C32 ORGANS OF THE SENSES. 



downwards. The front and lower parts of this canal are formed by a curved 

 plate of bone, which, in the foetus, exists as a separate ring (tympanic bone), 

 incomplete at its upper part. 



The skin lining the meatus is very thin, closely adherent to the cartilaginous 

 and osseous portions of the tube, and covers the surface of the membrana tympani, 

 forming its outer layer. After maceration, the thin pouch of epidermis, when 

 withdrawn, preserves the form of the meatus. The skin near the orifice is fur- 

 nished with hairs and sebaceous glands. In the thick subcutaneous tissue of the 

 cartilaginous part of the meatus are numerous ceruminous glands, which secrete 

 the ear wax, the ducts of which open on the surface of the skin. 



The arteries supplying the meatus are branches from the posterior auricular, 

 internal maxillary, and temporal. 



The nerves are chiefly derived from the temporo-auricular branch of the inferior 

 maxillary nerve. 



i MIDDLE EAR OR TYMPANUM. 



The middle ear or tympanum is an irregular cavity, compressed from without 

 inwards, and situated within the petrous bone. It is placed above the jugular 

 fossa, the carotid canal lying in front, the mastoid cells behind, the meatus auclito- 

 rius externally, and the labyrinth within. It is filled with air, and communicates 

 with the pharynx by the Eustachian tube. The tympanum is traversed by a 

 chain of movable bones, which connect the membrana tympani with the laby- 

 rinth, and serve to convey the vibrations communicated to the membrana tympani 

 across the cavity of the tympanum to the internal ear. 



The cavity of the tympanum measures about five lines from before backwards, 

 three lines in the vertical direction, and between two and three in the transverse, 

 being a little broader behind and above than below and in front. It is bounded 

 externally by the membrana tympani and meatus ; internally, by the outer surface 

 of the internal ear ; behind, by the mastoid cells ; and, in front, by the Eustachian 

 tube and canal for the Tensor tympani. Its roof and floor are formed by thin 

 osseous laminae, which connect the squamous and petrous portions of the temporal 

 bone. 



The roof is broad, flattened, and formed of a thin plate of bone, which separates 

 the cranial and tympanic cavities. 



The floor is narrow, and corresponds to the jugular fossa, which lies beneath. 



The outer wall is formed by the membrana tympani, a small portion of bone 

 being seen above and below this membrane. It presents three small apertures, 

 the iter chorda? posterius, the Glaserian fissure, and the iter chordee anterius. 



The aperture of the iter chordae posterius is behind the aperture for the membrana 

 tympani, close to its margin, at a level with its centre ; it leads into a minute canal, 

 which descends in front of the aquasductus Fallopii, and terminates in this canal 

 near the stylo-mastoid foramen. Through it the chorda tympani nerve enters the 

 tympanum. 



The Glaserian fissure opens just above and in front of the orifice of the mem- 

 brana tympani; in this situation it is a mere slit, about a line in length. It gives 

 passage to the long process of the malleus, the Laxator tympani muscle, and some 

 tympanic vessels. 



The aperture of the iter chordae anterius is seen just above the preceding fissure ; 

 it leads into a canal (canal of Huguier), which runs parallel with the Glaserian 

 fissure. Through it the chorda tympani nerve leaves the tympanum. 



The internal wall of the tympanum is vertical in direction, and looks directly 

 outwards. It presents for examination the following parts : 



Fenestra ovalis. Eidge of the Aquaxluctus Fallopii. 



Fenestra rotunda. Pyramid. 



Promontory. - Opening for the Stapedius. 



