228 



THE BRITISH NATURALIST. 



[October 



due to this kind of action, more or less modified by the action of the 

 higher nervous centres, that the immense number of movements in 

 response to stimulation of the sense organs take place. Such a process 

 occurs when we place our hands unexpectedly in hot water and 

 involuntarily jerk them out again. Here we have information given by 

 our sense organ, followed by the adjustment of our behaviour in 

 accordance- with that information. 



Whether we take a simple example like this, or a more complex one, 

 such as our action when our eyes show us some danger and we avoid 

 it, the condition reduced to its simplest form is the same. Our sense 

 organs receive an impression, they send a current along our nerves to 

 our nervous centre, and thence currents are sent to our muscles, and 

 certain movements result. It will be seen in dealing with the 

 simplest animals that this intervention of a definite nervous centre 

 is not essential. The nerve current may in them pass directly 

 from the part stimulated to the contractile part, or even the part 

 which receives the stimulus may combine in itself the function of 

 sensation and contraction. It should be noted, however, in this 

 connection that though in the highest animals most actions result 

 from impulses originating in, or at least passing through, the nerve 

 centres, nevertheless the contractile elements are sensitive to direct 

 stimulation. It is a familiar fact that muscle can be made to contract 

 by stimulation even when cut off from all nervous centres, so that 

 muscle, although a specially contractile tissue, has not, even in the 

 higher animals, quite lost its power of receiving and transmitting a 

 stimulus. 



The organs of touch, found scattered in great numbers all over 

 the body, are affected by the coarser jars and vibrations of objects 

 which come in contact with the skin. The organ of hearing also 

 is affected by vibrations in the surrounding air, more delicate than, but 

 in many respects allied to, the vibrations appreciated by the organs of 

 touch. The organs of touch are certainly sensitive to the concussion 

 caused by an explosion in the surrounding air, and this same concussion 

 is recognized by our hearing as a loud noise. Thus tactile and auditory 

 organs only differ in the degree of their sensitiveness. In the 

 sensitiveness of the retina to light we have another example of the ; 

 effect of vibrations ; but vibrations of a very different kind to those 

 of touch and sound. Organs of taste and smell are sensitive to 

 the chemical effect of certain substances which come in contact j 

 with them. It will be noticed that here also the difference between; 

 the actions of these two organs is one of degree rather than of! 

 kind. Especially is this the case in aquatic animals, where smell is 1 

 probably only a delicate form of taste, though often appreciated by,' 

 distinct organs. 



