68 THE SKELETON. 



situated at the upper and front part of the section. It is called the mastoid 

 antrum, and must be distinguished from the raastoid cells, though it communicates 

 with them. It is filled with air, and is lined with a prolongation of the mucous 

 membrane of the tympanum, which extends into it through an opening, by which 

 it communicates with the cavity of the tympanum. The mastoid antrum is 

 bounded above by a thin plate of bone, which separates it from the middle fossa 

 of the base of the skull on the anterior surface of the petrous portion of the 

 temporal bone ; below by the mastoid process ; externally by the squamous por- 

 tion of the bone just below the supra-mastoid crest ; and internally by the external 

 semicircular canal of the internal ear which projects into its cavity. The opening 

 by which it communicates with the tympanum is situated at the superior internal 

 angle of the posterior wall of that cavity, and opens into that portion of the 

 tympanic cavity which is known as the attic or epitympanic recess ; that is to say, 

 that portion of the tympanum which is above the level of the membrana tympani. 

 The mastoid cells, like the other sinuses of the cranium, are not developed 

 until after puberty; hence the prominence of this process in the adult: the 

 mastoid antrum, on the other hand, is of large size at birth. 



In consequence of the communication which exists between the tympanum and mastoid cells, 

 inflammation of the lining membrane of the former cavity may easily travel backward to that of 

 the antrum, leading to caries and necrosis of their walls and the risk of transference of the 

 inflammation to the lateral sinus or encephalon. 



Borders. The superior border of the mastoid portion is broad and rough, its 

 serrated edge sloping outward, for articulation with the posterior inferior angle of 

 the parietal bone. The posterior border, also uneven and serrated, articulates 

 with the inferior border of the occipital bone between its lateral angle and jugular 

 process. 



The Petrous Portion (TT^T/JOC, a stone), so named from its extreme density and 

 hardness, is a pyramidal process of bone wedged in at the base of the skull 

 between the sphenoid and occipital bones. Its direction from without is inward, 

 forward, and a little downward. It presents for examination a base, an apex, 

 three surfaces, and three borders, and contains, in its interior, the essential parts 

 of the organ of hearing. The base is applied against the internal surface of the 

 squamous and mastoid portions, its upper half being concealed ; but its lower 

 half is exposed by the divergence of those two portions of the bone, which brings 

 into view the oval expanded orifice of a canal leading into the .tympanum, the 

 meatus auditorius externus. The curved tympanic plate forms the anterior wall, 

 the floor, and a part of the posterior wall of this meatus, while the squamous 

 temporal completes it above and behind. The entrance to the meatus- is bounded 

 throughout the greater part of its circumference by the auditory 'process, which is 

 the name applied to the free rough margin of the tympanic plate, and which 

 gives attachment to the cartilaginous portion of the meatus. Superiorly the 

 entrance to the meatus is limited by the posterior root of the zygoma. 



The apex of the petrous portion, rough and uneven, is received into the 

 angular interval between the posterior border of the greater wing of the sphenoid 

 and the basilar process of the occipital ; it presents the anterior or internal orifice 

 of the carotid canal, and forms the posterior and external boundary of the fora- 

 men lacerum medium. 



The anterior surface of the petrous portion (Fig. 32) forms the posterior part 

 of the middle fossa of the skull. This surface is continuous with the squamous 

 portion, to which it is united by a suture, the petro-squamous suture, the remains 

 of which are distinct even at a late period of life. It presents six points for 

 examination : 1, an eminence near the centre, which indicates the situation of 

 the superior semicircular canal; 2, in front and a little to the outer side of this 

 eminence a depression indicating the position of the tympanum ; here the layer 

 of bone which separates the tympanum from the cranial cavity is extremely 

 thin, and is known as the tegmen tympani; 3, a shallow groove, sometimes 



