32 HARVEY. tion, Harvey conferred upon anatomy and medi- cine by his true doctrine of the circulation of the blood. A short statement of what is meant by the cir- culation of the blood, will enable us fully to ap- preciate the value and importance of this great discovery. And this may the more easily be done,, as the apparatus by which it is carried on, is, at this time of day, probably the best understood of any part of the animal economy. Of the utility of the circulation, every one will be immediately aware, when it is mentioned, that one of its chief purposes is to distribute to every part, every extremity, nook, and corner of the body^ the nourishment which is received into it, by one aperture : — What enters at the mouth, by means of this function, finds its way to the fingers' ends. To effect this difl[icult purpose, two things are necessary. 1st. A proper disposition of the blood- vessels, which has been not unaptly compared to the laying of the water-pipes in a populous city. 2d. The construction of the engine at the centre, viz., the heart, for driving the blood through them. In the case of the conveyance of water, one sys- tem of pipes is sufficient ; but in the living body another system of vessels is required, to reconvey the blood back to its source. The body, there- fore, contains two systems of blood-vessels, called arteries and veins. The next thing to be consi- dered, is the engine which works this machinery : for this purpose there is provided in the central part of the body a hollow muscle, viz., the heart, by the contraction of whose fibres the four cavi- ties of which it consists are squeezed together, so
