DEVELOPMENT OF THE ORGAN OF HEAEING. 55 



and inner side and the external cleft depression (meatus) and 

 developing squamosal to its outer (Fig. 39). As the internal 

 recess extends and widens outwards and backwards, the gelatinous 

 tissue is absorbed, so that the malleus and incus and developing 

 stapes, with the chorda tynipani, become surrounded by the 

 hypoblasts lining of the inner cleft recess and appear to be 

 situated within the cavity thus formed — the tympanum. The 

 tympanic plate forms its floor, the membrana tympani and 

 squamosal its outer wall, while the petro-mastoid forms its inner 

 wall and roof (Fig. 40). That part of the tympanum which lies 

 above the level of the membrana tympani is named the attic, 

 and contains the head of the malleus and body of the incus 

 (Fig. 36). 



In carnivora and some other mammals the floor of the tym- 

 panum, formed by the tympanic plate, is inflated into a bulla, 

 the tympanic bulla. Its meaning is unknown, but when a bulla 

 is developed the antrum of the mastoid is small or, absent. 



The Antrum of the Mastoid. — The antrum of the mastoid 

 represents the extreme outer or posterior end of the first cleft 

 recess (Figs. 39 and 40). It is formed at the same time and in 



tegmen tympani 

 squam.M / s xt- semicirc. canal 



petro-squam. fisJ for. ouale s i X^^^^5" trm 



mst-meat.procf W antrum ^^S^^/ ~ petro ~ mst 

 petro-mastoid^ ■:'.:, ■01 y^^^^^Kij^y 

 \i^S^ Eustachian tube \ »" 

 tympanum f or - rotundum 



tympanic ring 



Fig. 41. Fig. 42. Fig. 43. 



Fig. 41. — The temporal bone at birth showing the formation of the Antrum between 



the Squamosal and Petro-mastoid. 

 Fig. 42. — A transverse section showing how the Walls of the Antrum are formed. 

 Fig. 43. — Showing the outer aspect of the Petro-mastoid at birth after the Squamosal 



is removed. 



the same manner as the tympanum. Its use is unknown, but it 

 has frequently to be exposed by the surgeon to remove the effects 

 of chronic middle ear disease. At birth its outer wall is formed 

 by the thin post-auditory part of the squamosal (Figs. 41 

 and 42). The squamosal forming its outer wall is then only 

 2 mm. thick, but every year until the 20 th, or later, this plate 

 increases nearly 1 mm. in thickness, so that by the 20th year 



