THE CIRCULATION OF THE BLOOD. 249 



abstraction must be taken to indicate a condition of exhaustion of the 

 centre, and consequently of want of regulation of the peripheral resist- 

 ance. In the same way it might be thought that injection of blood into 

 the already full vessels would be at once followed by rise in the blood- 

 pressure, and this is indeed the case up to a certain point the pressure 

 does rise, but there is a limit to the rise. Until the amount of blood 

 injected equals about 2 to 3 per cent of the body-weight, the pressure 

 continues to rise gradually; but if the amount exceed this proportion, 

 the rise does not continue. In this case, therefore, as in the opposite 

 when blood is abstracted, the vaso-motor apparatus must counter- 

 act the great increase of pressure, but now by dilating the small ves- 

 sels, and so diminishing the peripheral resistance, for after each rise 

 there is a partial fall of pressure; and after the limit is reached the 

 whole of the injected blood displaces, as it were, an equal quantity which 

 passes into the small veins, and remains within them. It should be re- 

 membered that the veins are capable of holding the whole of the blood 

 of the body. 



Further, as we have seen, the amount of blood supplied to the heart, 

 both to its substance and to its chambers, has a marked effect upon the 

 blood-pressure. 



b. As regards quality. The quality of the blood supplied to the 

 heart has a distinct effect upon its contraction, as too watery or too 

 little oxygenated blood must interfere with its action. Thus it appears 

 that blood containing certain substances affects the peripheral resistance 

 by acting upon the muscular fibres of the arterioles, and so directly alter- 

 ing the calibre of the vessels. 



Proofs of the Circulation of the Blood. 



It seems hardly necessary at the present time to bring forward the 

 proofs that during life the blood circulates within the body; they are 

 so well known. It is interesting, however, to recount the main argu- 

 ments by which Harvey in the first instance established the fact of the 

 circulation; they were as follows: 



1. That the heart in half an hour propels more blood than the whole 

 mass of blood in the body; 



2. That the blood spurts with great force and in a jerky manner 

 from an opened artery, such as the carotid, with every beat of the 

 heart; 



3. That if true, the normal course of the circulation would explain 

 why after death the arteries are commonly found empty and the veins 

 full; 



4. That if the large veins near the heart be tied in a fish or snake, 

 the heart becomes pale, flaccid, and bloodless; and that on moving the 

 ligature, the blood again flows into the heart. If the artery is tied, the 



