THE BONES OF THE HEAD 73 



the mastoid foramen, usually placed near the posterior border, for 

 a large emissary vein, which passes between the lateral sinus 

 internally and the outermost tributary of the occipital vein, or 

 the posterior auricular vein externally. In addition to these, there 

 is the minute arterial fissure on the outer surface of the mastoid 

 process below its centre for the mastoid branch of the occipital 

 artery. 



The inner surface presents the deep sinuous sigmoid groove, which 

 lodges a part of the lateral venous sinus, and from which the mastoid 

 foramen opens. The genu or bend of this groove and its descend- 

 ing limb lie behind the mastoid antrum. 



The superior border, thick and serrated, articulates with the back 

 part of the inferior border of the parietal. Near its anterior part 

 it presents the parietal notch, which receives a portion of the 

 parietal bone. The posterior border, also thick and serrated, 

 articulates with the inferior border of the tabular portion of the 

 occipital. 



The interior of the mastoid portion contains a number of cavities 

 lined with mucous membrane, called the mastoid air cells. These 

 open into an irregular chamber, known as the mastoid antrum, which 

 is situated at the upper part of the posterior wall of the tympanum, 

 and is lined by mucous membrane continuous with that of the 

 tympanum and mastoid cells. The upper part of the antrum com- 

 municates with the upper part or attic of the tympanum by an 

 opening which faces that of the Eustachian tube, but the lower part 

 is shut off from the tympanic cavity, and its floor is on a lower level 

 than the floor of that cavity, which explains the difficulty in the 

 drainage of fluid. The roof of the antrum, called tegmen antri, is 

 continuous with the tegmen tympani, and both enter into the forma- 

 tion of the middle fossa of the base of the skull. The outer wall 

 is formed by the squamo-mastoid junction in the region of the supra- 

 meatal triangle ; the floor and inner wall are constructed by the 

 petro-mastoid portion ; and the posterior wall represents that part 

 of the mastoid portion which bears the genu or bend and descending 

 limb of the sigmoid groove. The mastoid cells extend from the 

 antrum into the mastoid portion in a backward and downward direc- 

 tion, and are subject to variety as regards number and size. They 

 are aiTanged in two groups — horizontal or superior, and vertical or 

 inferior. The former are purely pneumatic or air cells, but the 

 latter are of two kinds, the upper ones being pneumatic, and the 

 lower ones, which extend to the tip of the mastoid process, being 

 diploetic and of large size. Superiorly the cells extend forwards 

 over the roof of the external auditory meatus, upwards as high as 

 the supramastoid crest, and inwards for a certain distance into the 

 petrous portion. They may also extend into the jugular process 

 of the occipital in old persons. At birth the mastoid cells are not 

 developed, but the antrum is present. 



The petrous portion is so named from its rocky consistence, and 

 its direction is inwards and forwards into the base of the skull. It 



