of much wonder and doubt, and as might have been 

 expected, has elicited much wild and visionary specula- 

 tion. The ancients thought there was an innate fire in 

 the heart, which produced its motion, and a chemical 

 sect attributed it to an effervescence in the heart, caused 

 by the mixture of different kinds of blood. You smile 

 at such absurdities, but they can teach you an all-impor- 

 tant truth, which cannot be too strictly adhered to never 

 to reason on scientific subjects, without having first care- 

 fully considered all the facts connected with the case in 

 question, and then deducing cautiously the general prin- 

 ciples that arise from them. This is the basis of all true- 

 philosophy. The most that we know of the motions of 

 the heart is, that they are under the influence of the 

 nervous system in a very great degree, and particularly 

 of that portion of it which is connected with the spinal 

 marrow. 



Emily. What ground is there for even this fact ? 



Dr. B. In the first place, it is inferred from the fact, 

 that animals have been born without a brain, in which 

 the heart and the rest of the body were perfectly well 

 formed, a proof that the heart derives its nervous power 

 from the spinal marrow, and not the brain. Secondly, 

 it is inferred from the experiments of Le Gallois, who 

 ascertained that though a creature's head were cut off, yet 

 if the bleeding vessels were tied up, and artificial respi- 

 ration maintained by blowing air into the wind-pipe from 

 a pair of bellows, the heart would nevertheless continue 

 to act for a considerable time after the decapitation of 

 the animal. This is about the sum and substance of all 

 we know concerning the source of the heart's power. 



Emily. As the blood is incessantly revolving in a 

 circle, it must be continually returning at certain periods 

 to the place whence it started. Now, I am desirous to 

 know how long a time is required for a given quantity of 

 blood to complete the revolution. 



Dr. B. If you suppose that the arterial ventricle con- 

 tracts seventy-five times in a minute, expelling two ounces, 

 8 



