iv MECHANISM AND TELEOLOGY 305 



and the respiratory system responds in sympathy. Now 

 this, on the surface, throws them into strong contrast with 

 the parts of a machine, each of which, as we saw, must do 

 what it does irrespective of the working of the rest. But 

 it will hardly be supposed that the anti-mechanical view is 

 to win so cheap a victory as this. We have to ask how 

 the quickening of the heart and dilatation of the arteries 

 is effected, and here at once a further and special mechanism 

 is found. Heart, arteries and lungs are alike under the 

 control or partial control of nerves, and these nerves are 

 affected by the condition of the blood. Thus the respira- 

 tory nerves are traceable to a centre in the medulla, the 

 action of which responds accurately to the degree in which 

 the blood is oxidised. If the supply of oxygen falls off 

 the de-oxidised blood acts as a stimulus on the centre, 

 heightens the activity of the nerves which supply the 

 respiratory muscles and so quickens respiration. As the 

 normal state of oxidation is regained the stimulus falls off 

 and breathing resumes its normal course. A similar self- 

 balancing machinery can be indicated for the other pro- 

 cesses concerned. 



5. In these explanations, it is true, the phenomena of 

 nerve stimulus and reaction have to be employed. These 

 are peculiar to the living organism and have not as yet been 

 reduced either to mechanical or chemical terms. But on 

 this point once more we lay no stress. We take them pro- 

 visionally as mechanical in the sense that, given the muscle- 

 nerve arrangement, stimulus A will invariably produce 

 reaction a, and stimulus B reaction /3, with no regard to 

 results or concomitant circumstances. Once again we con- 

 centrate attention on the working of the process as a whole. 

 What we find is that the circulatory and respiratory organs 

 on the one hand, and the skeletal muscles which move the 

 limbs on the other, are not, as in the ordinary machine, 

 mutually c indifferent. 5 The working of each is intimately 

 affected by the working of the remainder. Not merely are 

 they arranged once for all so that by a regular rotation each 

 supplies or supplements the other, but on a far more com- 

 plex plan they are arranged so that variations of their 



