II
The prohibition to enter the manufactory of the human body is not so strict but that we may advance a little. We can see everything which goes in, that is, certain substances called food, which we all know, because they are publicly offered for sale; nor is it difficult to obtain permission to follow them through the mouth which forms the entry, and a long corridor, called the œsophagus, into a large, damp, warm cavity called the stomach, in which all substances are reduced to a very fine pulp; the whitish juice runs into certain little canals which flow into the circulating stream called blood, of which we have already spoken, and out of which every workman, every cell, draws what is necessary for its work. But no one has ever really discovered what is done with this appropriated material—the manner of its digestion and elaboration.
We know that the motor force of the manufactory is due to a change—a chemical operation, by which the energy of the substances introduced is transformed and appropriated by the cells that manifest it externally in that force which we call muscular contraction cerebral activity, &c.
The most important chemical operations performed in the manufactory are three in number. The first consists in the transformation of food into protoplasm, or cell-substance; the second, in the discharge of the energy accumulated in these cells; the third, in the elimination of those substances which the cells have exhausted and rendered useless.
If we make a careful comparison, by means of chemical analysis, between the composition of the substances introduced and those eliminated, we always find that the chemical energy of the latter is much reduced, from which we may know that it is food which puts everything in movement, for with nothing, nothing is made.
The walls of the building are often moistened by a fluid which trickles through in drops, and is called perspiration. Physiologists have constructed most costly apparatus in order to collect the smoke of the chimney and the air which escapes from the mouths of the innumerable ventilators called pores. Every little thing was studied and conscientiously analysed, and all were surprised that these elaborate and most intricate operations of life should result finally in products so simple. We may say that our body only produces carbonic acid, urea, and a few salts.