7-H THE EAR. 



calibre of the passage is smallest about the middle. The outer opening 

 is largest from above downwards, but the tympanic end of the tube 

 is slightly widest in the transverse direction. At the inner extremity the 

 tube is terminated by the membrana tympani, which is placed obliquely, 

 with the inferior margin inclined towards the mesial plane, and thus the 

 floor of the meatus is longer than its roof. 



The meatus is composed of a tube partly cartilaginous and partly osseous, 

 and is lined by a prolongation of the skin of the pinna. 



The cartilaginous part of the meatus forms somewhat less than half the 

 length of the passage. It is formed by the deep part of the cartilage of the 

 pinna, which has been already described. 



The osseous portion of the meatus is a little longer and rather narrower 

 than the cartilaginous part. At its inner end it 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, aod becomes gradually thinner towards the bottom of the pas- 

 sage. In the osseous part of the canal it adheres very closely to the 

 periosteum ; and at the bottom of the tube this lining is stretched over the 

 surface of the membrana tympani, forming the outer layer of that struc- 

 ture. After maceration in water, or when decomposition is advanced, the 

 epidermic lining of the passage may be separated and drawn out entire, and 

 then ib appears as a small tube closed at one end somewhat like the finger 

 of a glove. Towards the outer part the skin possesses flue hairs and seba- 

 ceous glands ; and in the thick subdermic tissue over the cartilage are many 

 small oval glands of a brownish-yellow colour, agreeing in form and struc- 

 ture with the sweat glands. The cerumen or ear-wax is secreted by these 

 glands, glandulce ceruminosce, 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 with arteries from 

 the posterior auricular, internal maxillary and temporal arteries; and with nerves 

 chiefly from the temporo-auricular branch of the fifth nerve. 



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

 infant, for the osseous part begins to grow out of the tympanic bone only at the period 

 of birth (p. 68), and thus the internal and middle parts of the ear are 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, placed between the 

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

 atmospheric air from the pharynx through the Eustachiau tube, and con- 

 tains a chain of small bones, by means of which the vibrations communicated 

 from 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 accord- 

 ing to the tension of the membrana tympani. The tympanum contains 

 likewise minute muscles and ligaments, which belong to the bones referred 

 to, as well as some nerves which end within this cavity, or pass through it to 

 other parts. 



The cavity of the tympanum may be considered as presenting for con- 



