114 THE CIRCULATION. 



the sympathetic nerves, on the other hand, accelerates and 

 strengthens the heart's action. 



Various theories have been proposed to account for these 

 peculiar results, but none of them are very satisfactory, and it 

 is probable that many more facts must be discovered before 

 any theory on the subject can be permanently maintained. 



The connection of the action of the heart with the other 

 organs, and the influences to which it is subject through them, 

 are explicable from the connection of its nervous system with 

 the other ganglia of the sympathetic, and with the brain and 

 spinal cord through, chiefly, the pneumogastric nerves. But 

 this influence is proved in a much more striking manner by 

 the phenomena of disease than by any experimental or other 

 physiological observations. The influence of a shock in arrest- 

 ing or modifying the action of the heart, its very slow action 

 after compression of the brain, or injury to the cervical por- 

 tion of the spinal cord, its irregularities and palpitations in 

 dyspepsia and hysteria, are better evidence for the connection 

 of the heart with the other organs through the nervous sys- 

 tem, than are any results obtained by experiments. 



Effects of the Heart's Action. 



That the contractions of the heart supply alone a sufficient 

 force for the circulation of the blood, appears to be established 

 by the results of several experiments, of which the following 

 is one of the most conclusive : Dr. Sharpey injected bullock's 

 blood into the thoracic aorta of a dog recently killed, after 

 tying the abdominal aorta above the renal arteries, and found 

 that, with a force just equal to that by which the ventricle 

 commonly impels the blood in the dog, the blood which he in- 

 jected into the aorta passed iu a free stream out of the trunk 

 of the vena cava inferior. It thus traversed both the systemic 

 and hepatic capillaries ; and when the aorta was not tied 

 above the renals, blood injected under the same pressure 

 flowed freely through the vessels of the lower extremities. A 

 pressure equal to that of one and a half or two inches of mer- 

 cury was, in the same way, found sufficient to propel blood 

 through the vessels of the lungs. 



But although it is probably true that the heart's action 

 alone is sufficient to insure the circulation, yet there exist 

 several other forces which are, as it were, supplementary to 

 the action of the heart, and assist it in maintaining the circu- 

 lation. The principal of these supplemental forces have been 

 already alluded to, and will now be more fully pointed out. 



