VESSELS OF ANIMALS. 107 



the body, a holloAV muscle, invested with spiral fibres run- 

 ning in both directions, the layers intersecting one another ; 

 in some animals, however, appearing to be semicircular 

 rather than spiral. By the contraction of these fibres, the 

 sides of the muscular cavities are necessarily squeezed to- 

 gether, 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 by a syringe 

 mto the arteries, and at each dilatation an equal portion is 

 received from the veins. This produces at each pulse a mo- 

 tion, 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-spoonfuls. 

 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 ventricle will at least contain one ounce 

 of blood. The heart contracts four thousand times in one 

 hour ; from which it follows, that there pass through the 

 heart, every hour, four thousand ounces, or three hundred 

 and fifty pounds of blood. Now the whole mass of blood is 

 said to be about twenty-five pounds ; so that a quantity of 

 blood equal to the whole mass of blood passes through the 

 heart fourteen times in one hour, which is about once in 

 every four minutes." 



Consider what an afiair this is, when we come to very 

 large aniiials. The aorta of a whale is larger in the bore 

 than the main pipe of the water- works at London bridge ; 

 and the water roaring in its passage through that pipe is 



