86 



THE HUMAN MOTOR 



Sensibility is the superior property which is assured by a special 

 co-ordinating system, the nervous system. 



It seems, in consequence, that the nervous tissue centralises 

 the effects of all excitati9ns, and perpetuates them all through 

 life. Hereditary influences, and acquired impressions are 

 perpetuated in this manner. Physically, the nervous system 

 maintains the animal substance in a peculiar state, in readiness 

 for reaction. All the protoplasmic molecules are drawn together 

 with a little more force when irritated, and offer a greater 

 resistance to separation. It is obvious that this state will affect 

 the elastic matter of the muscles, and not the rigid substance 

 of the bones. 



This internal activity of the protoplasm, this continuous 

 dynamic state, demands four external conditions, four factors, to 

 maintain it : 1. water ; 2. food ; the one for the accomplish- 

 ment of the chemical reactions, the other to repair organic waste 

 (see 99) ; 3. oxygen, a gas necessary for cellular combustion 

 and for physiological work ( 60), and also to repair fatigued 

 matter ( 147) and replenish the blood, the vehicle which per- 

 meates the whole organism of the body ; 4. heat, between certain 

 limits of temperature, the lower kinds of animals living at very 



variable temperatures and the 

 higher animals living at a tem- 

 perature said to be constant, or 

 homeothermic. If the variation 

 of temperature is great, death 

 follows, and it seems also that 

 maximum activity is manifested 

 at a constant temperature, which 

 is 37 '5 C for man. The reactions 

 of human life are characterised 

 by this "thermic optimum." 

 The variations of intensity of the 

 four preceding factors modify 

 the physical state of living 

 matter, this latter being without 

 physical constancy in the strict 

 sense of the word. Age, sex, 

 and past experience, with their 

 different influences, give different 

 characters to organisms and 

 complicate the problem of the 

 resistance of living materials. 



fw. 94. 



Section of femur showing the 

 curves of pressure (after 

 Culmann). 



66. The Bones: the Skeleton. The organs of 'locomotion ~are~ 

 the bones and the muscles ; the former being levers, and the 



