 75. sq. Haller had previously ascertained the 

 same thing, and, while allowing the influence of a vacuum, urged it as a proof, 

 that the vacuum was not efficient, but only auxiliary. EL Physiol. t. ii. p. 325. 



u Mr. Spry, Lancet. Jan. 1827. 



v Dr. David Williams, JEdinb. Med. and Surgical Journal^ 1 823, p. 528. 



x Journal de PhysioL t. i. p. 1 1 1 . 



y Statical Essays, vol. ii. p. 6. 



p 4 



