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_.
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.