394 



SPECIAL PHYSIOLOGY. 



the membrana tympani and the outer wall of the labyrinth. Its outer 

 boundary is formed by the membrana tympani, d; this is a thin semi- 

 transparent membrane, of an oval shape, which slopes from above, 

 downwards and inwards, and from behind, forwards and inwards, and 

 is fixed by its circumference to a slight groove in the bone. It is the 

 resemblance of this membrane to the head of a drum that has given 

 the name of drum of the ear, or tympanum, to the middle ear. This 

 membrane is composed of three layers ; an outer one, which is an ex- 

 tension of the skin lining the external meatus, an inner layer, similarly 

 derived from the mucous membrane lining the tympanum, and an in- 

 termediate layer, consisting of fibrous and elastic tissue, in which are 

 bloodvessels and nerves : the middle layer is said to consist of two 

 laminae, the outer of which is composed of radiating, and the inner of 

 annular fibres. (Toynbee.) The inner wall of the tympanic cavity 

 corresponds with the outer surface of the labyrinth. The tympanum 

 communicates, behind and above, with cells in' the mastoid process of 

 the temporal bone, called the mastoid cells, i; in front and below, it 

 opens into the Eustachian tube, e, a trumpet- shaped canal, partly os- 

 seous, partly cartilaginous, leading into the upper part of the pharynx. 

 There are also several small apertures, for the passage of vessels, 

 nerves, and minute muscles. 



The little bones, or ossicles of the ear, the smallest in the body, are 

 three in number ; they are stretched across the tympanic cavity, from 

 the membrana tympani to the inner wall of that chamber, Fig. 74, and 

 are named the malleus, the incus, and the stapes. The malleus, Fig. 

 75 a, 76 a, or hammer-like bone, is attached, by a somewhat twisted 



Pig. 75. 



Fig. 75. The three ossicles of the ear, found in the cavity of the tympanum (magnified). (Arnold.) a, 

 malleus, or hammer, its head ; h, handle of the malleus; the other process is the processus gracilis. b, the 

 incus, or anvil, consisting of a body and two processes, to the longer one of which, or loni? leg, is affixed 

 the tubercle or orbicular bone, o. c, the stapes or stirrup bone, consisting of head, bars, and foot-piece or 

 base. 



process, called its handle, h, to the inner surface of the membrana 

 tympani, near its centre; this attachment of the bone causes the mem- 

 brane to be drawn in, as it were, towards the tympanum. Another 

 long and very delicate process, called the processus gracilis^ descends 

 to the floor of the tympanum ; while its rounded part, or head, which 

 is also somewhat fixed to theo-oof of that cavity, is articulated with a 



