628 THE SENSES 



sonorous impulses. A state of excitement of this nerve, however induced, 

 gives rise to the sensation of sound. Hence the ringing and buzzing in the 

 ears heard by persons of irritable and exhausted nervous system, and by 

 patients with cerebral disease, or disease of the auditory nerve itself; hence 

 also the noise in the ears heard for some time after a long journey in a rattling, 

 noisy vehicle. Ritter found that electric currents also excite sounds in the 

 ears. From the above truly subjective sound we must distinguish those 

 dependent, not on a state of the auditory nerve itself merely, but on sonorous 

 vibrations excited in the auditory apparatus. Such are the buzzing sounds 

 attendant on vascular congestion of the head and ear or on aneurismal dilata- 

 tion of the vessels. Frequently even the simple pulsatory circulation of the 

 blood in the ear is heard. To the sounds "of this class belong also the buzz 

 or hum, heard during the contraction of the palatine muscles in the act of yawn- 

 ing, during the forcing of air into the tympanum so as to make tense the 

 membrana tympani. 



Irritation or excitement of the auditory nerve is capable of giving rise to 

 movements in the body and to sensations in other organs of sense. In both 

 cases it is probable that the laws of reflex action, through the medium of the 

 brain, come into play. An intense and sudden noise excites, in every person, 

 closure of the eyelids, and, in nervous individuals, a start of the whole body 

 or an unpleasant sensation throughout the body like that produced by an 

 electric shock. 



THE SENSE OF EQUILIBRIUM. 



Although the utriculus, sacculus, and semicircular canals form the major 

 part of the labyrinth and are closely associated with the cochlea in develop- 

 ment, there is increasing evidence that these structures are not concerned 

 with hearing, but rather with a sense of equilibrium. This view has been 

 strengthened by recent investigations into the anatomical relations of the 

 different elements in the auditory nerve, figure 435. 



These structures have each a special modification of the sensory epithelium 

 which receives the vestibular branch of the eighth nerve. These epithelial 

 areas are differentiations of the embryonic ear pit, which is derived from the 

 epiblast. In fishes which have well-developed semicircular canals and vesti- 

 bule, this sensory epithelium has a common origin from the embryonic anlage 

 which gives rise to the ear, the branchial sense organ, and the lateral line 

 organs, all of which probably have static functions. 



The Semicircular Canals. The semicircular canals are connected 

 with the utriculus, are three in number on each side, and have been already 

 shown to lie in space practically at right angles to one another. Each is 

 filled with endolymph, and each has a special organ, the crista acustica, 

 which receives a division of the vestibular branch of the eighth nerve. 



