CHAPTER XLII.

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_.

[Verse: "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.

[Verse: "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_.

[Verse: "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.

[Verse: "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_.