56 HUMAN EMBRYOLOGY AND MORPHOLOGY. 



the antrum is buried by a plate of bone about 20 mm. thick. 

 There is a great individual variation, however, in the thickness 

 of its outer wall. The antrum lies above and behind the level of 

 the external auditory meatus ; the post-auditory spine and supra- 

 meatal triangle formed by the post-auditory part of the squamosal 

 lie over it and serve as surface guides to it. The antrum opens 

 in front into the attic of the tympanum. The tegmen tympani 

 (Fig. 42) forms its roof and the petro-mastoid its floor and inner 

 wall. The aqueductus Fallopii runs down the inner wall of its 

 mouth (Fig. 43), and in its inner wall is situate the external semi- 

 circular canal. The petro-squamous suture in its roof (Fig. 42), 

 and the masto-squamous suture on its outer wall (Fig. 14 £), 

 become closed in the second year, and thus the escape of pus from 

 it is rendered more difficult. 



The Primitive Jugular Vein. — In the petro-squamous suture 

 a vein or sinus, frequently of considerable size, runs forwards from 

 the lateral sinus, and commonly ends in a tributary of the middle 

 meningeal vein. It receives as it runs along . venules from the 

 antrum and attic and may be the means of carrying infection from 

 the middle ear to the lateral sinus or to the meningeal veins 

 (Cheatle). The petro-squamous sinus represents the primitive 

 jugular vein, and may open in man, as it does in mammals 

 generally, at the post-glenoid foramen, situated at the outer end of 

 the Glaserian fissure, near the base of the zygoma. In the early 

 weeks of embryonic development the primitive jugular is the chief 

 vein from the skull, but very soon the internal jugular vein 

 enlarges and takes its place, and thus the blood of the lateral 

 sinus comes to pass out by the jugular foramen instead of by the 

 petro-squamous sinus and temporal canal at the base of the 

 zygoma. 



The Membrana Tympani. — As may be seen from Fig. 39, 

 the membrana tympani is of very considerable thickness until the 

 third month. It has an inner covering of hypoblast and an outer 

 of epiblast. In the mesoblastic tissue between the coverings lie 

 parts of the malleus, incus and chorda tympani. As the tympanic 

 cavity expands the mesoblastic tissue is compressed and absorbed, 

 and thus the handle of the malleus and chorda tympani come to 

 appear as if they lie on the membrane, although really within it. 

 The mucous lining of the tympanum covers them. The mem- 



