BOOK II. 

 THE HUMAN MACHINE. 



CHAPTER I. 

 THE HUMAN STRUCTURE. 



65. The Materials. We have first to consider the structure 

 of the matter which constitutes the organs of all living machines. 

 Man and animal do not differ one from the other as to the pro- 

 perties of that matter. But it must be said, first of all, that 

 animated substance or protoplasm does not possess any constant 

 and perfectly defined property; its elementary chemical carbonic, 

 hydrogeneous, oxygeneous, and azotic composition varies very 

 little, its physical characteristics evolve slowly and in a deter- 

 mined direction. As a rule, it hardens with age, it fixes and 

 produces solid elements, minerals, from which it forms new 

 material or tissue. 



We shall confine our attention chiefly to the organs of move- 

 ment and locomotion, that is to say, the bones, muscles, and 

 tendons. 



The continuous variation of which the protoplasm is the seat 

 causes it to pass through a succession of different states. This 

 evolution characterises the substances called colloidal or the 

 colloids, the gelatinous silica, for example, so that this silica, 

 left to itself, loses or throws off its aqueous components pro- 

 gressively, and coagulates more and more into the solid state, 

 this being a question of time, or, if it can be said, of age. 



Living matter is essentially irritable or excitable. It responds 

 to stimuli, whether of heat, electricity, light, mechanical shock, 

 or chemical action, the reaction being of an obscure nature, but 

 .allied to the phenomena of nutrition, f 1 ) 



In the animal, this irritability co-ordinates and adjusts itself, 

 all the reactions having a resultant appropriate to this object of 

 defence, it being understood that the word object does riot imply 

 a metaphysical finality, but a complex determinism which is 

 becoming clearer every day. 



(*) Hering Lotos, vol. ix., p. 35: 1889. 



