THE TEMPORAL BONES. 



127 



i bevelled at the expense of its inner table, except in front, where the margin is thick 

 ' and stout. There it articulates with the great wing of the sphenoid, its union with 

 ithat bone extending to near the anterior part of the summit of the curve, behind 

 which it is united to the parietal, overlapping the squamous border of that bone ; 

 i posteriorly the free margin of the squamous part ends at an angle formed between 

 it and the mastoid process called the incisura parietalis. 



Pars Tympanica. The tympanic part of the temporal bone forms the anterior, 

 i lower, and part of the posterior wall of the external acoustic meatus. Bounded 

 in front and above by the petro-tympanic fissure, it forms the posterior wall of the 

 | non-articular part of the mandibular fossa. Fused medially with the petrous part, its 

 lower edge, sharp and well defined medially, splits to enclose the root of the 

 projecting styloid process, and is hence called the vagina processus styloidei 

 (sheath of the styloid process). Laterally it unites with the anterior part of the 



Groove for middle 

 meningeal artery 



Arcuate eminence or 

 eminence of superior 

 semicircular canal 



Parietal notch 



Groove for superior 

 petrosal sinus 



Petro-squamous suture 



Carotid canal ^^^^Groove for sigmoid 



_ sinus 



"^^^^in x yJ^5>'' 



Styloid proces 



Inner surface of mastoid process 

 Groove for inferior petrosal sinus 



FIG. 137. THE RIGHT TEMPORAL BONE (Cerebral aspect). 



The squamous part is coloured blue ; the petro-mastoid part, red. 

 The styloid process and the zygoma are left uncoloured. 



mastoid process, and higher up with the descending process of the squamous part, from 

 both of which it is separated by the tympano-mastoid fissure, through which the 

 auricular branch of the vagus escapes. Its free border, which forms the anterior, 

 lower, and part of the posterior border of the external acoustic meatus, is usually 

 somewhat thickened and rough, and serves for the attachment of the cartilaginous 

 part of the external acoustic meatus. 



The meatus acusticus externus (external acoustic meatus) is directed obliquely 

 inwards and a little forwards, and describes a slight curve, the convexity of which 

 is directed upwards ; of oval form, its long axis, close to its orifice, is nearly vertical, 

 but, as it passes inwards, inclines somewhat forwards so as to give a twist to the 

 canal. The depth of the canal to the attachment of the membrana tympani 

 averages from 14 to 16 mm. The superior margin of the outer orifice overhangs 

 considerably the lower edge, but owing to the obliquity of the inner aperture, to 

 which the membrana tympani is attached, the superior wall of the osseous canal 

 only exceeds the length of the lower wall by one or two millimetres. 



Pars Petrosa et Pars Mastoidea. The petro-mastoid part of the temporal 



