1588 A MANUAL OF ANATOMY 



the malleus, incus, and stapes, are to be studied in situ. The insertions ol 

 the tensor tympani and stapedius muscles are to be made evident, and tlieir 

 actions studied. The stapedius muscle will be found to enter the tympanum 

 through an opening on the summit of the pyramid on the posterior wall. 

 The dissector need not expect to be able to show the ligaments of the tym- 

 panic ossicles, but he may make an effort to show the chorda tympani nerve. 

 This nerve enters the tympanum through the iter chordae posterius on the 

 posterior wall. It then passes forwards upon the membrana tympani, lying 

 between its mucous and fibrous layers, and it leaves the tympanic cavity by 

 passing through the iter chordae anterius, or canal of Huguier, at the inner 

 end of the fissure of Glaser. The inner wall of the tympanum, with the 

 fenestra ovalis, occupied by the foot -piece of the stapes and annular ligament, 

 the promontory, and the fenestra rotunda, closed by the secondary membrane 

 of the tympanum, is to receive careful attention. The dissector may note the 

 position of the attic or epitympanic recess, which lies above the level of the upper 

 margin of the membrana tympani, and which is divided incompletely into two 

 compartments, outer and inner, by the head and neck of the malleus, and the 

 body and short process of the incus. In connection with the oufer compart- 

 ment, he may try and familiarize himself with the pouch of Prussak, which 

 is bounded externally by the membrana flaccida, or Shrapnell's membrane. 

 For the pouches of Troltsch, see the description of the tympanum in the 

 text. The osseous part of the Eustachian tube is to be carefully studied, 

 and the 'mastoid cells are to be examined. For the latter purpose the mastoid 

 process of the temporal bone is to be laid open. 



The internal ear or labyrinth can only be effectually studied from specimens 

 which have been specially prepared. By means of the chisel and bone-pliers, 

 however, the dissector will obtain a fair amount of information regarding this 

 exceedingly complicated region. The meatus auditorius internus having been 

 already laid open, the roof of the labyrinth is to be removed in a fragmentary 

 fashion. The coiled cochlea, with its two and a half turns, is to be noted, its 

 base being directed towards the deep end of the meatus auditorius internus. 

 Behind the cochlea will be found the vestibule, and at the back part of the 

 vestibule the three semicircular canals are to be noted. 



Contents of the Meatus Auditorius Internus. — This meatus having been 

 already laid open, the auditory nerve, the pars intermedia of Wrisberg, the 

 meatal portion of the facial nerve, and the internal auditory artery are to be 

 studied. The auditory nerve will be found to break up into two divisions 

 — vestibular and cochlear. The meatal portion of the facial nerve will be 

 found on the upper and anterior aspect of the auditory nerve ; and the pars 

 intermedia of Wrisberg lies between the two. The auditory and facial nerves 

 are connected by two branches. The internal auditory artery is seldom in- 

 jected. 



Facial Nerve in the Aqueduct of Fallopius. — To make a complete dissection 

 of this very intricate aqueduct, the temporal bone requires to be decalci- 

 fied. An effort should, however, be made to display the aqueduct in the 

 natural bone. The tympanic cavity having been already exposed, the posi- 

 tion of the second portion of the aqueduct will be recognised by a ridge on the 

 upper part of the inner wall of the tympanum above the fenestra ovalis. It 

 is here to be laid open with the chisel and mallet. The first portion of the 

 aqueduct, which leads from the deep end of the meatus auditorius internus, 

 may also be laid open with the chisel and mallet in such a manner as to follow 

 the course of the facial nerve, which is horizontally outwards between the 

 cochlea and vestibule to the inner wall of the tympanum. The third or 

 vertical part of the aqueduct, which descends behind the tympanum, to the 

 stylo-mastoid foramen, can best be laid open by removing the mastoid process 

 of the temporal bone. This process is to be sawn across in the vertical-trans- 

 verse direction, posterior to the level of the stylo-mastoid foramen, into 

 which a probe may be passed for guidance. Another cut is to be made from 

 behind forwards, also in the vertical direction, to meet the preceding cut, 

 close to the stylo-mastoid foramen. The included bone having been removed, 

 the third part of the aqueduct may be reached and laid open with the chisel. 

 In dissecting the facial nerve in the aqueduct, the following structures are to be 



