OSTEOLOGY 





The inner surface of the mastoid portion presents a deep, curved groove, the 

 sigmoid sulcus, which lodges part of the transverse sinus; in it may be seen the 

 opening of the mastoid foramen. The groove for the transverse sinus is separated 

 from the innermost of the mastoid air cells by a very thin lamina of bone, and even 

 this may be partly deficient. 



Borders. The superior border of the mastoid portion is broad and serrated, for 

 articulation with the mastoid angle of the parietal. The posterior border, also 

 serrated, articulates with the inferior border of the occipital between the lateral 

 angle and jugular process. Anteriorly the mastoid portion is fused with the 

 descending process of the squama above; below it enters into the formation of 

 the external acoustic meatus and the tympanic cavity. 



A section of the mastoid process (Fig. 139) shows it to be hollowed out into a 

 number of spaces, the mastoid cells, which exhibit the greatest possible variety 

 as to their size and number. At the upper and front part of the process they are 

 large and irregular and contain air, but toward the lower part they diminish in 

 size, while those at the apex of the process are frequently quite small and contain 

 marrow; occasionally they are entirely absent, and the mastoid is then solid 

 throughout. In addition to these a large irregular cavity is situated at the upper 

 and front part of the bone. It is called the tympanic antrum, and must be distin- 

 guished from the mastoid cells, though it communicates with them. Like the mas- 

 toid cells it is filled with air and lined by a prolongation of the mucous membrane 

 of the tympanic cavity, with which it communicates. The tympanic antrum is 

 bounded above by a thin plate of bone, the tegmen tympani, which separates it 

 from the middle fossa of the base of the skull; below by. the mastoid process; later- 

 ally by the squama just below the temporal line, and medially by the lateral semi- 

 circular canal of the internal ear which projects into its cavity. It opens in front 

 into that portion of the tympanic cavity which is known as the attic or epitympanic 

 recess. The tympanic antrum fs a cavity of some considerable size at the time of 

 birth; the mastoid air cells may be regarded as diverticula from the antrum, 

 and begin to appear at or before birth; by the fifth year they are well-marked, 

 but their development is not completed until toward puberty. 



Petrous Portion (pars petrosa [pyramis]). The petrous portion or pyramid is 

 pyramidal and is wedged in at the base of the skull between the sphenoid and 

 occipital. Directed medialward, forward, and a little upward, it presents for 

 examination a base, an apex, three surfaces, and three angles, and contains, in 

 its interior, the essential parts of the organ of hearing. 



Base. The base is fused with the internal surfaces of the squama and mastoid 

 portion. 



Apex. The apex, rough and uneven, is received into the angular interval between 

 the posterior border of the great wing of the sphenoid and the basilar part of the 

 occipital; it presents the anterior or internal orifice of the carotid canal, and 

 forms the postero-lateral boundary of the foramen lacerum. 



Surfaces. The anterior surface forms the posterior part of the middle fossa of 

 the base of the skull, and is continuous with the inner surface of the squamous 

 portion, to which it is united by the petrosquamous suture, remains of which are 

 distinct even at a late period of life. It is marked by depressions for the convolu- 

 tions of the brain, and presents six points for examination: (1) near the center, 

 an eminence (eminentia arcuata) which indicates the situation of the superior semi- 

 circular canal; (2) in front of and a little lateral to this eminence, a depression indi- 

 cating the position of the tympanic cavity: here the layer of bone which separates 

 the tympanic from the cranial cavity is extremely thin, and is known as the 

 tegmen tympani; (3) a shallow groove, sometimes double, leading lateralward and 

 backward to an oblique opening, the hiatus of the facial canal, for the passage of 

 the greater superficial petrosal nerve and the petrosal branch of the middle men- 



