SEMICIRCULAR CANALS AND THE VESTIBULE. 379 



movement of the head may cause such variations in pressure the 

 theory offers a simple and satisfactory explanation of the mode 

 of excitation and the means by which the excitation may vary 

 appropriately under different conditions. While the endolymph 

 theory may be criticized easily, no other equally satisfactory theory 

 has been suggested to take its place. 



Functions of the Utriculus and Sacculus. These small sacs 

 contain sensory hair cells similar in general structure to those found 

 in the crista of the ampullary sacs. The collection of hair cells with 

 their supporting cells is designated as the macula, the macula utriculi 

 and 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 strik- 

 ingly 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 that need be attributed to the vestibular 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 



