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. 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. [Verse: "Though hand join in hand, the wicked shall not be unpunished; but the seed of the righteous shall be delivered."--PROVERBS XXI.] [Illustration: 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. D D. The point where the _ulnar canal_ and the _radial canal_, after having passed and supplied Wrist-town, form a junction, running through Palm-town, and in their course giving off branches to supply the four Finger-towns and Thumb-town. _For further explanations of the engraving, see 57._ 895. _How does the blood return to the lungs, after it has reached the extremities?_ The _veins_ constitute a system of vessels corresponding to the arteries. We may say that the arteries form _the down canal_, and the veins _the up canal_. The arteries, commencing in the great trunk of the _aorta_, branch off into large and then into smaller tubes, until they form _capillary_ or hair-like vessels, penetrating everywhere. [Verse: "As for man his days are as grass; as a flower of the field so he flourisheth."--PSALM CIII.]