vni BLOOD-STBEAM : MOVEMENT IN VESSELS 277 



it produces a transverse dilatation and elongation, which are in 

 relation with the pulsatile oscillations of pressure. The lengthening 

 of the arteries produces a movement of the vessels at each wave 

 of blood that traverses them, as is clearly visible in arteries 

 ligatured after amputation. Normally, however, since the arteries 

 are not free to elongate in the longitudinal direction, they become 

 laterally curved when rectilinear, and increase their curvature if 

 (as in old people) they have a winding course ; and when (as in 

 the aortic and pulmonary area) they form a free arch with a short 

 radius of curvature, the curvature tends at each systolic wave to 

 change, and to assume a longer radius, by a mechanism similar to 

 that of the metallic manometer of Bourdon. 



The elongation of the arteries, since it is plainly visible, was 

 known to the older surgeons; the transverse dilatation on the 

 other hand is a less obvious phenomenon, so that in the eighteenth 

 century some of the clinicians (De la Mure in particular) denied 

 it altogether, and held that the arterial pulse perceptible to touch 

 is the effect of simple vascular locomotion. Spallanzani (1773) 

 was the first to demonstrate the pulsatile dilatation of the aorta 

 in the salamander by an ingenious experiment. " The aorta " (he 

 wrote) " pulsates in its entire length, and in pulsating it dilates, 

 but not equally in all its parts. Where it arches, its diameter is 

 increased by a third, but elsewhere it increases only about a 

 twentieth. Although my eye informed me that in the pulsations 

 the increase in diameter or bulging of the aorta occurred more or 

 less at each point of the circumference, I employed the following 

 method to illustrate it: I passed the aorta 'through a small open 

 metal ring which, when closed, was of slightly larger diameter than 

 the aorta. When the aorta dilated in its pulsations, the empty 

 space between it and the ring became smaller ; when it was con- 

 stricted the space became larger. I then diminished the capacity 

 of the ring. Now, where the aorta bulged, that is where there 

 was the greatest dilatation, the circular space was lost during the 

 cardiac systole, being filled in every direction by the dilated 

 vessel : this proved decisively that the aorta in pulsating dilated at 

 all points of its circumference " (Dei fenomeni della circolazione, 

 Dissertazione terza}. 



In proof of the same phenomenon, Poiseuille (1828) introduced 

 a length of a large artery into a long chamber, having at both 

 ends a circular hole of the same diameter as the artery it was 

 to receive. The cover of the chamber (which could be closed so 

 as to become water-tight) was pierced by a vertical glass tube 

 provided with a millimetre scale. After the chamber containing 

 the artery had been filled with fluid, the fluid could be seen to rise 

 in the tube at each systolic wave, and to fall at each diastole. 

 Poiseuille measured the increase in arterial diameter by the highest 

 point of the rise. 



