THE TEMPORAL BONE 



83 



Internal Surface. The internal surface of the mastoid portion presents a deep, 

 curved groove, the sigmoid fossa, which lodges part of the lateral sinus; and into 

 it may be seen opening the mastoid foramen, which transmits an emissary vein 

 from the lateral sinus to the posterior auricular or occipital vein and a small 

 artery, the mastoid branch of the occipital artery. A section of the mastoid 

 process (Figs. 49 and 50) shows it to contain a number of cellular spaces, com- 

 municating with one another, called the mastoid cells (cellulae mastoideae), which 

 exhibit the greatest possible variety as to their size and number. At the upper 

 and front part of the bone these cells are large and irregular, and contain air. 

 They diminish in size toward the lower part of the bone; those situated at the 

 apex of the mastoid process are quite small, representing spaces of cancellous 

 bone, and usually containing marrow. Occasionally they are entirely absent, and 



Mastoid antrum 



Tegmen tympani 



Prominence of extl. semicircular canal 

 Prominence of facial canal 

 Fenestra ovalis 



Bristle in canal for Tensor tympani 

 Processus cochleariformis 

 Bristle in hiatvs Fallopii 



Mooid cells 



Carotid canal 



Bony part of Eustachiantube 

 Promontory 

 Bristle in pyramid 

 Fenestra rotunda 

 Sulcus tympanicus 

 is ^ e * n stylomastoid foramen 



FIG. 49. Section through the petrous and mastoid portions of the temporal bone, showing the communication 

 of the cavity of the tympanum with the mastoid antrum. 



the mastoid is solid throughout. In addition to these pneumatic cells may be 

 seen a large, irregular cavity, the mastoid antrum (Figs. 49 and 50), situated at 

 the upper and front part of the section. This must be distinguished. from the 

 mastoid cells, though it communicates with them. The mastoid cells are not 

 developed until after puberty, but the mastoid antrum is almost as large at birth 

 as it is in the adult bone. The antrum and cells are rilled with air, and are lined 

 with a prolongation of the mucous membrane of the tympanum, which extends 

 into them through an opening, by which they communicate with the cavity of 

 the tympanum. 



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

 inflammation of the lining membrane of the fnrmpr 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 t.n the lateral sinus nr pn 



