TIIK INTERNAL KMi 1057 



The tuba auditiva (Kustachian lube) (fig. 747) extends from the carotid (an- 

 terior) wall of the tympanic cavity interiorly, medially, and ventrally to the pharynx. 

 It is al'ont :i7 nun. ( I ."> in.) IOIILT. and throughout one-third of its length has a b'onv 

 wall, that of the other two-third,- being cartilaginous. '\ he osseous part (.-ee p 

 begins at the tympanic ostium on the anterior wall of the tympanic 1 1 is 



in relation medially and interiorly with the carotid canal, and gradually contract 

 to its irregular medial extremity, which is the narrowest point in the tube, and 

 is termed t lie isthmus. The cartilaginous part is (irmly attached to the o.-scous 

 and lies in a sulcus at the base of the angular spine of the sphenoid bone. It grad- 

 ually dilates in it- to the lateral wall of the pharynx, where its opening, phar- 

 yngeal ostium, is just posterior to the inferior nasal concha (turbinated bonei. The 

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

 as to form a troitirh-like structure, consisting of a medial and a lateral lamina, com- 

 pleted inferiorly by a membranous lamina formed of connective tissue. A small 

 1 )ort ion of the lumen in the superior part of the cartilaginous tube remains permanently 

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

 opened by the tensor and levator veli palatini muscles. The tnucoaa 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 mucous glands, especially near the 

 pharynx, where there is also some adenoid tissue. 



The development of the external and middle ear.- Embryologically the external and 

 middle curs have a common origin quite distinct from that which gives rise to the internal ear, and 

 are to be regarded as portions of the branchial arch apparatus, secondarily adapted to auditory 

 purposes. They are formed from the upper part of the first branchial cleft (see p. 9), and the 

 ailj.M (lit portions of the first and second branchial arches. 



I lie branchial cleft is represented in the human embryo by an external and a corresponding 

 internal or pharyngeal groove, separated by a thin membrane, there being no formation of an 

 actual cleft. The lower portion of the external groove closes, while the upper part persists as 

 the external acoustic meatus, the auricle being formed from the adjacent portions of the first 

 Utd second branchial arches. The first arch gives rise to the tragus, and the cms and upper portion 

 of the helix, and the second arch to the remainder of the helix, the anthelix, antitragus, and lobule. 



The membrane forming the floor of the groove and separating it from the pharyngeal groove 

 becomes the tympanic membrane, which is thus lined on its outer surface by ectoderm ana on its 

 inner by endoderni. The lips of the upper part of the pharyngeal groo've unite, so that the 

 portion of the groove is converted into a cavity, from which are formed the tympanic cavity and 

 the tuba auditiva (Kustachian tube); the lower portion of the groove remains open and is rep- 

 rv^cuted in the adult by the fossa in which the tonsil lies, and by the supratonsillar fossa. The 

 auditory ossicles and their muscles are derived from the neighbouring arches, the malleus and incus, 

 together with the tensor tympani, being derived from the first arch, while the stapes and stape- 

 clius come from the second. 



Tin- tympanic cavity is at first quite small, but later increases greatly, partly by the con- 

 densation of the loose areolar tissue which underlies its mucous membrane, the auditory ossicles 

 and their muscles being thus apparently brought within the cavity, and partly by the absorption 

 of the neighbouring bone. By this latter process the antrum and the tympanic and mastoid 

 cells arc formed, all these depressions or cavities being lined by mucous membrane continuous 

 with that of the tympanic cavity. 



THE INTERNAL EAR 



The internal ear is the essential part of the organ of hearing. It consists of a 



ity, the osseous labyrinth, contained .within the petrou> portion of the temporal 



bone, and enclosing a membranous labyrinth. The osseous labyrinth is divided 



into cochlea, vestibule, and semicircular canals, which have been described on 



pages 69 to 71. 



The membranous labyrinth, in which the acoustic nerve ends, lies within the 

 'nus labyrinth, the form of which it more or less closely resembles. It is much 

 smaller in diameter than the osseous, and its delicate walls are separated from the bone 

 by an endothelial-lined space which is tilled \\ith a fluid, the perilymph, while the fluid 

 which it itself contains is termed the endolymph. The portion of the membranous 

 labyrinth situated in the vestibule consists of two sacs, the utricle and saccule 

 (figs. 7~>~}. 7"i(i. 7">7). The utricle is an oval tubular sac. whose rounded end lies in the 

 superior and dorsal portion of the vestibule. It is here tiirhtly bound to the elliptic 

 receaa (t'uvea hemielliptica) by connective t issue and by the entrance of the filaments 

 of the utricular di vision of the acoustic nerve as they pass from the superior macula 

 cribrosa to the wall of the utricle. In the anterior part of the interior of the utricle, an 

 fi7 



