_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. 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. [Verse: "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_. But other exchanges take place. The blood, in addition to oxygen and carbon, contains _hydrogen_ and _nitrogen_. But it contains its four elements in _various forms of combination_, producing the following _materials_ for the use of the body: of 1,000 parts of blood, _about_ 779 are _water_; 141 are _red globules_; 69 are _albumen_; 3 are _fibrin_; 2 are _fatty matter_; 6 are various _salts_.