CHAPTER XLIII.

892. _Why do we know that the blood has become endowed with vital powers?_

Because, in the course of its formation, it has not only undergone change of condition and colour; but, if examined now by the microscope, it will be found to consist of millions of minute cells, or discs, which float in a watery fluid. The paste produced by mastication consisted of a crude admixture of the atoms of food; the cream (_chyme_) formed from this in the stomach, presents to the microscope a heterogeneous mass of matter, exhibiting no appearance whatever of a new organic arrangement; the milk (_chyle_) which is formed in the intestines is found to contain a great number of very small molecules, which probably consist of some fatty matter; as the chyle progresses towards the _thoracic duct_ (Fig. 50), it appears to contain more of these, and slight indications present themselves of the approach towards a new organic condition.

But wherever _vitalisation begins_, no human power can say with confidence. Yet there can be no doubt that the blood is both _organised_ and _vitalised_, and that it consists of corpuscles, or little cells, enclosing matters essential to life.

[Verse: "But they that wait upon the Lord shall renew their strength; they shall mount up with wings as eagles; they shall run and not be weary; and they shall walk and not faint."--ISAIAH XL.]

893. _Why does the blood circulate?_

Because all the bones, muscles, blood-vessels, nerves, glands, cartilages, &c., of which the body is composed, are constantly undergoing a _change of substance_. It is a condition of their life, health, and strength, that they shall be "_renewed_," and the blood is the great source of the _materials_ by which the living temple is kept in repair.

894. _How is the body renewed by the blood?_

Every drop of blood is made up of a large number of corpuscles, each of which contains some of the elements essential to the wants of the system.

Let us, to simplify the subject, consider the blood vessels of the body to be so many _canals_, on the banks of which a number of inhabitants live, and require constant sustenance. The corpuscles of the blood are the _boats_ which are laden with that sustenance, and when the heart beats, it is a signal for them to start on their journey. Away they go through the arch of the great _aorta_, and some of the earliest branches which it sends off convey blood to the arms. We will now for a moment dismiss the word _artery_, and keep up the figure of a system of canals, with a number of towns upon their banks.

Well, away go a fleet of boats through the _aorta_ canal, until they reach a point which approaches Shoulder-town; some of the boats pass into the _axillary_ canal and Shoulder-town is supplied; the other boats proceed along the _humeral_ canal until they approach Elbow-town, when another division of the boats pass into other branch canals and supply the wants of the neighbourhood; the others have passed into the _ulnar_ canals and the _radial_ canals until they have approached Wrist-town and Hand-town, which are respectively supplied; and then the two canals have formed a junction across the palm and supplied Palm-town, where they have given off branches and boats to supply the four Finger-towns, and Thumb-town.

[Verse: "Though hand join in hand, the wicked shall not be unpunished; but the seed of the righteous shall be delivered."--PROVERBS XXI.]

Between A and B the _brachial canal_, which gives off branches to supply Elbow-town, &c., and then divides into two main courses, diverging to the opposite sides of the arm, and sending a smaller canal down the centre.

D D. The point where the _ulnar canal_ and the _radial canal_, after having passed and supplied Wrist-town, form a junction, running through Palm-town, and in their course giving off branches to supply the four Finger-towns and Thumb-town.

_For further explanations of the engraving, see 57._

895. _How does the blood return to the lungs, after it has reached the extremities?_

The _veins_ constitute a system of vessels corresponding to the arteries. We may say that the arteries form _the down canal_, and the veins _the up canal_. The arteries, commencing in the great trunk of the _aorta_, branch off into large and then into smaller tubes, until they form _capillary_ or hair-like vessels, penetrating everywhere.

[Verse: "As for man his days are as grass; as a flower of the field so he flourisheth."--PSALM CIII.]

_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_.

Albumen and fibrin are a kind of flesh imperfectly formed, and probably are chiefly used in repairing the muscles. The red corpuscles contain the oxygen which goes to combine with the superabundant carbon, and develop heat; the fatty matters probably repair the fatty tissues, and glands that are of a fatty nature; and the various salts contribute to the bones, and to the chemical properties of those secretions which are formed by the glands, &c., while the great proportion of water is employed in cleansing, softening, and cooling the whole, or the living edifice, and it is the medium through which all the nutrition of the body is distributed.

900. _Why do we feel the pulse beat?_

Because every time that the heart contracts it send a fresh supply of blood to the blood-vessels, and the motion thus imparted creates _a general pulsation throughout the system_: but it is more distinctly perceived at the pulse, because there a rather _large artery lies near to the surface_.

[Verse: "Thy hands have made me and fashioned me: give me understanding, that I may learn thy commandments."--PSALM CXIX.]

901. _What becomes of the matter collected by the blood in the course of its circulation?_

We have already explained that carbon is thrown off from the lungs in the form of carbonic acid gas. But there are many other matters to be separated from the venous blood, and its purification is assisted by the action of the liver, which is supplied with a large vein, called the _portal vein_, which conveys into the substance of the liver, a large proportion of the venous blood, from which that organ draws off those matters which form the bile, and other matters which are transmitted with the bile to the bowels. The _liver_ and the _lungs_, therefore, are the great purifiers of the venous blood. But there are also smaller organs that assist in the same work.

A. The _aorta_.

B. Branches given off for the _aorta_ to supply one portion of the intestines.

C. Branches given off by the aorta to supply other portions of the intestines. A complete communication may be traced between these vessels from the origin of one to that of the other.

D. The _pancreas_, or sweetbread, a large gland that forms the pancreatic juice, which it pours in through the duct. (_See_ Fig. 50.)

E E E. The _large intestines_, forming the termination of the alimentary canal.