214 GENERAL OBSERVATIONS ON THE ARTERIES. 



this coat during the intervals of the heart's action, also con- 

 tributes to render the flow of blood more uniform. * 



The motion of the blood in the arteries appears to depend, 



1st, Upon the impulse given to it by the action of the heart. 



2dly, Upon the elasticity of -the arteries, in consequence of 

 which they first give way to the blood impelled into them, and 

 then react upon it ; and 



3dly, Upon the power of contraction in the arteries, or their 

 irritability. 



In the larger arteries the blood seems to move as it would 

 through an inanimate elastic tube, in consequence of the impulse 

 given by the heart, and kept up by the arteries themselves. In 

 the smaller vessels it seems probable that the motion of the 

 blood depends in a considerable degree upon the contraction 

 which arises from their irritability. 



The obvious effect of the elasticity of the arteries is to resist 

 distention and elongation, and to contract the artery to its 

 natural state, when the distending or elongating cause ceases to 

 act. But it must also resist the contraction induced by the mus- 

 cular fibres, and restore the artery to its natural size when the 

 muscular fibres cease to act after contracting it, as has been 

 observed by Mr. Hunter. 



It seems probable that all the fibres of which the artery con- 

 sists are nearly, but not completely circular ; for it is not certain 

 that there are any longitudinal fibres in the structure of an 

 artery. 



The internal coat of these vessels is very smooth, but 

 extremely dense and firm ; and seems to be rendered moist and 

 flexible by an exudation on its surface. It adheres very closely 

 to the contiguous fibres of the coat exterior to it, but may be 

 very readily peeled off from them. It is of a whitish color, 

 and, like the fibrous structure of the artery, is very elastic. 

 Like that substance, also, it is easily torn or broken ; and when 

 ligatures have been applied to arteries, it has often been 

 observed that the fibrous structure, and the internal coat have 



* Weber's Hildebrandt's Anatomic, tome iii. p. 68. P. 



