dan clemens had taken at the first very little interest in the peculiar studies and experiments of his teacher; indeed, they were to him, a kindly-affectionate boy, rather revolting; but after the successful operation upon rover, his feelings underwent a complete change; he was enraptured with the skill, firmness, and tender feeling manifested by rich, spent a great deal of time at the dissecting table, and manifested a strong desire to obtain, at least, some general knowledge in respect to the mechanism of his own frame.

one evening he was seated in the harness-room, watching rich, who was examining the stump of rover's leg, that had become sore from the pressure of the wooden substitute, and devising some way to remedy it, when he suddenly exclaimed,—

"mr. richardson, how do they cut off a man's leg?"

"very much as i did that dog's; only they use a tourniquet to compress the vessels and stop the[pg 206] circulation, then cut through the flesh, saw off the bones, and put ligatures on the ends of the arteries."

"what is it makes the great difference between the arteries and the veins, so that folks say, if you cut an artery, you'll bleed to death in no time. but they never speak so about veins; it's always arteries."

"i can't explain it to you, without telling you something about the heart, to start with."

"well, tell me. o, do tell me, please."

"you saw the hog's heart i had the other day. do you remember how it looked?"

"it looked something like an egg little end up."

"well, a hog's heart is very much like a man's, so that one will do to represent the other. you noticed that it was smooth, and stood out about its whole bigness clear from everything, except at the base, where it joined the body?"

"yes, sir."

"on each side of the base are two appendages, wrinkled, and shaped like an ear, denoting cavities within called from them the auricles, and into these cavities run several tubes that connect them to the parts adjacent. they are called auricles because they look so much like an ear."

"i know what they are. i saw the butcher cut them off, when he trimmed our hog's harslet: he called them deaf ears, and said they were poison."

"the heart is a hollow muscle, that contracts[pg 207] and dilates with great force. it is not dependent upon the will, but operates in virtue of a natural law. through the middle of the heart, from the base to the summit, runs a partition, leaving a chamber on each side, between which there is no direct communication: they are distinguished by the terms right and left auricles. in addition to this, there is a cross parting on each side, thus making four chambers, the two upper retaining the name of auricles, the two lower denominated ventricles.

"i will now explain to you the use of all this. the right auricle opens into the large trunk vein of the body, that, in connection with the others, brings back the blood from the extremities, after the arteries have distributed it. it has also another opening into the right ventricle below it. the auricle on the other side of the partition (the left) is pierced by four veins that enter the lungs, called pulmonary veins, and also by another passage communicates with the ventricle beneath it. now let us talk about ventricles. the right ventricle is entered by the great pulmonary artery that carries all the blood in the body through the lungs. the left ventricle is penetrated by the great artery, called the great aorta. in each of these cross partitions, there are valves that will permit blood to pass from the auricles into the ventricles, but not to return. there are also valves at the roots of the arteries that permit the[pg 208] blood to go from the heart into the arteries, but not to return. there are no valves at the roots of the veins that enter the auricles, nothing to obstruct the flowing of the blood from them into the auricles. thus the roots of the veins arise from the auricles, and the roots of the arteries from the ventricles. do you understand this description, because it is the foundation of all that follows—understand what a valve is?"

"yes, sir; the clapper in our pump-box is a valve; it lets the water come up out of the well into the pump, but it won't let a drop go back."

"well said; just so the valves in the partings of the heart permit the blood to pass from the auricles into the ventricles, but not to go back; thus, also, the valves placed at the roots of the arteries permit the passage of the blood from the ventricles into the arteries, but not the return of it to the heart. do you understand this?"

"yes, sir."

to make it more evident, rich drew the heart, the veins, and the arteries entering it, with chalk, and the main branches of both.

"now let us, for the clearer perception of what you wish to know, consider the march of the blood: and we might as well begin at the heart as anywhere."

"i think i can understand it better to commence there."

