634 ORGANS OF THE SENSES. 



process. These cavities vary considerably in number, size, and form ; they are 

 lined by mucous membrane, continuous with that covering the cavity of the 

 tympanum. 



The anterior wall of the tympanum, is wider above than below ; it corresponds 

 with the carotid canal, from which it is separated by a thin plate of bone ; it 

 presents for examination the 



Canal for the Tensor tympani. Orifice of the Eustachian Tube. 



Processus Cochleariformis. 



The orifice of the canal for the Tensor tympani, and the orifice of the Eustachian 

 tube, are situated at the upper part of the interior wall, being separated from 

 each other by a thin, delicate horizontal plate of bone, the processus cochleari- 

 formis. These canals run from the tympanum forward, inward, and a little 

 downward, to the retiring angle between the squamous and petrous portions of the 

 temporal bone. 



The canal for the Tensor tympani is the superior and the smaller of the two ; it 

 is rounded and lies beneath the upper surface of the petrous bone, close to the 

 hiatus Fallopii. The tympanic end of this canal forms a conical eminence, which 

 is prolonged backwards into the cavity of the tympanum, and is perforated at its 

 summit by an aperture, which transmits the tendon of the muscle contained in it. 

 This eminence is sometimes called the anterior pyramid. The canal contains the 

 Tensor tympani muscle. 



The Eustachian tube is the channel through which the tympanum communicates 

 with the pharynx. Its length is from an inch and a half to two inches, and its 

 direction downwards, forwards, and inwards. It is formed partly of bone, partly 

 of cartilage and fibrous tissue. 



The osseous portion is about half an inch in length. It commences in the lower 

 part of the anterior wall of the tympanum, below the processus cochleariformis, 

 and, gradually narrowing, terminates in an oval dilated opening, at the angle of 

 junction of the petrous and squamous portions, its extremity presenting a jagged 

 margin, which serves for the attachment of the cartilaginous portion. 



The cartilaginous portion, about an inch in length, is formed of a triangular 

 plate of cartilage, curled upon itself, an interval being left below, between the 

 non-approximated margins of the cartilage, which is completed by fibrous tissue. 

 Its canal is narrow behind, wide, expanded, and somewhat trumpet-shaped in 

 front, terminating by an oval orifice, placed at the upper part and side of the 

 pharynx, behind the back part of the inferior meatus. Through this canal 

 the mucous membrane of the pharynx is continuous with that which lines the 

 tympanum. 



The membrana tympani separates the cavity of the tympanum from the bottom 

 of the external meatus. It is a thin semi-transparent membrane, nearly oval in 

 form, somewhat broader above than below, and directed very obliquely down- 

 wards and inwards. Its circumference is contained in a groove at the inner end 

 of the meatus, which skirts the circumference of this part excepting above. The 

 handle of the malleus descends vertically between the inner and middle layers 

 of this membrane as far down as its centre, where it is firmly attached, drawing 

 the membrane inwards, so that its outer surface is concave, its inner convex. 



Structure. This membrane is composed of three layers, an external or cuticular, 

 a middle or fibrous, and an internal or mucous. The cuticular lining is derived 

 from the integument lining the rneatus. The fibrous layer consists of fibrous and 

 elastic tissues ; some of the fibres radiate from near the centre to the circumfer- 

 ence ; others are arranged, in the form of a dense circular ring, round the attached 

 margin of the membrane. The mucous lining is derived from the mucous lining 

 of the tympanum. The vessels pass to the membrana tympani along the handle 

 of the malleus, and are distributed between its layers. 



OSSICLES OF THE TYMPANUM. The tympanum is traversed by a chain of 

 movable bones, three in number, the malleus, incus, and stapes. The former is 



