THE MIDDLE EAR 715 



is fibrous. The rest of the tube is osseous and is a little longer and 

 narrower than the cartilaginous portion. Around the inner extremity 

 of the canal, except at its superior portion, is a narrow groove which 

 receives the greater portion of the margin of the membrana tympani. 



The skin of the external meatus is continuous with the integument 

 covering the pinna. It is very delicate, becoming thinner from without 

 inward. In the osseous portion it adheres closely to the periosteum, 

 and at the bottom of the canal it is reflected over the membrana tym- 

 pani, forming its outer layer. In the cartilaginous and fibrous portion 

 are short stiff hairs, with sebaceous glands attached to their follicles, and 

 the coiled tubes known as the ceruminous glands. The structure of 

 these glands and the properties and composition of the cerumen have 

 already been described in connection with the physiology of the glands 

 of the skin. 



General Arrangement of the Parts composing the Middle Ear. 

 A minute and purely anatomical description of the middle ear would be 

 out of place in this work, where it is desired to give only such an 

 account of the anatomy as will enable the student to comprehend the 

 physiology of the ear, reserving for special description certain of the 

 most important structures. It will be useful, however, to give a general 

 outline of the different parts, with their names. 



The middle ear presents a narrow cavity of irregular shape, 

 situated between the external ear and the labyrinth, in the petrous por- 

 tion of the temporal bone. The general arrangement of its parts is 

 shown in Fig. 185. The outer wall of the tympanic cavity is formed 

 by the membrana tympani (6, Fig. 185). This membrane is concave, its 

 concavity looking outward, and oblique, inclining usually at an angle of 

 forty-five degrees with the perpendicular. This angle, however, varies 

 considerably in different individuals. The roof is formed by a thin plate 

 of bone. The floor is bony and is much narrower than the roof. The 

 inner wall, separating the tympanic cavity from the labyrinth, is irregu- 

 lar, presenting several small elevations and foramina. The fenestra 

 ovalis, an ovoid opening near its upper portion, leads to the cavity of 

 the vestibule. This is closed in the natural state by the base of the 

 stapes and its annular ligament. Below is a smaller opening, the fenes- 

 tra rotunda, which leads to the cochlea. This is closed in the natural 

 state by a membrane called the secondary membrana tympani. The 

 principal portion of the tympanic cavity is sometimes called the atrium, 

 although this name is applied to other parts not connected with the ear. 

 From the general tympanic cavity is a prolongation, extending upward 

 and backward, in which are lodged the head of the malleus and a great 

 part of the incus. This is called the epitympanic recess, or attic. It 



