MOTION OF THE BLOOD IN THE ARTERIES. 557 



side 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 re- 

 mains. These two endowments exist, in various proportional degrees, in the 

 different parts of the Arterial system. Thus it was justly remarked by Hun- 

 ter, 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. On the other hand, the contractility 

 is most observable in the smaller vessels, where it is more required for regu- 

 lating the flow of blood towards particular organs. 



729. It is easily shown that the action of the Elasticity of the Arterial 

 tubes, is one of a purely physical character; and that its purpose is to con- 

 vert the intermitting impulses, which the fluid receives from the heart, into a 

 continuous current. The former are very evident in the larger trunks; but 

 they diminish with the subdivision of these, until they entirely disappear in 

 the capillaries, in which the stream is usually equable or nearly so. AVe may 

 imagine a powerful forcing-pump injecting water, by successive strokes, into 

 a system of tubes with unyielding walls ; the flow of fluid at the farther ex- 

 tremities of these tubes, would be as much interrupted as its entrance into 

 them. But if an air-vessel (like that of a fire-engine) were placed at their 

 commencement, the flow Avould be in a great degree equalized; since a part 

 of the force of each stroke would be spent upon the compression of the air 

 included in it; and this force would be restored by the elasticity of the air 

 during the interval, which would propel the stream, until directly renewed by 

 the next impulse. A much closer imitation of the natural apparatus would 

 be afforded, by a pipe which had elastic walls of its own ; if water were 

 forced by a syringe into a long tube of caoutchouc, for example, the stream 

 would be equalized before it had proceeded far. This effect is found to be 

 accomplished, at any point of the Arterial circulation, in a degree proportionate 

 to its distance from the Heart; and it is another effect of the same cause, that 

 the pressure of the blood upon the wall of the arteries (as shown by the ex- 

 periments of Poisseuille) is nearly the same all over the system. It is to the 

 distension of the arterial tubes, both in their length and calibre, that their 

 pulsation is due. Their elongation is the more considerable of the two effects ; 

 and it causes the artery to be lifted from its seat and to become curved. The 

 transverse dilatation has been denied by some physiologists ; but it has been 

 recently proved to take place, by an ingenious experiment of Poisseuille's. 

 The increase of capacity, however, is not more than l-10th ; so that the in- 

 crease of diameter will not be so much as l-20th, a quantity scarcely per- 

 ceptible to ordinary measurement. The transmission of the pulse-wave 

 through the whole system is not instantaneous, but takes place in an appre- 

 ciable time. The pulsation of the large arteries near the Heart, is synchron- 

 ous with the Ventricular systole ; but that of other arteries is somewhat later, 

 the difference varying with their distance, and amounting in some instances 

 to between l-6th and l-7th of a second. 



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

 Arteries possesses any property which can be likened to Muscular Contrac- 

 tility; and it will therefore be desirable to enter somewhat in detail into the 

 question. That it cannot be readily stimulated to contraction, through the 

 medium of its nerves, is universally admitted ; but the same is the case in 

 regard to the Muscular coat of the alimentary canal, which contracts most 



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