SEMICIRCULAR CANALS AND THE VESTIBULE. 389 



cerebellum forms the brain center in which the semicircular canal 

 impulses exert their influence upon co-ordinated muscular con- 

 tractions, the cerebellum forms the nerve center for the semi- 

 circular canals or the semicircular canals form a peripheral sense 

 organ to the cerebellum. Some such hypothesis seems to be 

 necessary to account for the general similarity between the effects 

 of lesions of the canals a/id of the cerebellum. Whether the 

 impulses from the canals are excitatory or inhibitory or both, as 

 regards their effect upon muscular contractions, is not clearly 

 apparent from the experimental evidence so far furnished, but 

 Ewald's suggestion that they serve to maintain reflexly the tonus 

 of the body musculature is perhaps the most acceptable view. 

 In regard to the means by which these nerves are normally stim- 

 ulated there is also much room for conjecture, but provisionally 

 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 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. 



