STIMULATION THE SENSES 119 



into the liquid. These hair-cells are connected with nerve fibres 

 and, when the apparatus moves, the hairs are dragged through the 

 water, since this does not move with them. The result is that they 

 are deflected, and their bases press upon the cells and nerves. The 

 process may be compared to " catching a crab " in rowing, the hairs 

 corresponding to oars, the nerve cells to the oarsman, and the walls 

 of the canal to the boat. 



It will be seen that the function of this apparatus is chiefly for 

 the perception of more rapid movements in space than could be 

 detected by the statocyst. If the movement is slow there will not 

 be much relative displacement of the liquid and the hairs, and when 

 at rest in any position there will be no stimulus at all. The stato- 

 cysts, on the other hand, are arranged to indicate permanent 

 changes of position, and are not very sensitive to rapid movements. 



As mentioned above, we do not depend entirely on the laby- 

 rinth for information as to position. In addition to differences of 

 pressure on that part of the skin in contact with solid objects, there 

 are receptor organs in the muscles themselves, which indicate 

 changes of tension in them, according to their position and that of 

 the parts to which they are attached. Moreover, the joints are 

 provided with nerves. Together, these form the receptor organs for 

 the so-called muscular sense. 



It will be noticed that we have here a distinct class of receptors, 

 affording information of the state of parts in our own bodies, as 

 distinguished from that of external objects. The former class is 

 known as that of the intero-ceptors ; the latter are the extero-ceptors. 

 Of the intero-ceptors, the most important are the proprio-ceptors 

 of the muscles, which afford information of the state of activity of 

 these organs. The centres receive messages, as it were, as to 

 whether the command has been obeyed, clearly of great import- 

 ance in the carrying out of complex movements, which depend on 

 a series of acts. 



The fact that certain sense-organs, especially the eye and the 

 hand, are provided with muscles capable of moving them in any 

 direction, is of much importance in the perception of direction in 

 space, and of the forms and distances of external objects. It is by 

 the co-ordination of these two organs that we learn, by experience, 

 how to interpret the information given by either. The size of the 

 image of an object on the retina would not inform us of the actual 

 size of the object unless we had, at some previous time, moved the 

 hand over it or some object of the same apparent size, and found 

 the muscular effort necessary. 



