526 THE AUDITORY APPARATUS 



already (p. 241). Its origin from the ganglion geniculi of 

 the facial nerve can now be seen. 



The communicating branch to the lesser superficial petrosal 

 arises from the ganglion geniculi, and unites with the fibres 

 of the tympanic nerve which issue from the tympanic plexus. 



The external petrosal nerve is not always present. It 

 joins the sympathetic plexus which accompanies the middle 

 meningeal artery. 



The nerve to the stapedius muscle arises from the facial as 

 it passes downwards posterior to the pyramid. It enters the 

 base of the pyramid and thus reaches the stapedius muscle. 



The communicating twigs to the auricular branch of the 

 vagus arise a short distance above the stylo-mastoid foramen. 



Chorda Tympani. The chorda tympani represents to a 

 large extent the sensory fibres set free from the trunk of 

 the facial nerve. It is the largest branch given off by the 

 facial during its passage through the canalis fadalis. It takes 

 origin a short distance above the stylo-mastoid foramen, and 

 arching upwards and forwards, in a narrow canal in the 

 petrous portion of the temporal bone (the canaliculus chordae 

 tympani), it appears in the tympanum by passing through 

 the tympanic aperture of the canaliculus chordae, below the 

 base of the pyramid, and close to the posterior margin of 

 membrana tympani. The bony tunnel which it occupies can 

 easily be opened up in a decalcified bone, but is somewhat 

 difficult to expose in the hard bone. After entering the 

 tympanum the chorda tympani runs forwards, upon the upper 

 part of the membrana tympani, under cover of the mucous 

 layer. It crosses the handle of the malleus on the medial 

 aspect near its root. Finally, reaching the anterior end of 

 the tympanic cavity it crosses the anterior process (O.T. 

 gracilis) of the malleus, passes above the tensor tympani, and 

 traverses the medial end of the petro-tympanic fissure, which 

 conducts it to the exterior of the skull. From its exit to 

 its junction with the lingual nerve the chorda tympani has 

 already been traced (p. 182). 



Nervus Acusticus. In the internal acoustic meatus the 

 acoustic nerve lies at a lower level than the facial, and at the 

 bottom of the passage it splits into two parts, termed the 

 cochlear and vestibular divisions. The two divisions again sub- 

 divide and supply the different parts of the labyrinth of the 

 ear through the foramina of the lamina cribrosa (Fig. 209). 



