was really furnishing the observer with a series of natural experiments on man in the form of disease. The physiologist removed a piece of the brain and watched the loss of sight or hearing, or the loss of motion which ensued. The physician, on the other hand, watches the same kind of loss of sight and hearing, or of motion, in his patient, and may perhaps conclude that here too a loss of brain-tissue is the cause. And this conclusion was confirmed by further observation. Perhaps this may be made a little clearer if we add a fact or two regarding the way in which these experiments of nature are conducted. The blood which is sent to the brain at every throb of the heart goes up in a set of tubes, which give off side branches, like the system of water-pipes which connect your basins with the reservoir. Each tube gets smaller as its branches are given off, until at the end, instead of one large pipe, there is an innumerable series of little end pipes, each throw-out its little stream.
Let us picture to ourselves the water-pipe system of a town set up on a frame aboveground, with the great main, the street mains, the house pipes, and the little pipes all over the houses, all in view, and we will have a sort of conception of the brain's vessels and its blood-supply. Now, it is easy to see that, if a stick or a mass of leaves start out from the reservoir into a main, they will go on and on till they reach a pipe too small to allow them to pass, and there they will lodge. If the stick gets into one's house pipe, one's entire house will be cut off from the water-supply; if the mass of leaves breaks up, a few particles may come in and plug up a pipe to one only of the basins. But in either case the basin will be as useless for washing purposes as if there were no reservoir at all. Now, something very similar to this occurs in disease. Little plugs sometimes come up to the brain from the heart in the blood, and lodge in the little vessels which conduct the blood to various parts of the brain; and when the part of the brain is thus cut off from its supply of nutrition, it gradually withers up and ceases to act.
But when it ceases to act, a loss of some one sense results, just as in the dog when a part of the brain was cut out a loss of some sense occurred. When these facts were studied in this way, it soon became evident that in some persons it was the sight, in others the hearing, in others some other sense, in others still the power of movement which was lost; and further study showed that the varying effect depended upon which part of the brain was deprived of nutrition and was withered, just as in the dogs the location of the part removed determined which sensation was destroyed; so that a striking parallel between the results of experiment and the results of disease can be drawn; and thus the conclusion is arrived at that what is true of animals is true of man, that in man as well as in
