THE PULSE. 



97 



[Roy experimented upon the elastic properties of the arterial wall. A portion of an artery, 

 so that it could be distended by any desired internal pressure, was enclosed in a small vessel 

 containing olive oil arranged in the same way as in tig. 62 for the heart. The variations of the 

 contents were recorded by means of a lever writing on a revolving cylinder. The instrument is 

 termed a sphygmotonometer. The aorta and other large arteries are most elastic and most dis- 

 tensible at pressures corresponding more or less exactly to their normal blood-pressure, while in 

 veins the relation between internal pressure and the cubic capacity is very different. In them the 

 maximum of distensibility occurs with pressures immediately above zero. Speaking generally, 

 the cubic capacity of an artery is greatly increased by raising the intra-arterial tension, say from 

 zero to about the normal internal pressure which the artery sustains during life. Thus in the 

 rabbit, the capacity of the aorta was quadrupled by raising the intra-arterial pressure from zero 

 to 200 mm. Hg., while that of the carotid was more than six times greater at that pressure than 

 it was in the undistended condition. The pulmonary artery is distinguished by its excessive 

 elastic distensibility. Its capacity (rabbit) was increased more than twelve times on raising the 

 internal pressure from zero to about 36 mm. Hg. Veins, on the other hand, are distinguished 

 by the relatively small increase in their cubic capacity produced by greatly raising the internal 

 pressure, so that the enormous changes in the capacity of the veins during life are due less to 

 differences in the pressure than to the great differences in the quantity of blood which they 

 contain.] 



Pathological. Interference with the nutrition of an artery alters its elasticity, [and that in 

 cases where no structural changes can be found]. Marasmus preceding death causes the arteries 

 to become wider than normal. In some old people they become atheromatous and even calcified. 



Cohesion. The cohesion of blood-vessels is very great, and in virtue of this 

 they are able to resist even considerable internal pressure without giving way. The 

 carotid of a sheep is ruptured only when fourteen times the usual pressure it is called 

 upon to bear is put upon it ( Volkmann). Given a vein and an artery of the same 

 thickness, a greater pressure is required to rupture the former than the latter. The 

 human carotid or iliac artery resists a pressure of 8 atmospheres, the 

 veins about the half of this. 



66. INVESTIGATION OF THE PULSE. [The characters of 

 the pulse may be investigated by (1) the eye (inspection) ; (2) the 

 finger (palpation) ; (3) instruments. 



Two or three fingers are placed over the course of the radial 

 artery, and the various phenomena in connection with the pulse are 

 noted. It takes much practice for the physician to acquire the 

 tactus eruditus, and notwithstanding the value of instruments, every 

 physician should make a careful study of the pulse-beat with his 

 finger. In order to feel the pulse-beat or to take a pulse-tracing, 

 there must be some resistant body, e.g., a bone behind the artery, 

 and a certain degree of pressure must be exerted on the artery.] 



The individual phases of the movement of the pulse can only 

 be accurately investigated by the application of instruments to the 

 arteries. 



(1) Poiseuille's Box Pulse-Measurer (1829). An artery is exposed and 

 placed in an oblong box filled with an indifferent fluid. A vertical tube with 

 a scale attached communicates with the interior of the box. The column of 

 fluid undergoes a variation with every pulse-beat. 



(2) He'risson's Tubular Sphygmometer consists of a glass tube whose lower 

 end is covered with an elastic membrane (fig. 71). The tube is partly filled 

 with Hg. The membrane is placed over the position of a pulsating artery, Fig. 71. 

 so that its beat causes a movement in the Hg. Chelius used a similar instru- Sphygmometer of 

 ment, and he succeeded with this instrument in showing the existence of the Herisson and 

 double beat (dicrotism) in the normal pulse (1850). Chelius 



(3) Vierordt's Sphygmograph (1855). In this, one of the earliest sphygmo- 

 graphs, Vierordt departed from the principle of a fluctuating fluid column, and adopted the 

 principle of the lever. Upon the artery rested a small pad, which moved a complicated system 

 of levers. At first he used a straw 6 inches long, which rested on the artery. The point of 

 one of the levers inscribed its movements upon a revolving cylinder. This instrument was 

 soon discarded. 



(4) Marey's Sphygmograph consists of a combination of a lever with an elastic spring. The 



