ON THE MOTION OP THE BLOOD. 73 



125. Since a supply of nerves and blood is requisite 

 to the action of the voluntary muscles, it has been 

 enquired whether these are requisite to the heart also.* 



The great influence of the nerves over the heart, is 

 demonstrated by the size of the cardiac nerves, and 

 by the great sympathy between the heart and most func- 

 tions, however different. A convincing proof of this, 

 is the momentary sympathy of the heart during the 

 most perfect health f with the passions, and with the 

 primes vice in various disorders. But the great import- 

 ance of the blood to the irritability of the heart, is evi- 

 dent from the great abundance of vessels in its mus- 

 cular substance. 



Nevertheless it is very probable, that the importance 

 of the nerves in this respect is greater in the voluntary 

 muscles, and of the blood in the heart. 



126. Besides these powers of the heart, there is ano- 

 ther, which is mechanical, dependent on structure, and 

 greatly contributing, in all probability, to sustain the 

 circulation. For when the blood is expelled from the 

 contracted cavities, a vacuum takes place, into which, 

 according to the common laws of derivation, the neigh- 

 bouring blood must rush, being prevented, by means 

 of the valves, from regurgitating. £ (D) 



* On this dispute consult R. Forsten, Question, select. Physiol. Lugd. 

 Rat. 1774. 4to. 



J. B. J. Behrcnd, Dissert, qua demonstratnr cur nervis carere. Mogunt. 

 1792. 4to. ; and on the other side, J. Munnik, Obscrvationes variee. 

 Groning. 1805. 4to. Lucre, 1. c. p. 37. tab. ii. 



f And how much more so when the heart is diseased, is shewn in Caleb Hil- 

 lier Parry's Inquiry into the Symptoms and Causes of the Syncope Anginosa, 

 commonly called Angina Pectoris, p. 114. Bath. 1799. 8vo. 



X Andr. Wilson, Inquiry into the moving powers employed in the Circulation 

 of the Blood. Lond. 1774. 8vo. p. 35 sq. 



