CHAPTER XLIV.
902. _Why when we cut our flesh does it heal?_
Because the blood coagulates over the cut, and throws out a kind of _lymph_, which forms an incipient flesh, and excludes the air while the blood-vessels are engaged in _repairing the part_.
[Verse: "And God said, Let us make man in our own image, after our likeness; and let them have dominion over the fish of the sea, and over the fowl of the air, and over the cattle, and over all the earth, and over every creeping thing."--GEN. I.]
903. _Why, since all the substance of the body undergoes change, do we preserve the same features throughout our lives?_
Because our substance changes in the _minutest atoms_; and each separate atom has a life of itself, the maintenance of which preserves the _unity and permanence of the whole_.
904. _Why do moles upon the skin continue permanent, while bruises and wounds disappear?_
Because moles are themselves _organised formations_, and repair themselves just as any other part of the body does. But bruises and wounds are the result of _accidental disturbances_, which in course of time become removed.
905. _Why do the marks of deep cuts sometimes remain?_
If the cut is so deep and serious as to destroy the _system of vessels_ which supply and repair the part, then it is evident that they cannot work so perfectly as when in their sound condition. Their functions are, therefore, interfered with, and instead of having flesh uniform with the other parts of the system; there results a _scar_, or a wound _imperfectly repaired_.
906. _Why when we hold our hands against a candle-light do we perceive a beautiful crimson colour?_
Because the fluids and vessels of the body are in some degree transparent, and the thin textures of the sides of the fingers allows the light to pass, and shows the beautiful crimson colour of the blood.
If the web of a frog's foot be brought in the field of a good microscope, and set against a strong light, the blood may be seen in circulation, with the most wonderful effect. Each vessel, and every globule of blood, can be seen most distinctly, and the junction of the arteries and veins can be clearly traced. The little boats of nutrition may be seen chasing each other in rapid succession, and when the animal exerts itself to escape, the flow of the blood increases; and not unfrequently, under these circumstances of agitation, have we seen two or three blood discs struggling together to enter a vessel that was too small for them. Again and again they have endeavoured to find a passage, until one of them happening to slip forward, got away, followed by the others!
[Verse: "Know ye that the Lord he is God: it is he that hath made us, and not we ourselves: we are his people, and the sheep of his pasture."--PSALM C.]
907. _Why does the flesh underneath the nails look red?_
Because the transparent texture of the nails enables us to see the colour of the _vascular structure_ that lies underneath the skin.
_Vascular_.--Full of vessels. In this instance, full of capillary blood-vessels.
908. _Why have we nails at our fingers' ends?_
Because they give _firmness to the touch_, and enable us to apply the extremities of the fingers to many useful purposes for which they would otherwise be unfitted. They enable us to _press the tips of the fingers_, where the highest degree of sensitiveness prevails, so as to bring _the largest amount of nervous perception_ into the sense of touch.
909. _Why do white spots occur upon the nails?_
Because the vascular surface underneath is attached to the horny texture of the nail; but by knocks and other causes, the nail sometimes _separates in small patches from the membrane_ below, and becomes _dry and opaque_.
910. _Why is there a circular line of whitish colour at the root of the nail?_
Because there the nail is _newly formed_ by the vascular substance out of which it grows, and has not yet assumed its proper horny and transparent nature.
911. _Why is the eyeball white?_
Because the blood-vessels that supply its surface are so very fine that they do not admit the _red corpuscles_ of the blood.
912. _Why does the eyeball sometimes become blood-shot?_
Because, under exciting causes of inflammation, the _blood-vessels become distended_, and the red corpuscles enter, producing a net-work of red blood-vessels across the white surface of the eye.
913. _Why are the lips red?_
Because the lips are formed of the _mucous membrane_ that lines the body internally, and covers the surface of most of the internal parts. This membrane contains a great number of minute red vessels, which give softness and moisture to the surface. A very beautiful illustration of the softness, moisture, and delicate colour of the mucous membrane is afforded by turning up and examining the under surface of the upper eyelid.
[Verse: "Hast thou not known, hast thou not heard, that the everlasting God, the Lord, the Creator of the ends of the earth, fainteth not, neither is weary? there is no searching of his understanding"--ISAIAH XL.]
914. _Why do delicate persons look pale and languid?_
Because, generally from the want of exercise and fresh air, their blood is deficient of the healthy proportion of _red corpuscles_.
915. _Why does exercise and fresh air impart to healthy persons a red and fresh appearance?_
Because the redness of the blood is due to the _amount of oxygen_ which it contains, and air and exercise _oxygenise_ the blood, and diffuse it throughout the system.
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?_
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_.