This section is from the book "The Human Body: An Elementary Text-Book Of Anatomy, Physiology, And Hygiene", by H. Newell Martin. Also available from Amazon: The Human Body.
1. A frog may be used to illustrate the beat of the heart. Anatomically a frog's heart differs in many respects from that of a mammal, but the phenomena of systole and diastole are essentially the same.
3. Laying the animal on its back, carefully divide with scissors the skin along the middle line of the ventral surface for its whole length. Make cross cuts at each end of this longitudinal one and pin out the flaps of skin.
4. Next pick up with forceps the remaining tissues of the ventral wall near its posterior end, and carefully divide them longitudinally a little on the left side of the middle line; being very careful not to injure either the viscera in the cavity beneath or a large vein (anterior abdominal) running along the wall in the middle line.
5. About the point where you see this vein passing from the wall to enter among the viscera of the ventral cavity, youwill come to the bony and cartilaginous tissues of the sternal region. Raise the posterior cartilage in your forceps, make a short transverse cut in front of the vein, and, looking beneath the sternum, note the pericardium with the heart beating inside it. Divide the fibrous bands which pass from the pericardium to the sternum, and with scissors cut away sternum, etc., taking great care not to injure the heart.
6. Push a rod about half an inch in diameter down the animal's throat so as to stretch the parts, and then picking up the pericardium in a pair of forceps, open it and gently cut it away from about the heart; push aside any lobes of the liver which lie on the latter organ. In the heart thus exposed noteó a. Its beat; a regularly alternating contraction (systole) and dilatation (diastole).
b. In consequence of the destruction of the spinal cord comparatively little blood now flows through the heart, but during the contraction you will be able to observe that the ventricular portion, which will be readily recognized, becomes paler; and during diastole again becomes deeply colored, getting more or less filled up with blood which shows through its walls.
c. Observe that each contraction starts at the auricular end and travels towards the ventricular; this may be more easily seen by-and-by, when the heart begins to beat more slowly.
7. The specimen may be put aside under a bell-jar with a wet sponge, or a piece of flannel soaked in water. If kept from drying the heart will go on beating for hours.
8. To demonstrate the action of the valves of the heart, obtain two uninjured sheep's hearts from a butcher. Remove them from the pericardium, taking care not to injure the vessels.
9. Cut off the apex of one heart so as to open the ventricles. Then fill up the stumps of the aorta and the pulmonary artery with water. As the water is poured in the semilunar valves will be seen to close up and block the passage to the ventricle, so that the stump of the vessel remains full for some time. The valves rarely act quite perfectly in a heart removed from the body and treated as above, but they will support the water column quite long enough to illustrate their action.
10. Carefully cut the auricles away from the other sheep's heart, taking great care not to injure the ventricles or the auriculo-ventric-ular valves. Then holding the ventricles, apex down, in one hand, pour water in a stream into them from a pitcher held about a foot above them. As the ventricles fill, the flaps of the mitral and tricuspid valves will be seen to float up and close the auriculo-ventricular orifice, illustrating their movement as the ventricle fills during its diastole in the natural working of the heart.
11. The manner in which the elasticity of the arteries and the friction resistance to flow in the capillaries together serve to turn a rhythmic into a steady flow may be readily demonstrated as follows:
Take an elastic bag such as is commonly sold with enema apparatus in drug stores, and having an entry and exit tube provided with valves In the exit tube place a piece of glass tubing six feet long. Put the entry tube of the bag in a basin of water. On pumping, an intermittent flow of water, corresponding to the strokes of the pump, will be obtained from the glass tube. Connect a very fine glass nozzle with the end of the long tube; on pumping, less water can be forced through, and the outflow is still rhythmic.
12. Replace the glass tube by a rubber tube of the black, highly elastic kind: on pumping we get again a rhythmic outflow. Now connect your narrow nozzle to the end of the rubber tube, and pump: the outflow will be nearly constant, because the rubber tube not being able to empty itself as fast as the water is pumped into it, becomes stretched, and in the Interval between two strokes of the pump It keeps on squeezing out the extra water accumulated in it. The longer and more elastic the tube, the quicker and stronger the stroke of the pump, and the narrower the exit, the more steady will be the outflow. In the body the heart keeps the arteries very tightly stretched all the time, and they keep up accordingly a steady flow into the capillaries. The experiment shows that to get such a steady flow two things are necessary : (1) that the tubes fed directly by the heart shall be highly elastic, and (2) that there shall be considerable resistance to the exit from their outflow ends.