MOTION OF THE BLOOD IN THE ARTERIES. 377 



extremities 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 would be in 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 walls of the arteries (as shown by the 

 experiments of Poisseuille) is nearly the same ail 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 one-tenth; so that the 

 increase of diameter will not be so much as one-twentieth, a quantity scarcely 

 perceptible 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 synchronous 

 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 one-sixth and one-seventh of a second. 



502. 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 

 vigorously on the direct application of stimuli to itself; and Valentin and 

 others have recently succeeded in producing evident contractions in both, by 

 irritation of the Sympathetic nerve, and of certain roots of the Spinal nerves 

 ( 209). Further, although many experimenters have failed in producing 

 contractions of this tissue, by stimuli directly applied to itself, yet others have 

 distinctly witnessed them ; and, in any question of this kind, the positive evi- 

 dence must be held to outweigh the negative. Thus Verschuir states, that he 

 has seen arteries contract, when stimulated by the mineral acids, by electricity, 

 and by the application of the point of a scalpel. Dr. Thomson also saw them 

 contract, on the application of ammonia, and when punctured with the point 

 of a fine needle, in the living body. It has been ascertained by the direct 

 and careful experiments of Poisseuille, that, when the artery is dilated by the 

 blood injected into it from the heart, it reacts with a force superior to the 

 impressing impulse ; and he has also* shown that, if a portion of an artery 

 from an animal recently dead (in which the vital contractility seems to be pre- 

 served), and one from an animal that has been dead some days (in which 

 nothing but the elasticity remains), be distended with an equal force, the 

 former becomes much more contracted than the latter, after the distending force 

 is removed. 



503. Several experiments also indicate the existence of a power of slow 

 contraction in the arteries, which has been distinguished by the appellation 



32* 



