1092 SPECIAL SENSE ORGANS 



The Antrum Tympanicum 



The aperture (aditus) in the upper part of the posterior wall of the tympanum 

 leads into the chamber termed the antrum tympanicum. This is a comparatively 

 large cavitj", of irregular form, lying mainly behind but also somewhat above and 

 lateral to the tj^mpanum, and extends to the medial end of the external auditory 

 meatus. It is lined by mucous membrane, continuous with that of the tympanic 

 cavity, and into it open the mastoid cells (cellulse mastoidese). These cells are 

 small, irregular cavities in the interior of the mastoid process and they com- 

 municate with one another freely. They vary exceedingly in their size and 

 asrangemeut. 



The antrum tympanicum has a roof, formed by the tegmen tyrapani, a posterior wall, 

 separating it from the bend of the transverse sinus, a lateral wall, lying about 10 mm. from the 

 surface of the head, a medial wall, and an anterior wall (see also p. 78). 



The Auditory (Eustachian) Tube 



The auditory tube [tuba auditiva] (Eustachian tube) (fig. 829) extends from 

 the carotid (anterior) wall of the tympanic cavity inferiorly, medially, and 

 anteriorly to the pharynx. It is about 37 mm. (1.5 in.) long. In the lateral 

 one-third of its length it has a bony wall, while in the medial two-thirds 

 this wall is cartilaginous. The osseous part (see p. 74) begins at the tympanic 

 ostium on the anterior wall of the tympanic cavity. It is in relation medially 

 and inferiorly with the carotid canal, and gradually contracts to its irregular 

 medial extremity, which is the narrowest point in the tube, and is termed the 

 isthmus. The cartilaginous part is firmly attached to the osseous and lies in a 

 sulcus at the base of the angular spine of the sphenoid bone. It gradually dilates 

 in its passage to the lateral wall of the pharynx, where its opening, pharyngeal 

 sotium, is just posterior to the inferior nasal concha (turbinated bone). The 

 walls of the cartilaginous part are formed by a cartilaginous plate which is folded 

 so as to form a trough-like structure, consisting of a medial and a lateral lamina, 

 completed inferiorly by a membranous lamina formed of connective tissue. 



A small portion of the lumen in the superior part of the cartilaginous tube remains per- 

 manently open; elsewhere the walls are in contact, except during deglutition, when they are 

 opened by the tensor veli palatini muscles. The mucosa of the osseous part is thin, and firmly 

 attached to the bony wall, but in the cartilaginous part it becomes thicker, looser, and folded, 

 and contains nmcous glands, especially near the pharynx, where there is also some adenoid 

 tissue. 



3. THE INTERNAL EAR 



The internal ear [auris interna] is the essential part of the organ of hearing. 

 It consists of a cavity, the osseous labyrinth, contained within the petrous portion 

 of the temporal bone, and enclosing a membranous labyrinth. The osseous 

 labyrinth is divided into cochlea, vestibule, and semicircular canals (seep. 80), 

 and the accompanying figures (338-838 )show their position and relations. 



It will be noticed that the vestibule forms a central chamber, from which the semicircular 

 canals and the cochlea branch off; the former from the superior and dorsal portion, and the latter 

 from the ventral and inferior. 



It will further be noticed that the bony wall of this vestibule shows depressions and ridges on 

 its interior, which are associated with parts of the membranous labyrinth, viz., an upper recess 

 for the utricle (fovea hemielliplica) and a lower recess for the saccule (fovea hcmispherica). 

 There arc openings in the bony wall for the entrance of nerves to the different parts of the 

 membranous labyrinth, and for the transmission of the ductus endolymph;iticus, as well 

 as the small openings of the semicircular canals (ducts) and the opening of the cochlear canal 

 (or duct). 



The membranous labyrinth, in which the auditory (acoustic) nerves (cochlear 

 and vestibular) end, lies within the osseous labyrinth, the form of which it more 

 or less closely resembles. Thus the membranous semicircular ducts lie within 

 the bony semicircular canals, the membranous cochlear duct within the bony 

 cochlea; while the vestibule contains two small membranous sacs, the utricle 

 and saccule, with their connections. The membranous structures are much 

 smaller in diameter than the osseous, and are partially sejiaratcd from the bone 

 by an endothclial-lined space which is filled with a fluid, the perilymph. The 



