OP ANIMAL BODIES. 93 



proceed, in other words to grow from the heart, like a plant 

 from its root, or the fibres of a leaf from its foot-stalk, (which, 

 however, were it so, would be only to resolve one mechanism 

 into another,) yet the venal, the returning system, can never 

 be formed in this manner. The arteries might go on shoot- 

 ing out from their extremities, i. e. lengthening and sub- 

 dividing indefinitely ; but an inverted system, continually 

 uniting its streams, instead of dividing, and thus carrying 

 back what the other system carried out, could not be refer- 

 red to the same process. 



II. The next thing to be considered is the engine which 

 works this machinery, viz. the heart. (PI. XVII. fig. 1.) 

 For our purpose it is unnecessary to ascertain the principle 

 upon which the heart acts. Whether it be irritation excited 

 by the contact of tlie blood, by the influx of the nervous 

 fluid, or whatever else be the cause of its motion, it is some- 

 thing, which is capable of producing, in a living muscular 

 fibre, reciprocal contraction and relaxation. This is the 

 power we have to work with ; and the inquiry is, how this 

 power is applied in the instance before us. There is pro- 

 vided in the central part of the body, a hollow muscle, in- 

 vested with spiral fibres, running in both directions, the 

 layers intersecting one another ; in some animals, however, 

 appearing to be semi-circular rather than spiral. By the 

 contraction of these fibres, the sides of the muscular cavities 

 are necessarily squeezed together, so as to force out from 

 them any fluid which they may at that time contain ; by 

 the relaxation of the same fibres, the cavities are in their 

 turn dilated ; and, of course, prepared to admit every fluid 

 which may be poured into them. Into these cavities are 

 inserted the great trunks, both of the arteries which carry 

 out the blood, and of the veins which bring it back. This 

 is a general account of the apparatus ; and the simplest idea 

 of its action is, that, by each contraction, a portion of blood 

 is forced as by a syringe into the arteries ; and, at each 

 dilation, an equal portion is received from the veins. This 

 produces, at each pulse, a motion and change in the mass 

 of blood, to the amount of what the cavity contains, which, 

 in a full grown human heart, I understand, is about an 

 ounce, or two table-spoons full. How quickly these changes 

 succeed one another, and by this succession, how sufficient 

 they are to support a stream or circulation throughout the 

 system, may be understood by the following computation, 

 abridged from Keill's Anatomy, p. 117. ed. 3. *' Each ven- 

 I 2 



