642 STRUCTURE OF THE BULB, PONS, AND MID -BRAIN [CH. XLIV. 



the paralysed side a characteristic, vacant look, from the absence of all 

 expression : the angle of the mouth is lower, and the paralysed half 

 of the mouth .looks longer than that on the other side ; the eye has 

 an unmeaning stare, owing to the paralysis of the orbicularis palpe- 

 brarum. All these peculiarities are exaggerated when at any time 

 the muscles of the opposite side of the face are made active in any 

 expression, or in any of their ordinary functions. In an attempt to 

 blow or whistle, one side of the mouth and cheeks acts properly, but 

 the other side is motionless, or flaps loosely at the impulse of the 

 expired air ; so, in trying to suck, one side only of the mouth acts ; 

 in feeding, the lips and cheek are powerless, and on account of 

 paralysis of the buccinator muscle, food lodges between the cheek 

 and gums. 



The motor fibres originate from a nucleus in the ventricular floor 

 below that of the fifth and to the outer side of that of the sixth 

 nerve. As they curve over the nucleus of the sixth, they give off a 

 bundle of fine fibres which cross the raphe, but their destination is 

 unknown. The facial nucleus receives collaterals from the sensory 

 tracts in the reticular formation. 



The seventh nerve, however, is not wholly motor. The geniculate 

 ganglion on it is of spinal type ; the fibres which arise from it pass 

 centrally into the pars intermedia of Wrisberg, which enters the pons 

 between the seventh and eighth nerves ; these, like other sensory fibres, 

 divide into ascending and descending branches ; the latter have been 

 traced down to the sensory nucleus of the glosso-pharyngeal nerve. 

 The peripheral branches of the geniculate ganglion cells pass into the 

 large superficial petrosal and chorda tympani, the gustatory fibres of 

 which they probably furnish. The origin of the secretory fibres of 

 the chorda tympani is still a matter of uncertainty. 



The eighth nerve (auditory) runs into the hinder margin of the 

 pons by two roots. One winds round the restiform body dorsal to 

 it, and is known as the dorsal or cochlear division ; the other passes 

 ventro-mesially on the other side of the restiform body, and is known 

 as the ventral or vestibular division. 



We will take these two parts separately. The fibres of the 

 cochlear nerve take origin from the bipolar nerve-cells of the spiral 

 ganglion of the cochlea; the peripheral axons ramify among the 

 hair cells of the organ of Corti, and the central axons pass towards 

 the pons ; as they enter they bifurcate, and some pass to and arborise 

 around a collection of nerve-cells situated between the two roots and 

 the restiform body, called the accessory auditory nucleus ; the remain- 

 ing fibres terminate similarly in a collection of cells in the grey matter 

 overlying the restiform body, and extending into the ventricular 

 floor in its widest part. This is called the ganglion of the root, and 

 the mass of grey matter is termed the acoustic tubercle. The auditory 



