also by cavities at the bottoms of their trunks—the heart. The branches of both trees being everywhere hollow, the contained fluid runs up the stem, and through smaller and smaller branches of the arterial tree, into the delicate vessels of the leaves, which may be taken to represent the capillaries. Passing through these into the twigs of the venous tree, the blood returns through larger and larger branches of this tree till it arrives at the trunk, and completes its circuit by again entering the trunk of the arterial tree through the cavities of the heart. Now the blood, in perpetually making this complete circuit of the body, performs three important functions: it serves to carry oxygen from the lungs to all the other parts of the body; it serves to supply all parts of the body with the nutritive material with which it is charged; and it serves to drain off from all the tissues of the body the effete products which they excrete, and to present these effete products to the organs whose function it is again to abstract them from the blood and expel them from the body. The two latter functions of the blood—those of nourishing and draining—I must consider more in detail. They are both performed in the capillaries, so that the object of the arteries and veins may be considered as merely that of conveying the blood to and from the capillaries. Moreover, both functions are performed by transfusion through the delicate walls of the capillaries—the nutritive material in the blood being thus transfused into the surrounding tissues, and the waste product of these tissues being transfused into the blood. Thus, in the various vascular tissues there is always a double process going on: first, that of receiving nourishment from the blood, whereby they are being constantly built up into an efficient state for the performance of their various functions; and, secondly, that of discharging into the blood the effete materials which the performance of these functions entails. Now, when any tissue or organ is in a state of activity in the performance of its function, the activity which it manifests entails a process of disintegration, which is the reverse of the process of nutrition; that is to say, when a tissue or organ is doing its work, it is expending energy which it has previously derived in virtue of the process of nutrition. Work is therefore, so to speak, the using up of nutrition; so that if the income of energy due to nutrition is equal to the expenditure of energy due to work, the tissue or organ will remain stationary as regards its capacity for further work, while, if the work done is in excess of the nutrition supplied, the tissue or organ will soon be unable to continue its work; it will become, as we say, exhausted, cease to work, and remain passive until it is again slowly and gradually refreshed or built up by the process of nutrition. Therefore all the tissues and organs of the body require periods of rest to alternate with periods of activity; and what is true of each part of the body is likewise true of the body as a whole—sleep being nothing other than a time of general rest during which the process of nutrition is allowed to gain upon that of exhaustion. Thus we
