1336 



CLINICAL AND TOPOGRAPHICAL ANATOMY 



here much less grave in the adult than would otherwise be the case, the inner layer (table), if now 

 separated from the outer, protecting the brain. Mr. Hilton showed that the absence of any- 

 external prominence here does not necessarily imply the absence of a sinus, as this may be formed 

 by retrocession of the internal layer. In old people these sinuses may enlarge by the inner 

 layer following the shrinking brain. Again, prominence of the supraciliary and frontal 

 eminences does not necessarilj- point to the existence of a sinus at all, being due merely to a 

 heaping up of bone. 



The mastoid cells are arranged in two groups, of the utmost importance in that 

 frequent and fatal disease, inflammation of the middle ear: — (A) The upper, or 

 'antrum,' present both in early and late life, horizontal in direction, closely adja- 

 cent to and communicating with the tympanum. (B) The lower, or vertical. 

 This group is not developed in early life. 



A. Tympanic antrum (fig. 1088). — This is a small chamber lying behind the 

 tympanum, into the upper and back part of which (epitympanic recess) it opens. 

 Its size varies, especially with age. Almost as large at birth, it reaches its maxi- 

 mum (that of a pea) about the third or fourth year. After this its size usually 

 diminishes somewhat, owing to the development of the encroaching bone around 



Fig. 1087. — Temporal Bone, showing Suprameatal Triangle. (Barr.) 

 The lower part of the transverse sinus is here placed too far back to be relied upon with con- 



stant accuracy. 



Root of zygoma 



Transverse 

 sinus 



Suprameatal 



liyiif Position for 

 -*^ perforating 

 vertical cells 



Line of facial 

 nerve 



it. Its roof, or tegmen, is merely the backward continuation of the tegmen 

 tympani. The level of this is indicated by the posterior root of the zygoma. 

 'The level of the floor of the adult skull at the tegmen antri is, on an average, less 

 than one-fourth of an inch above the roof of the external osseous meatus; in 

 children and adolescents, from one-sixteenth to one-eighth of an inch.' (Mac- 

 ewen.) In early life, when the bony landmarks, e. g. the suprameatal crest (fig. 

 1087), are little marked, the level of the upper margin of the bony meatus will be 

 the safest guide to avoid opening the middle fossa. 



The lateral wall of the antrum is formed by a plate descending from the 

 squamous bone. This is very thin in early life, but as it develops by deposit 

 under the periosteum, the depth of the antrum from the surface increases. 

 Macewen gives the average of the depth as varying from one-eighth to three- 

 fourths of an inch. The thinness of the outer wall in early life is of practical 

 importance. It allows of suppuration making its way externally — subperiosteal 

 mastoid abscess. This will be facilitated by any dela}^ in the closure of the petro- 

 and masto-squamosal sutures, by which this thin plate blends with the rest of the 

 temporal bone. Further, by the path of veins running through these sutures or 



