CHAPTEE VII. 



The Medulla (Continued). 



At the level where the Glosso-pharyngeus roots pass laterally to reach 

 the surface of the medulla one may recognize, on the median side of them, 

 a new nucleus: an acusticus nucleus. 



This is the region of the origin of that cranial nerve whose relationa 

 are least known in the lower vertebrates. The investigations of recent 

 years have finally made the Acusticus better understood in mammals. But 

 even in birds and still more so in lower vertebrates we have to depend on 

 what may be learned from sections. As yet, no one has attempted to study 

 so extremely complicated a nerve by the degeneration-method or the de- 

 velopment-method. But sections alone of this level of the medulla where 

 the structure is so complicated give, only too easily, occasion for misinter- 

 pretations. I will at present, then, confine myself only to that which has 

 been with certainty established. 



The Acusticus always contains one portion which passes to the vesti- 

 bule and one to the cochlea. The first, which supplies the labyrinth, is, as 

 numerous experimenters have demonstrated, of especial importance for the 

 maintenance of the equilibrium of the body. Ewald's experiments have 

 shown how every fluctuation of the fluid of the lymph leads, through the 

 agency of this nerve, to the disturbance of the equilibrium. They have 

 also proved that the vestibule is especially important for the maintenance 

 of muscle-tonus throughout the body. The cochlea is only slightly de- 

 veloped in fishes, bxit in birds it reaches a fair development. Corresponding 

 to these observations the majority of the fibers of the auditory nerve go to 

 the labyrinth and saccule. In mammals, for the first time, the portion of 

 the nerve which supplies the cochlea becomes large. 



In the brain one may recognize that in all lower vertebrates the major 

 part of the auditory nerve ends in a striking knob-like nucleus, which is 

 laterally located in the medulla where the peduncles of the cerebellum come 

 down. Lodged in the angle between the medulla and cerebellum and as 

 high up the vertebrate scale as to the birds, always covered with a formation 

 similar to the cortex of the cerebellum, lies this great nucleus of the Aeusti- 



(91) 



