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.
898. Why when we prick the flesh with a needle does it bleed?
Because the capillary arteries and veins are so fine, and are so thickly distributed all over the body, that not even the point of a needle can enter the flesh without penetrating the coats of several of these small vessels.
"Let every thing that hath breath praise the Lord. Praise ye the Lord."—Psalm cl.
899. What occurs during the circulation of the blood?
Not only do the various parts to which the boats are sent take from them whatever they require, but the boats collect all those matters for which those parts have no further use. The bones, the nerves, the muscles, &c., all renew themselves as the boats pass along; and all give something to the boats to bring back. One of the chief exchanges is that of oxygen for carbon, by which a gentle heat is diffused throughout the system. It is for this purpose that fresh air is so constantly necessary.