May 11, 1883.] 



SCIENCE. 



383 



At the upper part of the tube, extending 

 into the tympanum, is the receptacle of the 

 tympanic tensor muscle. This is formed by a 

 thin, cylindrical scroll, commonly open along 

 its fore-part, but closed in the recent condition 

 by fibrous membrane. The upper extremity 

 of the scroll tapers, curves outwardh', and 

 projects as a short conical process ■" into the 

 tympanum. Sometimes the receptacle forms 

 a complete osseous tube, open only at the 

 ends. 



The internal auditory meatus is a cylindri- 

 cal canal, about a third of an inch long, ex- 

 tending directlj- outward from near the middle 

 of the posterior surface of the petrosa. The 

 bottom of the meatus is directed forward and 

 outward, and is applied to the base of the coch- 

 lea and to the A'estibule. It is divided into two 

 fossae by a transverse ridge, which expands, at 

 the extremities, into the front and back walls 

 of the meatus. In the upper fossa, internally, 

 is the aperture of the facial canal,' bj- which 

 the facial nerve leaves the meatus. The facial 

 canal advances a short distance, and commu- 

 nicates, through the hiatus,' with the anterior 

 surface of the petrosa. Turning abruptly out- 

 ward and backward in the upper part of the 

 inner wall of the tympanum, it then curves 

 downward in the posterior wall, and ends under 

 the name of the stylo-mastoid foramen. It is 

 sometimes open along its course at the upper 

 part of the tj'mpanum, but is then closed in 

 the recent state bj' fibrous membrane. Outward 

 from the entrance of the facial canal is a con- 

 cave recess, with a circular or oval group of 

 minute foramina, which communicate with the 

 superior cribriform macula of the vestibule. 

 On the outer extremity of the transverse ridge 

 of the bottom of the meatus there is a variable 

 number of little pits, or foramina, usually' two 

 or three, which likewise communicate with the 

 macula just mentioned. 



The lower fossa of the meatus is impressed 

 with a band-like spiral tract,* which is pierced 

 along its course with numerous minute foram- 

 ina. These sometimes exhibit a slight ten- 

 dencj' to arrangement in little groups in 

 longitudinal series ; though I have never seen 

 a specimen exhibiting even a well-marked ap- 

 proximation to the regularitj' represented in 

 fig. 725 of Sappej''s Traits d'anatomie, sec- 

 ond edition. The foramina communicate with 

 canals of the modiolus, and transmit the fila- 

 ments of the cochlear nerve. 



Centrally, at the termination of the spiral 

 tract, there is usuall3' a conspicuously larger 



' Processus cocbloarifori 

 ■• H. Fallopii. 



foramen, which gives passage to the central 

 artery of the modiolus. 



Above the commencement of the spiral tract, 

 and just below the transverse ridge, is a recess, 

 variably distinct, sometimes scarcely marked 

 from the spiral tract, sometimes forming a 

 conspicuous depression or pit, with a little cir- 

 cular group of minute foramina, which commu- 

 nicate with the middle cribriform macula of the 

 vestibule. 



In the outer wall of the meatus, near the 

 recess just indicated, is the aperture of a nar- 

 row canal, which is directed outwardljs and 

 terminates in a group of minute foramina, which 

 communicate with the inferior cribriform mac- 

 ula of the ampulla of the posterior semicir- 

 cular canal. The three groups ^ of foramina, 

 communicating with the cribriform maculae, 

 transmit the filaments of the three divisions of 

 the vestibular nerve. 



The tympanic cavity occupies a position at 

 the fore-part of the petrosa, beneath the teg- 

 men, and closed in front by the tympanic pjate. 

 It communicates outward with the external 

 auditory meatus, outward and backward with 

 the mastoid antrum, and inward and forward 

 with the eustachian tube. Closed externally 

 by the tympanic membrane, it forms the tym- 

 panum, or ear-drum, — an air-chamber interme- 

 diate to the external auditory meatus and the 

 labyrinth. Within it are contained the ear- 

 ossicles. 



The tympanic cavitj^ is obliquely placed 

 parallel with the long axis of the petrosa. It 

 maj' be regarded as consisting of two portions, 

 — tlie main chamber, which maj' be named the 

 atrium, situated directl}' opposite the external 

 auditory meatus ; and a recess above this, 

 which maj' be distinguished as the attic (fig. 



1,6). 



The atrium '^ is discoid in shape, and is de- 

 fined outwardl)' by the prominent edge of tlie 

 tj'mpanic orifice of the external auditory 

 meatus. Its usual dimensions are about half 

 an inch obliquelj', fore and aft, and in height, 

 and about two and a half lines from within 

 outward. The inner wall is next the labyrinth, 

 and in great part is visible through the exter- 

 nal auditory meatus. It exhibits a conspicu- 

 ous smooth eminence, the promontory , caused 

 b_y the projection of the cochlea. The back 

 part of this arches over a deep recess, looking 

 backward and outward, and having at its inner 

 side an aperture, the round window^- wliich 

 communicates with the cochlea, and, in the re- 

 cent state, is closed bj- the cochleo-tympanic 



