182 THE MOTION OF THE BLOOD. 



of contracting equally with arteries, on the diminution of their 

 contents, the suction influence of the heart would constantly re- 

 duce their cavities to a smaller capacity than is compatible with 

 their functions. The collapse of the veins by pressure, during 

 the suction of the heart, is prevented by the fresh supply of 

 blood afforded by the vis a tergo, which does exist, although it is 

 not considered by him as of itself adequate to convey the blood 

 back to the right auricle. 



All allow that when the heart is relaxed its cavities enlarge, 

 though some ascribe this to its elasticity, and others regard it as 

 a necessary consequence of the arrangement of its fibres. Ex- 

 periment proves the same. Dr. Carson extracted the hearts of 

 some frogs, and immediately put them into water, blood-warm. 

 They were thrown into violent action, and, upon some occasions, 

 projected a small stream of a bloody colour through the trans- 

 parent fluid. The water could not have been projected unless 

 previously imbibed. It was thought that a stream of the same 

 kind continued to be projected at every succeeding contraction; 

 but that, after the first or second, it ceased to be observable, in 

 consequence of the liquid supposed to be imbibed and projected 

 losing its bloody tinge and becoming transparent, or of the same 

 colour with the fluid in which the heart was immersed. The 

 organ was felt to expand forcibly during relaxation, a fact 

 stated long ago by Pechlin , and subsequently by many others. 

 Indeed, some consider the expansion of the heart as a change 

 equally active with its contraction : conceiving, perhaps, that 

 different fibres may act alternately, and produce expansion and 

 contraction, just as the tongue may be retracted and protruded, 

 and the iris lessened or enlarged. 



Dr. Carson accounts, however, for the full dilatation of the 

 heart upon another principle, upon the consideration of which it 

 will be impossible to enter before the next section, where the 

 subject will therefore be prosecuted. 



" We must now enquire what powers are exerted by other 

 organs in assisting the circulation. The existence of some 

 secondary powers, and their ability to assist, or even in some cases 

 to compensate for, the action of the heart, are proved by several 

 arguments : v. c. the blood moves, according to many persons," 

 in some parts to which the influence of the heart cannot reach, 



De Corde. 



