THE ACTIVE FORCES OF LIVING ORGANISMS 79 

 In nutrition, as we saw in dealing with the amoeba, Chemical 



. action aa a 



there are two factors — a chemico-sensory or electro- cause of 

 chemical factor and a motor, in higher organisms impulses. 

 a vaso-motor and intestino-motor factor. We know 

 that both the bloodvessels and intestines have their 

 respective motor centres in the medulla and spinal 

 cord, and the point — a very difficult one — requiring 

 consideration is whether nerve-cells are affected by 

 the force liberated in the chemical processes of meta- 

 bolism and to what extent. A question which presents 

 itself, therefore, at the very outset of this inquiry is, 

 Does chemical action give rise to nervous impulses ? 

 To this an affirmative answer may be given. Not 

 only is it highly probable that our visual sensations 

 are due to chemical processes which are started in the 

 retina*— started in all probability by ethereal waves — 

 but it is almost certain that those of smell and taste 

 originate in a similar manner ; that is to say, through 

 chemical action. In the two latter cases a fluid is 

 secreted by the glands of the olfactory and buccal 

 mucous membranes, in which the particles to be 

 smelt or tasted are respectively dissolved. Chemical 

 force thus generated or liberated is at once transformed 

 into nervous impulses. With these facts before us it 

 seems natural to suppose that when chemical action 

 takes place in the stomach, intestines, and other parts, 

 as it does with greater rather than with less intensity, 

 nervous impulses would in like manner result. Indeed, 



* Foster, loc. ait, pp. 1252, 1253. 



