390 THE SPECIAL SENSES. 



In recent years the view that has been most discussed is that ad- 

 vanced by Breuer, namely, that these organs give us information 

 regarding the position of the head when at rest and when mak- 

 ing progressive that is, non-rotary movements, supplementing, 

 therefore, the functions of the semicircular canals on the supposition 

 that these latter act especially in movements of rotation. Or, as it 

 is sometimes expressed, the sacs form a static and the canals a dy- 

 namic organ of equilibrium. According to this view, the otoliths 

 act as a means of mechanical stimulation of the hairs. Being 

 heavier than the endolymph, they press upon the hairs with a force 

 varying with the position of the head and thus give rise to sensations 

 or reflexes which are adapted to the maintenance of equilibrium. 

 Since the planes of the two sacs are different, they may be differ- 

 ently affected by the same position or movement. So also in pro- 

 gressive movements forward the weight of the otoliths may be im- 

 agined to exercise a stress of some sort upon the hairs. This theory 

 has been the subject of much investigation, numerous experiments 

 having been made chiefly upon fishes and invertebrates.* Accord- 

 ing to some observers destruction of these sacs or section of their 

 nerves is accompanied by a distinct interference with the fish's nor- 

 mal equilibrium: the animal swims at times upon its back or side 

 and apparently loses its normal means of judging correctly its posi- 

 tion. In many invertebrates there is present a sac, known as the 

 otocyst, containing hair cells and otoliths. Its structure resembles 

 that of the vestibular sacs of the mammalian ear, and it has been 

 assumed that it has a similar function. Experiments by numerous 

 observers have indicated that when the otoliths are removed the 

 animal shows disturbances in equilibrium, particularly in the matter 

 of the compensatory movements exhibited during rotation. Others, 

 however, deny these facts and state that invertebrates without oto- 

 cysts make compensatory movements when rotated and that in 

 those with otocysts compensatory movements and maintenance of 

 normal equilibrium persist after destruction of the sacs. A very 

 ingenious experiment reported by Kreidl seems to show that the oto- 

 liths may affect the hairs by their weight. When the palsemon, a 

 crustacean, molts it casts off the inner lining of the otocyst, together 

 with the otoliths. The otocysts in these animals lie at the base of 

 the antennules and open freely to the exterior. After molting the 

 animal by means of its claws places fine grains of sand in the otocyst 

 to act as otoliths. Taking advantage of this peculiarity, Kreidl 

 placed the animal, after molting, upon finely powdered iron, with 

 the result that some of the iron granules were deposited in the oto- 



* Consult the following papers: Sewall, "Journal of Physiology/' 4, 339, 

 1884; Lee, ibid., 15, 311, 1893, and "American Journal of Physiology," 1, 

 128, 1898; Lyon, 'American Journal of Physiology/' 3, 86, 1900. 



