916. _How is the blood propelled through the arteries?_
By the very powerful contraction (and alternate dilation) of the thick _muscles of the heart_, assisted also by the _muscular cords of the blood-vessels_ themselves, and in many instances by the _compression of the muscles_ in which the arteries lie embedded.
917. _Why are the capillary arteries capable of receiving the great quantity of blood sent out through the larger vessels?_
Because the capillary vessels are _so numerous_, that though they are infinitely smaller, they are capable of receiving in their minute tubes _the whole of the quantity of blood_ transmitted to them through the larger vessels.
918. _Why, when we sit with our legs crossed, do we see the foot that is raised move at regular intervals?_
Because the pressure upon the muscles of the leg retards the progress of the blood until it forces _itself through the compressed vessels_, and thereby imparts a pulsation which moves the leg and foot.
919. _Why are capillary blood-vessels found in every part of the system?_
Because it is _through these small vessels alone_ that the substances of the body are renewed and changed. Even the larger blood-vessels _do not sustain themselves upon the blood which they contain_, but receive into their coats numerous capillary vessels by which they are nourished.
[Verse: "All my bones shall say, Lord, who is like unto thee, which deliverest the poor from him that is too strong for him, yea, the poor and the needy from him that spoileth him?"--PSALM XXXV.]
920. _How much blood does the human body contain?_
From _twenty-five_ to _thirty-five_ pounds. (_See_ 623.)
921. _How does the blood ascend in the veins, in opposition to gravitation?_
In addition to the muscular coats of the veins, and the influence of muscular action upon them, there are in the veins numerous semi-circular valves, which are not found in the arteries. These valves extend from the sides of the veins in such a manner that they allow the free passage of the blood upwards, but a backward motion of the blood would expand the cup-like valves and stop the passage; so that the blood can only move in one direction, and that _towards the heart_.
922. _How frequently does the total amount of blood circulate through the system?_