110 ON THE PRINCIPLE OF LIFE, 



in accounting for the circulation of the blood ; but the moment we reflect 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 gravita- 

 tion, instead of being able to avail itself of its assistance, is produced with- 

 out any muscular propulsion that we are able to discover, and that even the 

 arteries do not, when uninfluenced by pressure, appear to change their diame- 

 ter 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 dis- 

 tinct and independent, and which the laws of physical force are altogether 

 incompetent to explain. Dr. Young, in his excellent Croonian lecture, read 

 before the Royal Society in 1809,f has endeavoured to revive the mechanical 

 theory ; but he is still compelled to admit a variety of phenomena in the ani- 

 mal machine, and especially in the circulatory system, which are altogether 

 unaccountable upon any of the known principles of common hydraulics, and 

 which can never fail to reduce us 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 the 

 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 produced in 

 the heart by that very contraction or systole which has just propelled the 

 blood returned from the lungs into the arterial system. Whence the heart 

 iteelf becomes alternately a forcing and a suction pump ; being the former 

 in respect to the arteries, and the latter in respect to the veins.;}: 



Upon a moderate estimate, the common labourer may be said to employ a 

 force capable of raising a weight of ten pounds to the height of ten feet in 

 a second, and continued for ten hours a day. A moderate horizontal weight 

 for a strong porter, walking at the rate of three miles an hour, is 200 pounds : 

 the chairman walks four miles an hour, and carries 150 pounds. The daily 

 work of a horse is equal to that of five or six men upon a plane ; but from 

 his horizontal figure in drawing up a steep ascent, it does not exceed the 

 power of three or four men. In working windmills, twenty-five square feet 

 of the sails is equivalent to the work of a single labourer; whence a full-sized 

 mill, provided it could be made to work eight hours a day, would be equiva- 

 lent to the daily labour of thirty-four men. A steam engine of the best con- 

 struction, with a thirty inch cylinder, has the force of forty horses ; and as 

 it acts without intermission, will perform the work of 120 horses, or of 

 600 men ; every square inch of the piston being equivalent to the power of a 

 labourer. 



There are many muscles given to us which the common customs and habits 

 of life seldom render it necessary to exert, and which in consequence grow 

 stiff and immoveable. Tumblers and buffoons are well aware of this fact ; 

 and it is principally by a cultivation of these neglected muscles that they are 

 able to assume those outrageous postures and grimaces, and exhibit those 

 feats of agility, which so often amuse or surprise us. 



The same muscles of different persons, however, though of the same length 

 and thickness, and, so far as we are able to trace, composed of the same 

 number of fibres, are by no means uniformly possessed of the same degree 

 of power ; and we here meet with an express deviation from the law of 

 physical mechanics ; as we do also in the curious fact, that whatever be the 

 power they possess, they grow stronger in proportion to their being used, 

 provided they are well used, and not exhausted by violence or over-exertion. 



I have calculated the average weight carried by a stout porter in this me- 

 tropolis at 200 pounds ; but we are told there are porters in Turkey, who by 

 accustoming themselves to this kind of burden from an early period, are 

 able to carry from 700 to 900 pounds, though they walk at a slower rate, 

 and only carry the burden a short distance. " The weakest man can lift with 

 his hands about 125 pounds, a strong man 400. Topham, a carpenter, men- 



* See Lect vm p. 91, as also the Author's Study of Medicine, vol. ii. p. 16. Edit. 2d, 1825. 

 t On the Functions of the Heart and Arteries, Phil. Trans. 1809, p. 1. 

 t See Study of Med. vol. ii. p. 19. Ed. 2d. 



