in] of the New Physics. 79 



" Two effects follow the beat of the heart, the filling of the 

 " arteries with the blood driven into them, and the exit of the 

 "same blood from the same arteries. Certainly these two 

 " events cannot take place at the same time ; for the one 

 "consists in an expansion, the other in a constriction of the 

 " same arteries, and these two being opposed in nature cannot 

 "take place at the same time. Whence it must be that the 

 "filling of the arteries takes place first, and that their con- 

 striction and emptying follows afterwards. 



" The filling and distension however which take place first 

 " cannot be carried out without a violent extension of the 

 " transverse fibres of the said arteries. Now we know from other 

 " sources that all fibres of vessels, and all fibres of muscles, of the 

 " intestines, of tendons, of membrane, and of the true skin resist 

 " extension, and when extended possess a power of contracting 

 " like that of a strung bow. Nay, indeed, we see that all fibres 

 "when placed in their natural surroundings possess some 

 "amount of active tension, for when they are divided they 

 "contract of their own accord and become shorter. This would 

 "not happen if these fibres existed in a condition of equili- 

 brium between extension and contraction; like the cord of an 

 "unstrung bow they would suffer neither contraction nor 

 " extension. 



" But if all fibres in a natural condition undergo some 

 "amount of extension it follows that, when the arteries are 

 " filled with blood, the transverse fibres, owing to the enlarge- 

 " ment of the cavity, must become much more elongated and in 

 "consequence undergo a much greater extension. And since 

 "the said expansion of the arteries is succeeded by a con- 

 striction which cannot take place without a shortening of the 

 " circular fibres, and indeed such a shortening is proper to and 

 "part of the very nature of these fibres, it follows that the 

 " arteries after their violent expansion, due to their being filled 

 "to distension, cannot do other than exercise by the law of 

 "nature that mechanical force which they possess. This 

 "squeezing the arteries like a rope twisted circularly round 

 "them expels with force the blood through their terminal 

 " orifices." 



