SEMICIRCULAR CANALS AND THE VESTIBULE. 413 



in the sacculus, the macula sacculi. Lying among the hairs of the 

 hair cell are found masses of small crystals of calcium carbonate, 

 the otoliths or otoconia. In this respect the structure of the 

 macula differs strikingly from that of the crista. The position and 

 connections of the utriculus and sacculus lead at first naturally to 

 the supposition that they are stimulated by the sound waves of the 

 perilymph, and are, therefore, concerned in the function of hearing. 

 The accepted views regarding the functions of the cochlea in hearing 

 make this organ sufficient for all auditory purposes, and there is no 

 specific part of this process tfhat need be attributed to the vestibu- 

 lar sacs. It was, indeed, at one time suggested that their structure 

 adapts them to respond especially to short and irregular vibrations, 

 but no cogent reasons or facts have been advanced to support this 

 view. The fact that the sacs are so closely connected with the 

 semicircular canals suggests rather that the functions of these organs 

 are similar and that like the canals, therefore, they influence the 

 contractions of the muscles and function as organs of equilibrium. 

 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 



* 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 Psychology," 3, 86, 1900. 



