Work of the Heart. — Work done by an engine may be measured by the weight it can lift through a certain distance, or the distance through which it can lift a certain weight. Thus if one were to say a load of 100 pounds was lifted 1 foot high, a perfectly accurate idea of the work done would be gained, and it would amount to the same thing if a weight of 1 pound were lifted 100 feet high. Thus work done is measured by "foot-pounds," that is, the number of pounds weight lifted multi plied by the number of feet through which it was lifted gives the work done. The same method can be applied to measure the work done by the heart. With every contraction of the ventricle 6 ounces of blood are forced into the aorta. It has been found that if the blood were thrown out freely, it is sent out of the ventricle with such force that it would rise to a height of 6 feet. Thus with every stroke of the ventricle the work done is equal to raising 6 ounces 6 feet high, or, what is the same thing, 36 ounces 1 foot high. But 36 ounces are 2f pounds ; therefore, the force exerted by the ventricle at each beat is equal to 21 foot-pounds. Suppose the heart beats 70 times a minute, 2f multiplied by 70 are equal to a force of 157i foot-pounds exerted by the left ventricle in one minute. Multiply 157i by 60, the number of minutes in an hour, and then by 24, the number of hours in a day, and the result is 226,800 foot-pounds of work performed by the left ven tricle in a day. If we add to this the work done by the right ventricle, it would equal, by a rough estimate, 300,000 foot-pounds per day, that is, 300,000 pounds lifted 1 foot high ; that I, a sufficient force is developed by the heart in ne day to raise the body of a man, weighing 150 munds, 2000 feet in the air. That force seems normous. It is worth noting the conditions of is development. They are: proper nourishment, onveyed to the heart's substance by its own yatem of coronary vessels, and a regular alter tation of work and rest, for we have seen that luring the period of its round the heart works hree-fifths of the time and rests two-fifths.
Nervous Control of the Heart.—The regu ar rhythmic movement of the heart is main ained by nervous influence. If a frog be sud lenly killed by a blow on the head, and the lest be immediately opened, the heart will be 'ound still beating. If a long piece of straw, ixed at one 'end, be laid over the heart, the 'ree end will move up and down, showing and the movement. More than this, he frog's heart can be entirely removed from is body, and attached to tubes filled with tourishing fluid, from which the heart is re Aenished at intervals. By this means the heart ain be kept beating vigorously for a whole day >r more, and observations made. It has been ascertained by these and similar means that, it various places in the substance of the heart, there are nervous ganglia, that is, masses of nervous matter. It is from certain of these that there proceed, at regular intervals, dis larges of nerve energy, which excite the movement. All of the ganglia, however, do not seem to excite movement; the business of me of them seems to be to restrain. If the exciting ganglia had it all their own way, the heart would go on contracting at a speed that would be quickly exhausting, and if the re straining ganglion had it all its own way, the heart would stand still. The one influence,
however, modifies the other, and the result is a moderate and regular activity of the heart.
Now these nervous arrangements are within the substance of the heart itself ; but the organ is subject to influences from outside of itself. Two nerves are connected with the heart, the pneumogastric nerve (p. 152) and the sympa thetic (p. 152). If the pneumogastric be excited by electrical shocks or in other ways, the heart slows, and, if the excitement be strong enough, stops beating, in a condition of complete relaxa tion and fully distended ; if the sympathetic be stimulated, the heart quickens its movement, beats faster and faster, until, if the stimulus be strong enough, it stops, but this thee in a condition of complete and rigid contraction. It would, therefore, appear As if the sympathetic were connected with the exciting ganglia of the heart, and as if the pneumogastric were con nected with the restraining ganglion.
Now let us observe how these nervous rela tions act in ordinary life. A person is the sub ject of some emotion, and his heart is beating faster than usual; that means that the excite ment is communicated from his brain by sym pathetic nerves to the heart, which it stimulates to increased activity. When a person receives a blow on the stomach which causes him to faint, the explanation is that the blow has pro duced a profound impression on certain nerves in the belly, which have conveyed the impression to the brain, and from the brain the impression has, in turn, been carried down to the heart by the pneumogastric uervee, causing the heart to cease beating for an instant. This is of the nature of a reflex action (p. 132). If the effect on the heart be so great as to restrain its move ments for any appreciable time, death is the result. It is in a similar way that sudden shocks of any kind, severe pain, &c., cause fainting, the restraining influence exerted on the heart by the stimulation of the pneumo gastric momentarily suspending its movements.
THE There are three kinds of blood-vessels, capil laries, arteries, veins, which differ from one another in various particulars.
The Structure of Capillaries will be first described, since they are the most delicate vessels. They are very fine tubes formed by long flattened cells united edge to edge. This is shown by staining with nitrate of silver a fine tissue, such as the inner membrane of the brain, the pia mater (p. 136), which capillaries in abundance. On then examining the tissue with the aid of a microscope, delicate vessels are seen traversing it in all directions. The nitrate of silver stains darkly the cement substance between the cells, and thus the fact of the vessel being made up of cells is revealed. This is shown in Fig. 137, a, where the clear areas, mapped out by the irregular dark lines, are the cells. A smaller vessel similarly stained is seen at b. At d in the figure (where the cells are not mapped out by staining) there is repre sented a smaller capillary giving off branches so fine that the blood corpuscles would require to travel through them in single file. These are the finest branches, and have no greater dia meter than the to the of an inch. In b and d a small body is seen in the centre of each elongated cell. It is a nucleus (p 53).