836 



THE OEGANS OF SENSE. 



mucous surface are small and proceed from the anterior tympanic branch of the internal maxillary, 

 and from the stylo-mastoid branch of the posterior auricular. The veins from the cutaneous surface 

 open into the external jugular ; those from the mucous surface partly into the venous plexus on the 

 auditory tube, and partly into the transverse sinus and veins of the dura mater. The lymph 

 vessels, like the blood-vessels, are arranged in two sets, cutaneous and mucous, which, however, 

 communicate freely with each other. Kessel has described as lymphatics the spaces between the 

 branches of Gruber's dendritic fibres. The lateral surface of the membrane receives its nerves 

 from the auriculo-temporal branch of the trigeminal and from the auricular branch of the vagus ; 

 the medial surface, from the tympanic branch of the glossopharyngeal. 



ANTRUM TYMPANICUM ET CELLULJE MASTOIDE.E. 



Antrum Tympanicum. The tympanic antrum (O.T. mastoid antrum) is an air- 

 space situated above and behind the tympanic cavity. It is nearly as large in the 

 new-born child as in the adult. In the adult its measurements are length from 



Tympanic antrum, the medial 



wall of which is related to the 



lateral semicircular canal 



edial part of posterior wall of external 

 acoustic meatus left in situ 



Points to the recessus epitympanicus 



Mastoid air-cells 



Facial nerve 



Facial canal laid open, displaying the facial nerve within 

 FIG. 712. 



Preparation to display the position and relations of the tympanic antrum. The greater part of the posterior 

 wall of the external acoustic meatus has been removed, leaving only a bridge of bone at its medial ex- 

 tremity ; under this a bristle is displayed, passing from the tympanic antrum through the iter to the 

 cavity of the tympanum. 



12 to 15 mm., height from 8 to 10 mm., and width from 6 to 8 mm. It is roofed 

 in by the tegmen tympani, and its floor and medial wall are formed by the pars 

 petrosa and pars mastoidea of the temporal bone, while laterally it is closed by 

 the junction of the thin outer part of the squama with the pars mastoidea. It 

 communicates with the epi tympanic recess by a triangular or rounded opening, 

 on the medial wall of which, immediately above and behind the canalis facialis, is 

 a smooth, convex area of bone indicating the position of the arnpullated extremities 

 of the superior and lateral semicircular canals. At birth the lateral wall of the 

 antrum has a thickness of only 1-2 mm., but by the ninth year this has increased 

 to about 10 mm. Coincident with the growth of the mastoid process the mastoid 

 air-cells are developed downwards and backwards as diverticula from the antrum, 

 and present the greatest possible variation in different skulls. 



Cellulae Mastoidese. The mastoid air-cells may be large, comparatively few 

 in number, and involve the whole mastoid process, in which case the compact 

 bone surrounding them is extremely thin, and the innermost cells are separated 



