The reason why : $b a careful collection of many hundreds of reasons for things which, though generally believed, are imperfectly understood. A book of condensed scientific knowledge for the million
John Stuart Mill · en
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?_
The blood circulates once through the body in about _two minutes_.
If, therefore, we estimate the amount of blood at twenty-four pounds,
it follows that no less than _twelve pounds of blood pass through the
heart every minute_; and it is estimated that if the blood moved with
equal force in a straight line it would pass through _one hundred and
fifty feet in a minute_.
CHAPTER XLV.
923. _How many bones are there in the human body?_
There are _two hundred and forty-six_, and they are apportioned to the
various parts of the body in the following numbers:--
Head 8
Ears 6
Face 14
Teeth 32
Back-bone and its base 26
Chest, &c. 26
Arms and Hands 64
Legs and Feet 62
Small moveable bones 8
[Verse: "Our bones are scattered at the grave's mouth, as when one
cutteth and cleaveth wood upon the earth."--PSALM CXLI.]
924. _Of what substances are the bones composed?_
One hundred parts of bone consist of