8o THE NERVOUS SYSTEM 



dorsal to the bundles of the pyramidal tract and medial 

 to the sensory nucleus of the trigeminal (Fig. 42). After 

 they leave the nucleus, the efferent fibres pass backwards 

 and curl round the nucleus of the sixth nerve, at the 

 same time forming an elevation in the floor of the fourth 

 ventricle (p. 56). They then turn forwards and pass through 

 the whole substance of the pons, finally emerging near its 

 lower border (Fig. 8). 



The motor part of the facial, the nervus intermedius, and 

 the eighth nerve pass laterally together through the sub- 

 arachnoid space and enter the petrous part of the temporal 

 bone through the internal acoustic meatus. 



At the bottom of the internal acoustic meatus, the facial 

 and the nervus intermedius enter a small canal, in which 

 they pass laterally to the medial wall of the tympanic cavity. 

 At this point the canal bends sharply backwards, and a 

 small swelling, termed the geniculate ganglion^ is situated 

 on the facial nerve. It is from this ganglion that the nervus 

 intermedius arises. The course taken by the sensory fibres 

 of the seventh is described on page 84. 



As the facial canal passes backwards along the medial 

 wall of the tympanum, the facial nerve is separated from 

 the middle ear only by a thin plate of bone, which may 

 readily become necrosed in otitis media. Opposite the aditus 

 to the tympanic (mastoid) antrum (p. 206), the nerve makes 

 a second bend, this time in a downward direction, and 

 reaches the stylo-mastoid foramen on the inferior surface of 

 the skull. As it descends in the last stage of its passage 

 through the temporal bone, the facial nerve is joined by the 

 chorda tympani and it gives off the nerve of supply to the 

 stapedius muscle. 



The stapedius muscle arises within the posterior wall of the 

 tympanic cavity and passes forwards to be inserted into the 

 neck of the stapes (p. 205). It would appear to act as an 

 antagonist of the tensor tympani, for the condition of 

 hyperacousis results when the stapedius is paralysed. In this 



