ON THE STUDY OF PHYSIOLOGY. 61 



movements, susceptible of rigid calculation, necessarily 

 succeeding each other in the organs from the time that life 

 commences. These movements he referred to an impul- 

 sive power in the heart, renevi^ed by the influence of the 

 nervous fluid brought from the brain. In this explanation 

 the body is an hydraulic machine, in which the heart per- 

 forms the office of a piston : the beautiful construction and 

 endless variety of the animal organization are reduced to an 

 assemblage of pipes, canals, levers, pulleys, and other me- 

 chanism. The treatises on physiology, of this period, were 

 filled with mathematical problems, long calculations, and 

 algebraic formulae. 



This system maintained its ground for a long time, in 

 defiance of observation and common sense. In palliation 

 of what strikes us now as so extravagantly erroneous, it 

 must be observed, that many things in the animal economy 

 admit of explanation on these principles. The structure 

 and motions of the joints are purely mechanical ; and the 

 degree of eff'ect produced by the muscles of a limb, like 

 the acting force of a moving power applied to a common 

 lever, depends entirely on the relative situation of their 

 tendinous insertions to the centre of motion, and the rela- 

 tion which the course of their fibres bears to the axis of 

 the moving bone. All these things may be as exactly de- 

 termined by calculation as the operation of common levers : 

 but the contraction of the living fibre, or original moving 

 force, cannot be subnrltted to calculation — cannot be in the 

 slightest degree elucidated by mechanics. 



The valves of the heart and blood-vessels act mechani- 

 cally, and operate as well in the dead, as in the living body. 

 The swelling of the veins of the lower limbs in the erect pos- 

 ture and the turgescence of the same vessels in the head and 

 neck, when they are held in a dependent attitude, will con- 

 vince us, that, although the blood flows through living canals, 

 its motion is not withdrawn from the all-pervading influ- 

 ence of gravity. 



The transparent parts of the eye act on the rays of light 

 according to the common laws of optics ; and bring them to 



