Every drop of blood is made up of a large number of corpuscles, each of which contains some of the elements essential to the wants of the system.
Let us, to simplify the subject, consider the blood vessels of the body to be so many canals, on the banks of which a number of inhabitants live, and require constant sustenance. The corpuscles of the blood are the boats which are laden with that sustenance, and when the heart beats, it is a signal for them to start on their journey. Away they go through the arch of the great aorta, and some of the earliest branches which it sends off convey blood to the arms. We will now for a moment dismiss the word artery, and keep up the figure of a system of canals, with a number of towns upon their banks.
"Though hand join in hand, the wicked shall not be unpunished; but the seed of the righteous shall be delivered."—Proverbs xxi.
Well, away go a fleet of boats through the aorta canal, until they reach a point which approaches Shoulder-town; some of the boats pass into the axillary canal and Shoulder-town is supplied; the other boats proceed along the humeral canal until they approach Elbow-town, when another division of the boats pass into other branch canals and supply the wants of the neighbourhood; the others have passed into the ulnar canals and the radial canals until they have approached Wrist-town and Hand-town, which are respectively supplied; and then the two canals have formed a junction across the palm and supplied Palm-town, where they have given off branches and boats to supply the four Finger-towns, and Thumb-town.
Fig. 52.—ILLUSTRATION OF THE SYSTEM OF CANALS THAT SUPPLY THE FORE-ARM WITH BLOOD.
Between A and B the brachial canal, which gives off branches to supply Elbow-town, &c., and then divides into two main courses, diverging to the opposite sides of the arm, and sending a smaller canal down the centre.