THE TYMPANUM. 631 



presents a narrow groove, which extends round the sides and floor 

 of the meatus, but is deficient above ; into this the margin of the 

 membrana tympani is inserted. 



The skin of the meatus is continuous with that covering the pinna, 

 but is very thin, especially in the osseous part, and becomes gradually 

 thinner towards the bottom of the passage. In the osseous part it 

 adheres very closely to the periosteum, and at the bottom of the tube is 

 stretched over the surface of the membrana tympani, forming the outer 

 layer of that structure. After maceration in water, or when decompo- 

 sition has advanced, the epidermic lining of the passage may be 

 separated and drawn out entire, and then it appears as a small tube 

 closed at one end somewhat like the finger of a glove. Towards the 

 outer part the skin possesses fine hairs and sebaceous glands ; and in 

 the thick subdermic tissue over the cartilage are many small oval glands 

 of a brownish yellow colour, agreeing in form and structure with the 

 sweat glands, but larger. The cerumen or ear-wax is secreted by these 

 glands, glcmdukc ccrumiiwscc, and their numerous openings may be seen 

 to perforate the skin of the meatus. These accessory parts are absent 

 over the bony part of the tube. 



"Vessels and nerves. — The external auditory meatus is supplied w'itli arteries 

 from the posterior auricular, internal maxillary and temporal arteries ; and v/ith 

 nerves chiefly from the temporo-auricular branch of the fifth nerve. The prin- 

 cipal branches of the arteries coui-se along the tipper and back wall of the canal. 

 The nerves break up into numerous branches on reaching the osseous part. 



State in the infant. — The auditory passage is in a very mdimentaiy state in 

 the infant, for the osseous part begins to grow out of the temporal bone only 

 at the period of birth, and thus the internal and middle parts of the ear arc 

 brought much closer to the surface than in the adult. 



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, j^laced between 

 the inner end of the external auditory canal and the labyrinth. It 

 receives the atmospheric air from the pharynx through the Eustachian 

 tube, and contains a chain of small bones, by means of which the vibra- 

 tions communicated fi'om without to the membrana tympani are in part 

 conveyed across the cavity to the sentient part of the internal ear, and 

 by which also pressure is maintained on the contents of the internal ear, 

 varying in amount according to the tension of the membrana tympani. 

 The tympanum contains 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. 

 A roof and floor, an outer and inner wall, and an anterior and posterior 

 boundary are commonly described. 



The roof of the tympanum is formed by a thin plate of bone, which 

 may be easily broken through so as to obtain a view of the tympanic 

 cavity from above ; it is situated on the upper surface of the petrous 

 portion of the temporal bone, near the angle of union with the squa- 

 mous portion, from which in its development it is derived. The ^oor 

 is narrow, in consequence of the outer and inner boundaries being 

 inclined towards each other. 



The Older tvall is formed, to a small extent, by bone, but mainly by 



