883. Why does some portion of the food we eat nourish the system, while other portions are useless?

Because most food contains some particles that are indigestible, or that, if digested, are innutritious, and not necessary for the system. The liver is the organ by whose secretion the useful is separated from the useless; for when the bile enters through the duct (Fig. 49) and mixes with the grey cream coming from the stomach, it remains no longer a grey cream, but turns into a mass coloured by bile, having upon its surface little globules of milk, small, but very white. Those minute globules of milk (chyle) are the nutritious particles derived from the food; the other portion, coloured with bile, is the useless residue, or rather the bulk from which the nutrition has been extracted.

"God hath made of one blood all nations of men for to dwell on all the face of the earth."—Acts xvii.

884. Why does the milky, or nutritious matter, separate from the innutritious, upon admixture with bile?

Because the bile contains an oily matter which repels the watery milk of nutrition.

The pancreatic juice also enters through the same duct with the bile. But its precise use is not understood. It is a fluid much like the salivary secretion of the glands of the mouth.

A B. Jugular veins which return blood from the head to the heart.

C. The superior venÆ cava, or trunk vein, which pours the blood returned from the upper part of the system into the heart. There is a similar large vessel which meets this one and brings back blood from the lower part of the body, and they both pour the blood into the right side of the heart.

D E. The branches of the venous system which bring back the blood from the arms.

F F. The great aorta, the blood vessel which conveys arterial blood from the heart, and gives off branches that supply every part of the body.

G. Another large vein which returns the blood from the muscles of the chest, &c.

H H. The thoracic duct, which receives the newly dissolved food from the small absorbents, that collect it from the intestines. It conveys this nutrition (called chyle) upward along the back, until it reaches where the duct turns into the junction of two veins, and pours its contents into the veins bringing blood back to the heart. The nutrition, therefore, is at this moment mixed with the venous blood, and is sent to the lungs to be oxygenised.

"But now hath God set the members in the body, every one as it pleased him."—1 Corinthians xii.

885. How is the nutrition taken away from the bilious residue?

The muscular threads (or hands, as we figuratively call them) continue to push forward the digested matter through a long tube, called the alimentary canal, or bowels. This canal is some thirty feet in length, and is folded in various layers across the abdomen, and tied to the edge of a sort of apron, which is gathered up and fastened to the back-bone. All along this alimentary canal those muscular hands are pushing the digested mass along. But upon the coat or surface of the canal there are millions of little vessels called lacteals, which look out for the minute globules of milk as they pass, and absorb them, which means that they pick them up, and carry them away. There is an immense number of these little vessels, all busily at work picking up food for the system.

Then there is a large vessel, called the thoracic duct, which comes down and communicates with those little vessels (it is a sort of overseer, having a large number of workmen,) and collects the produce of their toil, and carries it upwards to the part where it passes from the organs of digestion into the vessels of circulation.886. What becomes of the nutrition, when it has entered the vessels of the circulation?

It is sent through a large vein into the heart, entering that organ on the right side, from which the heart propels it into the lungs, mixed with venous blood; and the venous, or blue blood, is sent into the lungs, taking with it the milk, the formation of which we have traced.887. Why are the venous blood and the chyle sent to the lungs?

Because the venous blood, in its circulation through the body, has parted with its oxygen, and taken up carbon, and it requires to get rid of the carbon, and take up more oxygen. The chyle, also, now combined with the blood, requires oxygen, and having obtained it, is converted into bright red blood, and the blue blood of the veins, having got rid of its carbon, which formed the carbonic acid of the breath, has again become bright red blood. We must therefore, in pursuing our description, cease to speak of blue, or venous blood, and of white milk, or chyle, for the two have now combined, and, with the oxygen of the air, have formed arterial blood.

"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.

A. The heart.

B B. The lungs.

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 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.

"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.

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.