412 THE SPECIAL SENSES. 



at least it seems permissible to adopt the view that variations 

 in the pressure of the endolymph upon the hairs of the hair cells, 

 especially in movements of rotation, constitute the immediate 

 cause of their excitation. Granting that changes in position or 

 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 cristae of the ampullary sacs. Each collection of hair cells, 

 together with the supporting cells, is designated as a macula. One 

 of these is found in the utriculus, the macula utriculi, and another 

 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 that 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- 



