MOVEMENT OF THE BLOOD IN THE ARTERIES. 



483 



most important influence upon the movement of blood through them, in virtue 

 of the physical and vital properties of their walls, or rather of their middle or 



Fig. 141. 



Web of Frog' 's foot, stretching between two toes, magnified 3 diameters ; showing the bloodvessels 

 and their anastomoses : 1, 1, veins ; 2, 2, 2, arteries. 



fibrous coat, wflich alone is possessed of contractile properties. We find in this 

 coat a layer of annular fibres, composed of muscular fibre cells, mingled with 

 areolar tissue. 1 On the outside of this, is a layer of yellow elastic tissue, which 

 is much thicker in the larger arteries, in proportion to their size, than in the 

 smaller. To this last tissue is due the simple elasticity of the arterial walls, 

 which is a physical property that persists after death, until a serious change 

 takes place in their composition; whilst to the one first mentioned, we are to 

 attribute the property which they unquestionably possess (in common with proper 

 muscular tissue) of contracting on the application of a stimulus, so long as their 

 vitality remains. These two endowments are possessed in various proportional 

 degrees, by the different parts of the Arterial system. Thus, it was justly re- 

 marked by Hunter, that elasticity, being the property by which the interrupted 

 force of the Heart is made equable and continuous, is most seen in the large 

 vessels more immediately connected with that organ ; whilst on the other hand, 

 the contractility is most observable in the smaller vessels, where it is more 

 required for regulating the flow of blood towards particular organs. 



513. It has been denied by many Physiologists, that the middle coat of the 

 Arteries possesses any property that can be likened to muscular Irritability ; 

 but no reasonable doubt can any longer exist on this point. That the walls of 

 arteries cannot be readily stimulated to contraction through the medium of their 

 nerves is universally admitted ; but the same is the case with regard to the 

 muscular coat of the Alimentary canal which contracts most vigorously on the 

 direct application of stimuli to itself; and Valentin and others have succeeded 

 in producing evident contractions in the Aorta, by irritation of the Sympathetic 

 nerve, and of the roots of the cervical nerves of the Spinal system. Further, 

 although many experimenters have failed in producing contractions of this tissue 

 by stimuli directly applied to itself, yet such contractions may be so easily 



1 Kolliker, in "Kolliker and Siebold's Zeitschrift," 1849. 



