CIRCULATION OF THE BLOOD. 113 



tunics. It is obvious, that whether an artery be tied over a 

 hollow tube, or its tunics be converted into a bony and un- 

 yielding canal, the interruption to the pulsific power in the 

 inferior part of the vessel must be the same. 



I have known another instance in which a portion of the 

 aorta near the heart was found converted into bone, in the body 

 of a nobleman, a man of great muscular strength. The experi- 

 ment of Galen, therefore, or, at all events, a state analogous 

 to it, not effected on purpose but encountered by accident, 

 makes it sufficiently to appear, that compression or ligature of 

 the coats of an artery does not interfere with the pulsative 

 properties of its derivative branches ; and indeed, if the experi- 

 ment which Galen recommends were properly performed by any 

 one, its results would be found in opposition to the views which 

 Vesalius believed they would support. 



But we do not therefore deny everything like motion to the 

 tunics of the arteries ; on the contrary, we allow them the same 

 motions which we concede to the heart, viz., a diastole, and a 

 systole or return from the distended to the natural state ; this 

 much we believe to be effected by a power inherent in the coats 

 themselves. But it is to be observed, that they are not both 

 dilated and contracted by the same, but by different causes and 

 means; as may be observed of the motions of all parts, and of 

 the ventricle of the heart itself, which is distended by the 

 auricle, contracted by its own inherent power ; so, the arteries 

 are dilated by the stroke of the heart, but they contract or 

 collapse of themselves. 1 



You may also perform another experiment at the same time : 

 if you fill one of two basins of the same size with blood issuing 

 per saltum from an artery, the other with venous blood from a 

 vein of the same animal, you will have an opportunity of per- 

 ceiving by the eye, both immediately and by and by, when the 

 blood in either vessel has become cold, what differences there 

 are between them. You will find that it is not as they believe 

 who fancy that there is one kind of blood in the arteries and 

 another in the veins, that in the arteries being of a more florid 

 colour, more frothy, and imbued with an abundance of I know 

 not what spirits, effervescing and swelling, and occupying a 



1 Vide Chapter III, of the Disquisition on the Motion of the Heart and Blood. 



8 



