600 



THE NERVOUS SYSTEM. 



Inasmuch as in the examination of the ear and the treatment 

 of its diseases it is necessary to introduce an aural speculum (Fig. 

 377), a knowledge of the direction and shape of the canal is essen- 

 tial. Its greatest diameter is at the external orifice and is vertical ; 



FIG. 376. Muscles of the auricle : 1, attollens aurem ; 2, attrahens aurem ; 3, 

 Tetrahens aurem; 4, helicis major; 5, helicis minor; 6, tragicus, with 6', its acces- 

 sory portion ; 7, an ti tragicus ; a, spine of helix; b, concha (Testutj. 



its smallest diameter is in the middle. At the tympanic end the 

 greatest diameter is horizontal. The direction of the canal is 

 obliquely forward, inward, and downward. Before introducing 

 the speculum the helix of the ear is raised upward and backward 

 so as to straighten the canal as much as possible. 



Middle Ear. This is called also the tympa- 

 num (Fig. 378). 



Membrana Tympani (Fig. 379). This mem- 

 branous structure separates the tympanic cavity 

 from the external auditory canal. Its shape is 

 oval, and the direction of its long axis is down- 

 ward and inward ; its diameter along this axis 

 is about 9 mm. It is composed of three layers : 

 An external or cuticular, which is an extension 

 of the integument that lines the external audi- 

 f j I'll tory canal ; an internal, mucous, a continuation 



of the mucous membrane lining the tympanic 

 cavity ; and a middle, fibrous, made up of both 

 fibrous and elastic tissues. There are two vari- 

 eties of these fibers radiating, which radiate from the center to 

 the circumference ; and circular, which form a ring at the circum- 

 ference. The membrana tympani is set into a groove in a ring 

 of bone, except at the upper part, where it is attached to the wall 

 of the canal. This portion of the membrane is not so tense as 



FiG. 377.-Aural 

 speculum. 



