508 MEATUS AUDITORIUS. TYMPANUM. 



The Transversus auriculas, partly tendinous and partly muscular, 

 extends transversely from the convexity of the concha to that of the 

 helix, on the posterior surface of the pinna. 



These muscles are rudimentary in the human ear, and deserve only 

 the title of muscles in the ears of animals. Two other muscles are 

 described by Mr. Tod,* the obliquus auris and contractor meatus, or 

 trago-helicus. 



The MEATUS AUDITORIUS is a canal, partly cartilaginous and partly 

 osseous, about an inch in length, which extends inwards and a little 

 forwards from the concha to the tympanum. It is narrower in the 

 middle than at each extremity, forms an oval cylinder, the long 

 diameter being vertical, and is slightly curved upon itself, the con- 

 cavity looking downwards. 



It is lined by an extremely thin pouch of epidermis, which, when 

 withdrawn after maceration, preserves the form of the meatus. Some 

 stiff short hairs are also found in its interior, which stretch across the 

 tube, and prevent the ingress of insects and dust. Beneath the epi- 

 dermis are a number of small ceruminous follicles, which secrete the wax 

 of the ear. 



Vessels and Nerves. The pinna is plentifully supplied with arteries; 

 by the anterior auricular from the temporal, and by the posterior 

 auricular from the external carotid. 



Its Nerves are derived from the anterior auricular of the fifth, the 

 posterior auricular of the facial, and the auricularis magnus of the cer- 

 vical plexus. 



MIDDLE EAR OR TYMPANUM. 



The tympanum is an irregular bony cavity, compressed from without 

 inwards, and situated within the petrous bone. It is bounded exter- 

 nally by the meatus and membrana tympani ; internally by the base of 

 the petrous bone ; and in its circumference by the petrous bone and 

 mastoid cells. 



The Membrana tympani is a thin and semi-transparent membrane 

 of an oval shape, the long diameter being vertical. It is inserted into 

 a groove around the circumference of the meatus near its termination, 

 and is placed obliquely across the area of that tube, the direction of the 

 obliquity being downwards and inwards. It is concave towards the 

 meatus, and convex towards the tympanum, and is composed of three 

 layers, an external epidermic, middle fibrous and muscular, and internal 

 mucous, derived from the mucous lining of the tympanum. 



The tympanum contains three small bones, ossicula auditus, viz., 

 the malleus, incus, and stapes. 



" The Anatomy and Physiology of the Organ of Hearing," by David 



