[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.]
_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?_
Because large veins lie underneath the skin, through which the blood of the fingers and hand is _conveyed back to the heart_.
897. _Why are the veins more perceptible than the arteries?_
Because the arteries are buried _deeper in the flesh, for protection_. It would be _more dangerous to life to sever by accident an artery than a vein_. A person might bleed longer from a vein than from an artery, without endangering life; because the arteries supply the _life sustaining blood_. The Almighty, therefore, has buried the arteries for safety.