Thousands of volumes have been written about this struggle, and those who only read an epitome of it in the treatises of physiological chemistry are yet astonished at the power of the human mind, and at the incredible, almost superhuman, difficulties with which it has to cope. Never was there, nor will there be, any war even faintly resembling in ardour, perseverance, and power of intellectual means, this siege of centuries, which seeks to close every issue to Nature and force her to reveal the secret of her chemical operations. It is wonderful to see how attacks are prepared in advance, what cunning traps are laid, and by what subtile signs the road of ingress is divined. Who can describe the joy of all besiegers when one step forward has been made, or a glimmer of light is detected in the darkness? The exultation and applause which welcome every invention which resolves, or analyses, or recomposes a molecule, every instrument which enables us to break one crumb off the immense rock of the Unknown? And who shall tell the tale of pain, of disappointment, bitterness, and error; of the forgotten or unknown deeds of heroism, of existences lost in the obscurity of the schools, of the laboratories, of the hospitals; of those who die unnoticed in these last trenches where spectators and witnesses to shout applause are lacking? And yet this deters no one; all press onward. The soldiers of science renew their vigilance and redouble their caution, the phalanx of the living draws closer together, returning with fresh courage to the assault. Nothing can resist this unremitting war, this wonderful harmony of aim, this iron will of man; we will die on the battle-field with the certainty that others will take up our weapons and that the victory will be ours. 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.