162 NATURAL THEOLOGY. 



ner. The arteries might go on shooting out from 

 their extremities — ?.e.,lengtliening and subdividing 

 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 referred to the same process. 



II. The next thing to be considered is the engine 

 which works this machinery — viz., the heart. For 

 our purpose it is unnecessary to ascertain the prin- 

 ciple upon which the heart acts. Whether it be 

 irritation excited by the contact of the blood, by the 

 influx of the nervous fluid, or whatever else be the 

 cause of its motion, it is something which is capa- 

 ble of producing, in a living muscular fibre, recip- 

 rocal contraction and relaxation. This is the power 

 w^e have to work w^ith ; and the inquiry is, how this 

 power is applied in the instance before us. There 

 is provided, in the central part of the body, a hollow 

 muscle, invested with spiral fibres, running in both 

 directions, the layers intersecting one another ; in 

 gome animals, however, appearing to be semicir- 

 cular 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 arc inserted the 

 great trunks, both of the arteries w^hich carry out 

 the blood, and of the veins wliich bring it back. 



