2 12 ON THE FUNCTIONS OF THE HEAhT AND ARTERIES. 



III. 



The Croonian Lecture. On the Functions of the Heart and 

 Arteries. By Thomas Young, M. D. For. Sec, R. S, 



( Concluded from p. 68. J 



Functisnsof -UL Shall proceed to inquire, in the third place, into the na- 

 ili€ muscular tore and extent of the functions, which are to be attributed 

 coats of thenar- ^^ ^^^ muscular fibres of the coats of the arteries; and I 

 teries. apprehend, that it will appear to be demonstrable, that 



effe^^t onTbe'^ ^^^^ ^^~ much less concerned in the progressive motion of 

 niotioii of the the blood, than is almost universally believed. The argu- 

 J'^^^jV^" '^ inents, which may be employed to prove this, are nearly 

 posed. the same that I have already stated, in examining the mo- 



Arguments to ^JQjj jjj- g^ fjuicl carried alonij before a moving body in an 

 prove this» = n j 



open canal ; but in the case of an elastic tube, the velocity 



of the transmission of an impulse being rather diminished 

 than increased by an increase of tension, the reasoning is 

 still stronger and simpler; for it may here be safely assert- 

 ed, that the anterior parts of the dilatation, which must be 

 forced along by any progressive contraction of the tube, 

 can only advance with the velocity appropriate to the tube, 

 and that its capacity must be proportionate to its length 

 and to the area of its section : now the magnitude of its 

 section must be limited by that degree of tension, which is 

 sufficient to force back through the contraction what re- 

 mains of the displaced fluid ; and the length, by the differ- 

 ence of the velocity appropriate to the tube, and that with 

 which the contraction advances : consequently, if the con- 

 traction advance with the velocity of a pulsation, as any 

 contractile action of the arteries must be supposed to do, 

 this length necessarily vanishes, and with it the quantity of 

 the fluid protruded ; the whole being forced backwards, by 

 the distending force which is exerted by a very small di- 

 Suppositirn, lated portion immediately preceding the contraction. It 

 t!lctioirfo*ll*ows ™ig^t indeed be imagined, that the contraction follows $he 

 the pulsation pulsation with a velocity somewhat smaller than its own,; 

 T»ui tibveo- ^^^^ ^j^j^ opinion would stand on no other foundation than 



mere 



