406 



PHYSIOLOGY 



CHAP. 



To cerebellum 



the sense-organs; here we need only say that the physiological 

 expression, acoustic nerve, applies only to the cochlear branch and 

 not to the whole nerve, since the vestibular branch has nothing 

 to do with hearing. With the earliest experiments of Flourens 

 (1828-30), who may be called the founder of the physiology of the 

 semicircular canals, the important fact became evident that all 

 lesions or injuries of the labyrinth are followed by specific motor 

 disorders without loss of hearing ; while deafness without motor 

 disturbance is the effect of destroying the cochlea. To prove that 

 the organs innervated by the vestibular nerve have quite a 



different function from the 

 cochlea, we may cite the 

 facts adduced by Bateson 

 (1890), Kreidl (1895), Lee 

 (1898), and others, showing 

 that fishes, which have no 

 cochlea, have no proper 

 sense of hearing, i.e. they 

 do not react to ordinary 

 sound vibrations. When 

 fish are excited by ex- 

 plosions or other loud noises 

 this cannot be due to stimu- 

 lation of the labyrinth, 

 because approximately the 

 same reactions are ex- 

 hibited when the labyrinth 

 has been removed. These 

 sounds therefore excite the 

 tactile sense in fishes the 

 auditory sense, as we shall 

 presently see, being only a 

 specialisation of this. 

 Doubt was, however, cast on these experiments on hearing in 

 fishes by Parker (1903). He noted that certain kinds of fish, 

 although devoid of cochlea, reacted to the vibration of a violin 

 string, or even to the note of a tuning-fork transmitted through 

 water, by modifications in the movements of their fins and their 

 respiratory rhythm. According to Parker, tjiese reactions dis- 

 appear almost entirely on destroying the labyrinth. 



As the two branches of the 8th nerve are analogous to two 

 dorsal spinal roots, so the *7th cerebral nerve the Facial corre- 

 sponds to a large ventral root, or more properly to the union of a 

 number of such roots. It takes origin in the lower part of the 

 pons from a nucleus of large ganglion cells, which lies at about 

 the level of the 6th nerve, and somewhat higher and more 

 ventral than the nucleus of the vestibular branch of the 8th 



Fio. 213. Plan of roots of acoustic nerve. (Thane.) 

 The outline represents a section at the junction of 

 the bulb with the pons : VIII.M, vestibular division ; 

 vin. L, cochlear division of and itory nerve ; N. vn i. ACC, 

 accessory nucleus ; G.L.R., lateral nucleus ; N.VIII.D, 

 dorsal nucleus ; AV, bulbo-spinal root of 5th nerve. 



