CIRCULATION. 



C63 



blood occasionally appears to exert within an 

 aneurismal tumour; giving rise to its peculiarly 

 hard pulsation on every side, and assisting the 

 ravages by absorption which are frequently the 

 consequence of the larger internal aneurisms. 

 The pressure in an aneurism is obviously to be 

 measured by the extent of its internal surface 

 multiplied into the force with which the blood 

 moves in the part of the artery where it opens 

 into the aneurismal sac. 



c. Arterial pulse. The arterial pulse, or suc- 

 cession of beats felt by the finger placed over 

 an artery, depends upon the impulse of the 

 left ventricle being communicated along the 

 arterial tube and the column of blood which 

 it contains. 



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 of the 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 who 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 



* Dissert, sur la Dilatation des Arteres. Paris, 

 1770. 



t Dr. C. H. Parry's Inquiry into the Nature of 

 the Arterial Pulse. Bath and Lond. 1816. Dr. 

 Chas. Henry Parry's Additional Experiments. 

 Lond. 1819. 



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 occur 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 being applied round a part of an 

 artery, has proved distinctly the occurrence of 

 lateral enlargement, and estimated its extent 

 in the larger arteries at 1-1 1th 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- 

 tural 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 performed 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 



* Sur la dilatation des Arterea ; Magendie's 

 Journ. vol. ix. p. 44; and Breschet's Repert. 

 1828. 



