CHAP, xiii.] CIRCULATION. 209 



dilate ; and that is the cause of their vacuity in the dead 

 body. 1 



It seems idle at the present day to debate whether the arteries 

 dilate when they are traversed by the sanguineous stream which 

 each contraction or systole of the cardiacal ventricles drives into 

 their canals. But the fact at one time was the subject of contro- 

 versy among physiologists. Spallanzani demonstrated the dila 

 tation of the great arterial trunks, and especially of the general 

 trunk of the arterial tree, the aorta, by surrounding this vessel 

 with a metallic ring. 2 He saw that during the cardiacal systole 

 the aortical diameter augmented a third, in immediate proximity 

 to the heart, and a twentieth only in the rest of its passage. 

 Other* experimentalists have demonstrated by analogous means 

 similar facts in the large arterial trunks, for instance, the carotid ; 

 but the dilatation becomes less and less considerable in proportion 

 as the artery is more distant from the heart and of a narrower 

 calibre. In the small arterial branches, as Magendie has re- 

 marked, the dilatation is no longer perceptible. 3 



If the arterial dilatation is unquestionable in arteries of con- 

 siderable calibre, and if it assuredly depends on the passage of the 

 sanguineous stream, we may ask oursejves whether the pulsation, 

 the arterial pulse has the same cause. Tho older physiologists, 

 Haller, Spallanzani, and so on, have pretended, and wrongly, 

 that the pulsation is simultaneous in the whole arterial tree, and 

 consequently could not be attributed to the movement of trans- 

 lation of the sanguineous fluid, but rather to the shock, to the 

 transmission from point to point in the liquid column, of the 

 rapid agitation of the vibratory wave determined by the ventri- 

 cular systole. We now know that the isochronism of the arterial 

 beatings does not exist; but it is nevertheless probable that 

 the vibratory transmission contributes its part to the production 

 of the pulse. 



1 Yulpian, loc. cit., p. 324. 



2 Spallanzani, Experiences sur la Circulation. 



3 Magendie, Journal de Physiologie, t. I., p. 113. 



P 



