72 LAY SERMONS, ESSAYS, AND REVIEWS. [v. 



In this case, simple observation yields us a knowledge of the 

 existence of the blood from some accidental haemorrhage, we will 

 say ; we may even grant that it informs us of the localization of 

 this blood in particular vessels, the heart, &c., from some acci 

 dental cut or the like. It teaches also the existence of a pulse 

 in various parts of the body, and acquaints us with the structure 

 of the heart and vessels. 



Here, however, simple observation stops, and we must have 

 recourse to experiment. 



You tie a vein, and you find that the blood accumulates on 

 the side of the ligature opposite the heart. You tie an artery, 

 and you find that the blood accumulates on the side near the 

 heart. Open the chest, and you see the heart contracting with 

 great force. Make openings into its principal cavities, and you 

 will find that all the blood flows out, and no more pressure is 

 exerted on either side of the arterial or venous ligature. 



Now all these facts, taken together, constitute the evidence 

 that the blood is propelled by the heart through the arteries, and 

 returns by the veins that, in short, the blood circulates. 



Suppose our experiments and observations have been made on 

 horses, then we group and ticket them into a general proposition, 

 thus : all horses have a circulation of their blood. 



Henceforward a horse is a sort of indication or label, telling us 

 where we shall find a peculiar series of phenomena called the 

 circulation of the blood. 



Here is our general proposition, then. 



How, and when, are we justified in making our next step a 

 deduction from it ? 



Suppose our physiologist, whose experience is limited to horses, 

 meets with a zebra for the first time, will he suppose that this 

 generalization holds good for zebras also ? 



That depends very much on his turn of mind. But we will 

 suppose him to be a bold man. He will say, &quot; The zebra is 

 certainly not a horse, but it is very like one, so like, that it 

 must be the ticket or mark of a blood-circulation also ; and, 

 I conclude that the zebra has a circulation.&quot; 