"from the right ventricle of the heart, springs[pg 209] the pulmonary artery, which, separating into several branches, some of them not larger than hairs, carries the blood into all portions of the lungs, where they communicate with the terminations of the pulmonary veins, which, receiving the blood from the arteries, bring it back to the left auricle, uniting, as they approach the heart, into four large veins, called the pulmonary veins. from the left ventricle rises the main artery (or great aorta), which, receiving all the blood of the body poured into it by the pulmonary veins, distributes it over the trunk and limbs, branching in every direction, the divisions gradually becoming smaller and smaller as they approach the extremities: here they communicate with the extremities of the veins which bring back the blood to the right auricle. so much for the aqueducts; now we will look at the action of the force-pump itself. the heart is a hollow muscle. all the valves and division walls we have been talking about are muscular in their texture, and moved by a network of muscles and minute tendons, tough and elastic, like the gizzard of a fowl, and capable of contraction and expansion. we will suppose the right auricle to be full of blood that has been brought by the veins from the fingers, toes, the substance of the heart itself, the lungs, and the liver, and poured into it. this blood is dark-colored; called black blood. it has washed the whole body. the instant it enters the auricle, that organ contracts and forces[pg 210] it into the ventricle below it; the valve holds it there: then the ventricle contracts and forces it into the pulmonary artery; the valve of the artery holds it there: the auricle expands, fills, again contracts, fills the ventricle, that, in its turn, forces the blood into the artery, and thus, by successive leaps, it passes into and through the lungs, enters the pulmonary veins, and is by them brought back to the left auricle. it is now no longer black blood, but bright, red, arterial blood: before it was venous."

"what makes it red?"

"i don't know. it is supposed by being brought in contact with the air in the cells of the lungs. when the auricle receives this red blood, it contracts, forces it into the left ventricle beneath, then the ventricle in its turn contracts, forces it into the main artery, and by this and its branches it is carried to the extremities, to come back in one continual round, as long as life lasts. it is life; for the moment the heart ceases to contract and dilate, insensibility takes place, and death instantly follows."

"it seems to me that the left side of the heart has a great deal more work to do than the right, for the left has to force the blood into the main artery, and all over the body, to the toes, the fingers, the brain: but the right ventricle only has to force it through the lungs that are close by, touch the heart, and it is a short route."

[pg 211]

"true, and for this reason, the muscles of the left ventricle, which force the red blood of the great circulation through the main artery, are much more numerous and stronger than those of the right, which has so much less work to perform. it is the powerful contraction of the muscles of the left ventricle, causing the point of the heart to strike the fifth or sixth rib, that creates the throb you can feel; they exert power enough to send all the blood of the body through the heart twenty-three times in an hour."

"i had no idea matters were going ahead inside of me at that rate."

"you must bear in mind that i have described these things separately, but in the order of nature, it is quite another matter. the red blood from the lungs arrives at the left, and the black blood from the veins at the right auricle at the same instant; both auricles contract at once, and force the blood into their respective ventricles; both ventricles contract together and force the blood into the arteries; and thus it goes on in a person of the feeblest pulse; these alternate motions occupy, when in a state of health, but a second; the pulse at your wrist is the throb of the artery, the stroke of the heart. what do you suppose now is the force of that stroke, when the left ventricle contracts?"

"i'm sure i don't know."

"well, the blood has been known to spurt more[pg 212] than five feet from the artery of the neck (carotid) when first cut. you see, now, why it is so dangerous to wound a large artery: the blood spurts at every stroke of the heart, while in the veins there is no such pressure or direct connection; besides, as the veins are designed not to carry the blood from the heart, but to bring it back, they are also furnished with numerous valves that favor the flow of blood towards the heart, but not from it."

"there is one thing i can't understand. when a man's leg is cut off, all the arteries and veins cut, how does the blood get back to the heart when the ends of the arteries are tied, and there is no communication between them and the veins?"

"by a provision of nature, there are many minute twigs and branches given off by the arteries all along their course, scarcely observable when the circulation is in its normal state, that are connected with veins equally small; those become enlarged by pressure, and renew the connection."

"it seems to me, mr. richardson, that the heart is like two pumps in two wells, side by side, only one throws a bigger stream than the other, and with more force."

"ay, daniel; but your mother's pump bears no comparison to the heart. during the time i have been with her, the spear has worn off, the boxes have been new leathered, and the cracks in the pump that sucked air have been covered with putty[pg 213] and lead; but this pump runs eighty, and sometimes a hundred years without the pause of a second."

"why don't the muscles of the heart get tired, just as my legs do, and want to rest?"

"they do rest, and just as long as they work; rest a second, and work a second, day and night. the other muscles are in a state of tension all day, and then rest at night."

"well, i mean to know how i am made up, before i am much older."