When a ligature is put upon an artery, no pulse is felt beyond the place where the artery is obstructed, but it is distinct up to that place. This experiment at once shews the dependence of the pulse on the systole of the ventricle, and establishes that this phenomenon is not dependent on the progressive motion of the blood, since, in that part of the artery placed on the side dale ligature next to the heart in which the pulse is distinct, the blood is at rest. Nor does the pulse appear in ordinary circumstances to depend upon lateral distension of the arteries, for such distension occurs to so small a degree as is quite insufficient to account for the produc tion of the pulse. Arthaud,* a French surgeon, was the first vvho sustained, in opposition to the opinion prevalent at the time he wrote, the view that the arteries are not laterally di lated at each systole of the heart, and that the pulse is not to be explained by such dilatation. Arthaud shewed that when an artery is laid bare, no perceptible enlargement of its calibre takes place at the time when the heart con tracts and the pulse is felt. We have already stated that the arterial system being fuller of blood at one time than another must be dilated to admit the blood propelled into the aorta from the ventricle ; and it seems to follow from the observations of Arthaud, which have been ably confirmed by the interesting expe riments of the late Dr. Parry,t that the en largement of the capacity of the arteries is effected principally by their elongation. Ac cording to these experimenters, when one of the larger arteries is laid bare, the eye does not distinguish any lateral - enlargement corres ponding to the systole of the ventricle, and Parry measured with great care the artery at the time of each pulse and between the beats without being able to detect the slightest differences in its size ; but though not percep tibly distended laterally, the artery undergoes a certain change of place, for at each systole of the ventricle it is propelled in a direction outwards from the heart, and during the di astole it returns to its former situation. This locomotion of the artery, as it is called, is obviously produced by the distension and elongation of the larger arteries near the heart. A considerable elongation of the arteries may also easily be seen at all sudden incurvations of these vessels. The bend of the curved part is generally increased and projected further out wards during the systole; and we observe that a straight part of an artery, if fixed at its opposite ends, is bent at the time of the pulse in conse quence of its elongation. In many persons in a state of health the arteries may be seen to move under the skin, although not exposed. This motion is generally perceived at places where there is a sudden bend of an artery, or where the artery lies upon an unyielding part, as bone, &c. and in some individuals an ap pearance of 'dilatation or lateral enlargement even may be perceived in some of the larger arteries. Although these circumstances shew that the pulse is not attributable to a lateral dilatation of arteries, yet it would appear that such an enlargement does oecur in a small degree, for it is occasionally perceptible to the eye in the arteries when laid bare ; and M. Poiseuille,* by means of a small apparatus, capable of beincr applied round a part of an artery, has proved distinctly the occurrence of lateral enlargement, and estimated its extent in the larger arteries at 1-11th of their dia meter.
The finger laid upon an exposed artery does not feel any pulse, unless the artery be com pressed, and when the arteries are in their na tuml situation covered by the integuments, it is only when they lie upon a hard part, as a bone, and when the sides of the artery are brought nearer to one another by pressure, that the pulse is perceptible. Those instances in which this does not appear to be the case, as well as those in which the dilatation occasionally seems to occur below the integuments, may in like manner depend upon the artery being subjected to pressure of superjacent parts at the place ob served. It is also sufficiently obvious that the
pulse does not depend upon any active change of the artery itself, or upon any vital contraction and dilatation of the vessels,for the exact appear ance of the living pulse may be produced in the arteries of a dead animal by injecting water into the arteries with a syringe,_if care be taken to imitate with the strokes of the piston the beats of the left ventricle of the heart. A fur ther proof of this, and an excellent illustration of the nature of the pulse, is obtained from the curious experiment perthrmed by Bichat of connecting the bloodvessels of a living animal with those of a dead one, the result of which is the production of a pulse in the vessels of the dead animal connected with the arteries of the living one. In those instances in which a communication has been established between an artery and a contiguous vein in consequence of a wound, or in what is called Aneurismal Varix, the vein pulsates exactly like an artery.
Many have remarked that the pulse in the arteries of the extremities is a little later than the beat of the heart on the ribs and the pulse in the arteries in the immediate neighbourhood of the heart. This retardation has of late been more distinctly pointed out by Dr. M'Donnell of Belfast,'' and by Weber of Leipsig.t It is much more marked in some persons than in others, and is always most perceptible when the circulation is slowest. With a little atten tion we can thus observe a distinct succession in the occurrence of the beat of the apex of the heart at the ribs, the pulse in the carotid, facial, radial, and posterior tibial arteries, the interval between each of which, though very small, being yet appreciable by the finger. \Veber states that the retardation of the pulse in the foot after that of the beat of the heart amounts to not more than one-seventh part .of a second. We have ourselves confirmed by experiments on several individuals the most of these facts relating to the later pulse in the more remote arteries. The cause of the retar dation is obviously the elasticity and yielding of the arterial parietes ; for were the arteries rigid tubes, it is manifest that the impulse of the heart would be felt at one arid the same in stant of time throughout the whole of the branches ; but as these vessels yield to disten sion, that part of them to which the distending force is immediately applied is first dilated, and this dilatation does not reach immediately the remote parts.
The pulse has been correctly compared to the propagation of an undulation or wave on the surface of water; for the successive im pulses of the heart are first given to the column of blood in the commencement of the aorta; this column communicates these impulses to the arterial parietes and tends to distend them. The parietes re-act against this distending force and compress the adjoining part of the column of blood, from which the impulse passes to the next part of the aorta ; and so the pulse; gradu ally passing on from the trunks to the smaller. branches, becomes less and less perceptible as the force of the heart is equalized by the elastic resistance of the coats of these vessels.t The pulse is still perceptible in very small arteries : Haller§ states that he was unable to perceive any in small arteries of one-sixth of a line in diameter,—an observation which does not, however, prove the flow of the blood to be uniform or without jerks even in vessels of this size, for Spallanzani observed pulsations in arteries of this small size; and the microsco pic observation of the circulation in transparent parts by Haller himself, Spallanzani, and others, shews that the visible impulse of the heart is communicated to the blood in the smallest of those vessels, which have distinctly the characters of arteries.