78 



THE SKELETON 



tympanum; and (3) the Eustachian tube. They point out that "the cleft is primarily con- 

 tinuous, and however much it may be altered in shape and modified in parts to form these three 

 cavities, that continuity is never lost." It will be clear that the mastoid antrum, according to 

 this view, is not an outgrowth from the tj^mpanum, but is simply the lateral end of the middle- 

 ear cleft. 



The tympanic cavity maj^ be divided into three parts. The part below the 

 level of the superior margin of the external auditory meatus is the tympanum 

 proper ; the portion above this level is the epitympanic recess or attic ; it receives 

 the head of the malleus, the body of the incus, and leads posteriorly into the 

 recess known as the mastoid antrum. The third part is the downward extension 

 known as the hypotympanic recess. 



The tympanic or mastoid antrum. — -The air-cells which in the adult are found in the 

 interior of the mastoid portion of the temporal bone open into a small cavity termed the 

 mastoid antrum. This is an air-chamber, communicating with the attic of the tympanum, 

 and separated from the middle cranial fossa by the posterior portion of the tegmen tympani. 

 The floor is formed by the mastoid portion of the petrosal, and the lateral wall by the squamosal, 

 below the temporal ridge. In children the outer wall is exceedingly thin, but in the adult it is 

 of considerable thickness. The lateral semicircular canal projects into the antrum on its 



I 



Fig. 



98. — Temporal Bone at Birth Dissected from above and behind to show the Semi- 

 circular Canals and the Mastoid Antrum. (Enlarged \.) 



^r^ 



Opening into tympanum 



Superior semicircular canal 



Mastoid antrum 



Lateral semicircular canal 



Posterior semicircular canal 



medial wall, and is very conspicuous in the foetus. Immediately below and in front of the 

 canal is the facial nerve, contained in the facial canal. 



The mastoid antrum has somewhat the form of the bulb of a retort (Thane and Godlee) 

 compressed laterally, and opening by its narrowed neck into the attic or epitympanic recess. 

 Its dimensions vary at different periods of life. It is well developed at buth, attains its maxi- 

 mum size about the third year, and diminishes somewhat up to adult life. In the adult the 

 plate of bone which forms the lateral wall of the antrum is 12 to 18 mm. (i to f in.) in thickness, 

 whereas at birth it is about 1.8 mm. d',, in.) or less. The deposition of bone laterally occurs, 

 therefore, at average rate of nearly 1 mm. a year in thi<-kness. In the adult the antrum is 

 about 12 mm. (^ in.) from front to back, 9 mm. (f in.) from above downward, and 4.5 mm. 

 (i\ in.) from side to side. 



A canal occasionally leads from the mastoid antrum through the petrous bone to open in 

 the recess which indicates the position of the floccular fossa; it is termed the petro-raastoid 

 canal. (Grubcr.) 



The facial (Fallopian) canal. — This canal begins at the anterior angle of the superior fossa 

 of the internal auditory meatus, and passes forward and laterally above the vestibular portion 

 of the internal our for a distance of 1.5-2.0 mm. At the lateral end of this portion of its course 

 it becomes dilated to accommodate the geniculate ganglion, and then turns abruptly back- 

 ward and runs in a horizontal ridge on the medial wall of the tympanum, lying in the angle 

 between it and the toginon tympani, immediately above the fenestra vestibuli, and extending 

 as far backward as the entrance to the ma.stoid antrum. Here it comes into contact with the 

 inferior a.speot of the projo(;t,ion formed by the lateral scmichcular canal, and then turns verti- 

 ftally downward, running in t he angle between the medial and posterior walls of the tympanum 

 to terminate at the stylo-ina.stoid foramen. 



The canal is traversed f)y the facial nerve. Numerous openings exist in the walls of this 

 passage. At its abrupt bend, or genu, the greater and smaller superficial petrosal nerves escape 

 from, and a branch from the middle meningeal artery enters, the canal, and in the vertical part 

 of its course the cavity of the pyramid opens into it. There is also a small orifice by which the 

 auricular branch of the vagus joins the facial, and near its termination the iter chordse posterius 

 for the chorda tympani nerve leads from it into the tympanum. 



