528 



PRINCIPLES OF GENERAL PHYSIOLOGY 



chitinous covering as they grow too large for it. Along with it, in sonic specie-, 

 the inner lining of the statocvsts comes awav, naturallv taking the statoliths also. 

 These latter must therefore be replaced by new grains of sand or similar bodies. 

 Kreidl placed the animals under such circumstances that the only grains available 

 were iron filings, which were duly taken into the statocvsts. On bringing a magnet 

 into various positions with relation to the animals, the iron filings were attracted and 

 pressed against various points of the lining of the statocyst, and the animals showed 

 by their movements that the effect on them was the same as if, under normal 

 conditions, they had been turned into such a position that the weight of the grains 

 would have excited the cells in question. 



A detailed account of the properties of the statocysts of Pecten will lie found in an article 

 by von Buddehbroek (1911). 



In addition to organs of this kind, the vertebrate pos-e e- a remarkable 



organ, the labyrinth or 

 semicircular canals. The 

 statocysts are known in the 



vertebrate as utricle and 

 saccule. There are three 

 semicircular canals on each 

 side, and from Fig. 168 it 

 will be seen that these loops 

 are arranged in the three 

 dimensions of space. This 

 fact, in itself, suggests that 

 they are concerned with the 

 sense of position, but Crimi 

 Brown (1874) and Cyon 

 (1873) were the first to 

 draw attention to the rela- 

 tion. It is, howe\er, to the 

 experiments of Mach (IS?.")) 

 and of Breuer (1891) that 

 we owe the clear present 

 ment of their modi- of 

 action. Flourens (1828) had 

 obtained, on section of the 

 semicircular canals, peculiar 

 movements of the head, 

 dill'ering according to the 

 canal injured. It appear- 

 that their structure is such 

 as to enable rapid change- 

 of position of the head in 



space to be appreciated and their particular direction to be known. The receptor 

 cells lining a part of the tube of which each of these loops is composed, and 

 especially those of the dilated ampulla at the end of each, have long delicate hairs 

 projecting into the liquid filling the tube. Since there is free circulation all round 

 the circle, any movement of the head of the animal, to which the base of the hair 

 cell is attached, will move these cells through the liquid, owing to the inertia of 

 the latter not allowing it to share the movement at once. The result of this 

 relative movement is to drag the sensitive hairs through the more or less stationary 

 liquid, bending them and thus exciting the nerve endings at their attached ends. 



The investigations of Ewald (1892), chiefly on the semicircular canals of the pigeon, will 

 l>e found of much interest. They show how the sense of the position of the head in space is 

 disturbed by injury or stimulation of various component parts of the labyrinth mechanism. 

 The recent work of Wilson and Pike (1912) is a valuable contribution to the knowledge of the 

 effects of stimulation and of extirpation of the labyrinth in various mammals. 



The reflex tonic contraction, especially of the muscles concerned in the maintenance of 

 posture, is controlled by the labyrinth, and will be described in the next chapter. 



FHJ. 168. SEMICIRCULAR CANALS OF MAX. Photograph 

 of an enlarged model made by Tramond, Paris. The 

 three arcs are seen to lie in the three dimensions of 

 space. The membranous canals are shown inside the 

 bony canals, which are partly cut away. The cochlea 

 is at the bottom. 



