706 PHILOSOPHICAL TRANSACTIONS. [aNNO 1702. 



only gives a prevalence to one of two powers before equilibrated, so here it 

 serves to enable those muscles to lift up a weight, too ponderous for their 

 strength not so assisted ; and therefore as soon as that assistance is withdrawn, 

 the costae are again depressed by the mere gravitation of the atmosphere, which 

 would otherwise remain elevated through the natural tendency of those muscles 

 to contraction. 



This is plainly proved from the Torricellian experiments, and those made on 

 animals in Mr. Boyle's engine ; where, as soon as the air is withdrawn, and 

 the pressure thereby taken off, the intercostal muscles and diaphragm are con- 

 tracted, and the ribs elevated in an instant, and caimot by any power of the 

 will be made to subside, till the air is again let in to bear them forcibly down. 



It were scarcely worth while to take notice here of a mistake of the learned 

 Dr. Willis,* were it not for the great authority of the man, which is almost 

 sufficient to keep error in countenance. The Doctor having observed that the 

 fibres of the external and internal intercostal muscles ran in a contrary order, 

 as it were, decussating each other, takes occasion from thence to fancy, that 

 there was an opposition in their office, and that, as the external served to raise 

 up the ribs, the internal drew them down again ; forgetting that, when a coi)- 

 tractile body is fastened at the several ends to points unequally moveable, let 

 the contraction happen in what part or manner soever, the more moveable point 

 must be drawn towards the less moveable ; by which rule, whether external or 

 internal intercostals be contracted, the lower ribs will be forced to approach the 

 upper, that is, be raised up. 



As in the elevation of the costae, the blood, by the passage that is opened 

 for it, is in a manner solicited into the lungs, so in the depression of them, by 

 the subsidence of the lungs and the contraction of the blood-vessels, both 

 which are consequent to it, the blood is forcibly driven, as it were, with an 

 embolus, through the pulmonary vein, into the left ventricle of the heart. And 

 this, together with the general compression of the body by the weight of the 

 atmosphere, which surrounds and presses on its whole surface, is that power 

 which causes the blood to mount in the veins, after the force impressed upon 

 it by the heart is broken and spent, and which is sufficient to force the heart 

 from its natural state to dilatation. 



He who can compute the weight of a column of air, equal to the surface of 

 the whole body, will readily grant it a power sufficient for the effects here 

 ascribed to it. And when he considers, that the bodies of animals are com- 

 pressible machines, he will find that it must of necessity affect them in the man- 



* De Respirationis Organ, et Usu.— Orig. 



