"My flesh and my heart fainteth; but God is the strength of my heart, and my portion for ever."-PSALM LXXIII.

888. What becomes of the arterial blood thus formed?

It is sent back from the lungs to the right side of the heart, from which it is sent into the great trunk of the aorta, and from thence it passes into smaller blood-vessels, until it finds its way to every part of the system.

C. The aorta, and on either side of the aorta the vessels which convey the venous blood to the lungs to be oxygenised, and the corresponding vessels which return it to the heart, after it has undergone that operation. (For aorta see Fig. 50.)

D. The trachea, or large air passage, through which the air passes into the spongy texture of the lungs, when we breathe.

E E. Arteries and veins, being the trunks of the vessels that supply the head, &c.

889. Why does the chest expand when we breathe?

Because the lungs consist of millions of hollow tubes, and cells, which, having been emptied by throwing off carbonic acid gas and nitrogen, become compressed, and the atmospheric air

s

'All the while my breath is in me, and the spirit of God is in my nostrils, My lips shall not speak wickedness, nor my tongue utter deceit."-JOB XXVII.

flowing into these millions of spaces, and filling the lungs, just as water fills and swells a sponge, causes them to expand, and occupy greater room.

890. How does the blood communicate with the air in the lungs?

Through the sides of very minute vessels, of which, perhaps, a fine hair gives us the best conception. But these vessels are twisted and wound round each other in such a curious manner, that they form millions of cells, and by being twisted and wound, a much greater surface of air and blood are brought to act upon each other, than could otherwise be accomplished.

891. Why does the blood which is thus formed, impart vitality to the parts to which it is sent?

Because the blood is itself vitalised-is, in fact, alive, and capable of diffusing life and vitality to the organisation of which it forms a part.

This is a very wonderful fact, but no less true than wonderful, that dead matter which, but a little while ago, was being ground by the teeth, softened by the saliva, and solved by the gastric juice and bile, has now acquired life. Nobody can tell the precise stage or moment when it began to live. But somewhere between the stomach and the lungs, melted by the gastric juice, softened by the secretion of the pancreas, separated by the bile of the liver, macerated by the muscular fibres of the bowels, taken up by the absorbents, warmed by the heat of the body, and ærated in the lungs, it has by one, or by all of these processes combined, been changed from the dead to the living state, and now forms part of the vital fluid of the system.

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,

"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.

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.

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

"Though hand join in hand, the wicked shall not be unpunished; but the seed of the righteous shall be de ivered."-PROVERBS XXI.

pass

boats 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.

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.

DD. The point where the ulnar canal and the radial canal, after having passed and supplied Wristtown, form a junction, running through Palm-town, and in their course giving off branches to supply the four Finger-towns and Thumbtown.

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.

"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.

899. What occurrs 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 es along; and all give something to the boats to bring back. One of the chief exchanges is that of oxygen for