IRRITABIWTY, AND MUSCULAR POWER, 



the fallacy of their reasoning. Among those who have adopted this mode 

 of explanation, and have perused it with most acuteness, and may be re- 

 garded as the fathers of the school, I may be allowed to mention Borelli 

 and Keil ; but while the former, in order to account for the circulation of 

 the blood in man, calculated the force with which the heart contracts to 

 be equal to not less than a hundred and eighty thousand pounds weight at 

 every contraction, the latter could not estimate it at more than eight ounces. 



In like manner BoreUi, in applying the same theory to the power with 

 which the human stomach triturates, or, as we now call it, digests its food, 

 calculated it, in conjunction .with the assistance it receives from the aux- 

 ihary muscles, which he conceived to divide the labour about equally with 

 itself, as equal to two hundred sixty-one thousand one hundred and eighty- 

 six pounds ; and Pitcairn has made it very little less, since he estimates the 

 moiety contributed by the stomach alone at one hundred and seventeen 

 thousand and eighty-eight pounds ; which gives to these organs jointly a 

 force m©re than equal to that of twenty mill-stones ! " Had he," said Dr. 

 Monro, " assigned five ounces as the weight of the stomach, he had been 

 nearer the truth."* 



The fallacy of this theory, however, and especially as it applies to the 

 stomach, has been completely exposed in our own day, by the well ascer- 

 tained fact, that though the muscular coat of the stomach in most animals 

 bears some part in the process of digestion, this important operation is 

 almost entirely performed by a powerful chemical solvent secreted by tl^ 

 stomach itself for this very purpose, and hence denominated the gastric 

 juice ; and which answers all the purposes of the most violent muscular 

 pressure we can conceive, and with a curious simplicity of contrivance. 



The laws of physical force will '^ertainly better apply to the action of the 

 heart and arteries than to that of the stomach, and in some measure assist 

 us in accounting for the circulation of the blood ; but the moment we re- 

 flect that one half of this very circulation, that I mean which depends upon 

 the veins, and which has for the most part to contend against the attraction 

 of gravitation, instead of being able to avail itself of its assistance, is pro- 

 duced without any muscular propulsion that we are able to discover, and 

 that even the arteries do not, when uninfluenced by pressure, appear to 

 change their diameter in a state of health,! we are necessarily driven to the. 

 conclusion, that there is in animal statics, as well as in animal mechanics,, 

 a something distinct and independent, and which the laws of physical force 

 are altogether incompetent to explain. Dr. Young, in his excellent Croo- 

 nian lecture, read before the Royal Society in 1809,| has endeavoured to 

 revive the mechanical theory ; but he is still compelled to admit a variety 

 of phaenojmena in the animal machine, and especially in the circulatory 

 system, which are altogether unaccountable upon any of the known ffm- 

 ciples of common hydraulics, and which can never fail to reduce u^ to the 

 same result. 



So far, therefore, as we at present know, the circulation of the blood is 

 performed by a double projectile power ; one moiety being dependent on 

 tho action of the living principle in the heart, and perhaps the arteries ; and 

 the other moiety on the common law of hydraulics, or the vacuum pro- 

 duced in the heart by that very contraction or systole which has just pco- 



* Comp. Anat. pref. p. viii. note. 



t See Lect. VIII. as also the author's Study of Medicii^, vol. ii. p. 16. Edit, seeond, 

 1825. J i I 



t On the Functions of the Heart and Arteries, Phil. Trans. 1809, p. L 



