THE PULSE. 127 



to the square-root of the weight. This is apart from any elastic after-effect. Veins 

 may be extended to at least 50 per cent, of their length without passing the limit 

 of their elasticity. 



[Roy has made careful experiments 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 inclosed in a small vessel containing olive oil. The small vessel with 

 oil was arranged in the same way as in Fig. 33 for the heart. The variations of 

 the contents were recorded by means of a lever writing on a revolving cylinder. 

 The aorta and other large arteries were found to be most elastic and most distensible 

 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 clistensibility occurs with pressures imme- 

 diately 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 2UO mm. Hg., while that of the carotid was more than six times as great 

 at that pressure as 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 (Roy).] 



Pathological. Interference with the nutrition of an artery alters its elasticity 

 [and that in cases where no structural changes can be found]. Marasmus pre- 

 ceding death causes the arteries to become wider than normal (Roy). Age also 

 influences their elasticity in some old people they become atheromatous and 

 even calcined. [The ratio of expansion of strips of the aortic wall to the weights 

 employed to stretch them, remains much the same from childhood up to a certain 

 age (Roy).] 



Cohesion. Blood-vessels are endowed with a very large amount of 

 cohesion, in virtue of which 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). A greater pressure is required to rupture a 

 vein than an artery with the same thickness of its wall. 



66. The PulseHistorical. 



Although the movement of the pulse in the superficially placed arteries was 

 known to the ancients, still the pulse, as it was affected by disease, was more 

 studied by the older physicians than the normal pulse. Hippocrates (460 to 377 

 B.C.) speaks of the former as o-^uyuos, while Herophilus (300 B.C.) contrasted the 

 normal pulse (TraX/uos) with the pulse of disease (o-^uyjuos). He lays special stress 

 upon the relative time occupied by the dilatation and contraction of the arterial 

 tube, and compares these phenomena with the notes of music. He established the 

 fact that the rhythm of the pulse varies in the newly-born, in the adult, and in 

 the aged. Further, he distinguished the size, fulness, quickness, and. frequency of 

 the pulse. Erasistratus (f280 B.C.), a contemporary of Herophilus, made correct 



