80 



THE EAR. 



meatus. The meatus of the foetus is for the most part closed by apposition of 

 its upper and lower walls, but towards the end of foetal life it becomes occupied with 

 epithelial scales and ear-wax. The cartilage of the external ear and meatus appears 

 to be formed in two or three pieces, the fissures of Santorini marking the junctions 

 of these component portions (Burkner). 



THE MIDDLE EAR OR TYMPANUM. 



The tympanum or drum, the middle chamber of the ear, is a narrow irregular 

 cavity in the substance of the temporal bone, placed between the membrane occlud- 

 ing the inner end of the external auditory canal and the outer bony wall of the 



Fig. 89. THE RIGHT TEMPORAL BONE SAWN IN A CORONAL PLANE THROUGH THE TYMPANUM, VESTIBULE, 



AND AUDITORY MEATUS, THE POSTERIOR PART OF THE SAWN BONE HAVING BEEN TURNED ROUND 



so AS TO DISPLAY BOTH SURFACES OF THE SECTION. Natural size. (Testut.) 

 A, Anterior aspect ; B, posterior aspect. 



labyrinth. Its width between these boundaries varies from about 2 mm. to 4 mm., 

 being narrowest opposite the middle of the membrane, and narrower below and in 

 front than above and behind. It measures about 15 mm. from above down and 

 about the same from before back. 



The vertical measurement includes the so-called reccssus epitympanicvg or anfJifus ad 

 antrum, which lodges the head of the malleus and the greater part of the incus : this, by 

 Bezold and some other authors is excluded from the tympanum proper, which without 

 it measures about 9 mm. In other words, the entrance to the mastoid antrum and mastoid 

 cells is about 9 mm. above the bottom of the tympanic cavity. The orifice of the Eustachian 

 tube is about 4 mm. above the lowest part of the floor. 



The tympanum contains a chain of small bones (by means of which the vibra- 

 tions communicated from without to the membrana tympani are conveyed across 

 the cavity to the internal ear) and also certain minute muscles and ligaments, which 

 belong to the bones referred to, as well as nerves, some of which end within the 

 cavity, whilst others merely pass through it to other parts. The cavity is otherwise 

 filled with air, for it communicates with the atmosphere through the Eustachian 

 tube, which leads into the pharynx. The bony walls of the cavity are by no means 



