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.

"As for man his days are as grass; as a flower of the field so he flourisheth."—Psalm ciii.

The capillary extremities of the arteries, unite with the capillary extremities of the veins, and the blood passes from the one set of vessels into the other. As the arteries become smaller from the point where they receive the blood, so the veins grow larger; the venous capillaries, pour their contents into small vessels, and these again into larger ones, until the great venous trunks are reached, and the blood is passed again into the heart as at first described. ([Fig. 50].)

896. Why do we see blue marks upon our arms and hands?