THE TEMPORAL BONE. 



177 



expands forward and deeper than the original ring, making the front part of the 

 tympanic plate, bounding the cavity of the tympanum and the Eustachian tube 

 externally. The tympanic cavity, or the middle ear, lies between the petro-mastoid 

 and the tympanic portion, the roof and floor being developed from the former. 

 (3) The sqicamous portion is external and above. It forms a part of the side of the 

 skull, the roof of the external meatus where the tympanic portion is deficient, the 

 articulating surface for the jaw, and a part of the mastoid process. There is also the 

 long, s\^n<\^x styloid process, which is a part of the hyoid bar of the second visceral 

 arch of the embryo. It begins as an ossification of a distinct piece of cartilage, but 

 joins the petro-mastoid. The following description is that of the adult bone. 



The Squamous Portion.' — Most of this is a thin vertical layer forming part 

 of the wall of the skull, joined below by a horizontal one which forms a small part of 

 the base of the skull, the articulating surface for the jaw, and the roof of the external 



Fig. 196. 



Eminentia articuiaris 



Zygoma 



Glenoid fossa 



Postglenoid tubercle 

 Fissure of Glaser 



Tympanic plate 



External auditory Meatus 



Eustachian tube 



Carotid canal 



Cochlea 



Semicircular canal 

 Facial canal 



Antrum 



Groove for lateral sinus 

 Horizontal section through right temporal bone, seen from below. 



auditory meatus. The edge of the vertical part is convex except below. The upper 

 and posterior borders overlap the parietal bone by a broad bevelled surface. The 

 anterior border joins the great wing of the sphenoid, overlapping above and over- 

 lapped below, where it passes into the horizontal part. The posterior angle of the 

 vertical portion sends downward the postauditory process, from which the upper part 

 of the mastoid, including some of the mastoid cells, is developed. The squama, 

 mastoid sntzire, separating this from the mastoid portion, is usually lost in the second 

 year. When it persists, it shows that the anterior portion of the mastoid down to 

 the lower border of the external meatus, or even lower, is formed from the squamosal. 

 Its surface is smoother than that of the mastoid proper. A small, particularly 

 smooth, but inconstant patch situated on the level of the upper part of the meatus, 

 one centimetre or more behind it, marks the position of the antrum. The thick- 

 ness of the bone at this place, which is that of note-paper in the infant reaches 



^ Pars squamosa. 

 12 



