ii8 INTRODUCTION TO GENERAL PHYSIOLOGY 



action was made clear by an ingenious experiment with the crayfish, 

 which sheds the inner lining of its statocysts along with its shell. 

 The cavity of the statocysts is in communication with the outside 

 by a small pore ; grains of sand are normally taken in to replace 

 those lost when the lining is shed. If nothing but iron filings is 

 available, these are taken in and can then be caused to press upon 

 various parts of the wall of the statocyst by bringing a magnet near 

 the animal, which then proceeds to turn over in the way it would 

 have done if this part of the statocyst had been brought in the 

 normal way to be the lowest part of the sac. In the vertebrate, 

 these functions appear to be undertaken by those parts of the 

 internal ear^ known as utricle and saccu/e, wjiich have structures 

 similar to those of the statocysts of loweF animals, although some 

 oT>M-rvers hold that the former have auditory functions. It must 

 be remembered that the touch and pressure receptors in the skin 

 and the muscles serve to give us information of that part upon 

 which pressure is being exerted, or of what part is being stretched, 

 and thus indirectly of the relation of our bodies to the vertical 

 direction. 



The second kind of position-receptor is that interesting organ 

 known as the labyrinth or sem_t-circular canals, present only in 

 vertebrates. There are three of these on each side of the head, 

 forming a part of the internal ear, although they have nothing to 

 do with the perception of sound. Each is in the form of a hollow 

 ring, and the three are connected together at one part, so that there 

 is communication between their internal space. The plane of each 

 ring is at right angles to that of the two others, so that the three 

 canals are situated in the three dimensions of space, corresponding 

 to the length, breadth, and thickness of an object (E., p. 214). 



Suppose now that the whole arrangement is moved quickly in 

 a plane which coincides with that of one of the canals. The liquid 

 contained in it will not partake, to any great extent, in the move- 

 ment of the walls, because the friction between the layers of the 

 liquid is not sufficiently great to convey the motion to the whole 

 mass at once. In other words, the walls are moved along, leaving 

 the liquid behind. Those canals which are at right angles to the 

 plane of movement will, naturally, not be affected in such a way as 

 to cause relative displacement of the liquid and the walls. But if 

 the direction of movement is such as to have components affecting 

 more than one canal, the effect on each will be inversely propor- 

 tional to the angle which its plane makes with that of the 

 movement. 



How is this effect made to stimulate nerve-endings? At one 

 end of each canal there is a dilated portion, and on one part of its 

 wall there is a protruding mass of cells with long hairs reaching 



