700 ORGANS OF SENSE. 



The orifice of the canal for the Tensor Tympani, and the orifice of the Eusta- 

 chian tube, are situated at the upper part of the anterior wall, being separated 

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

 cochleariformis. These canals run from the tympanum forward^ inward, and a 

 little downward, to the retiring angle between the squamous and petrous por- 

 tions 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 per- 

 forated 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 commu- 

 nicates 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 

 margins of the cartilage, which is completed by fibrous tissue. Its canal is 

 narrow behind, wide, expanded, and somewhat trumpet-shaped in front, termi- 

 nating by an oval orifice, at the upper part and side of the pharynx, behind 

 the back part of the inferior meatus. Through this canal the mucous mem- 

 brane 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 iu 

 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 (cuti- 

 cular), a middle (fibrous), and an internal (mucous). The cuticular lining is de- 

 rived from the integument lining the meatus. The fibrous layer consists of 

 fibrous and elastic tissues ; some of the fibres radiate from near the centre to 

 the circumference ; 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 

 tympaui along the handle of the malleus, and are distributed between its layers. 



OSSICLES OF THE TYMPANUM. (Fig. 380.) 



The tympanum is traversed by a chain of movable bones, three in number, 

 the malleus, incus, and stapes. The former is attached to the membrana tym- 

 pani, the latter to ther fenestra ovalis, the incus brin.ir placed between the two, 

 to both of which it is connected by delicate articulations. 



The? MaHt'ii^ so named from its fancied resemblance to a hammer, consists of 

 a head, neck, and three processes : the handle, or manubrium, the processus 

 gracilis, and the processus brevis. 



The h'd'f is tin- lur-c upper extremity of the bone ; it is oval in shape, and 

 articulates posteriorly with the incus, being free in the rest of its extent. 



