298 THE CAT. [CHAP. ix. 



fenestra ovalis is a rounded prominence marked by grooves for nerves 

 and called the promontory. 



Another opening, called the fenestra rotunda, lies below and 

 behind the promontory, and is, naturally, closed by membrane. 



Above the fenestra ovalis is a small depression or fossa, in which 

 the stapedius muscle has its organ. 



The anterior part of the tympanum gradually narrows and 

 becomes the proximal part of the Eustachian tube, which, as before 

 said, is the canal which places the cavity of the middle part of the 

 ear, in communication with the mouth. 



The EUSTACHIAN TUBE proceeds forwards and inwards and 

 slightly downwards from the tympanum to the pharynx. Its 

 anterior or distal part consists of cartilage and fibrous membrane. 



The tympanum is crossed by four small bones which proceed 

 inwards from the inner surface of the tympanic membrane to the 

 fenestra ovalis. These are the AUDITORY OSSICLES. 



The outermost of these is called, from its shape, the malleus 

 (Fig. 138, B), and consists of a quadrangular thickened portion, the 

 head, with a rounded articular surface (as). Adjacent to this is a 

 part called the neck (n), beneath which it expands, on one side, into 

 a lamina of bone (/). From this extends a long, delicate, pointed 

 process (py), called the processus gracilis. From the inner side of 

 the neck a large process (pm) projects, for the tendon of the 

 tensor tympani muscle. Opposite this is a slight prominence, the 

 processus brevis (pb), and from between these two last mentioned 

 processes there extends a long curved production, the manubrium 

 (emn), which is fixed on the middle of the tympanic membrane. 



The second bone is called the incus or anvil (Fig. 138, c), also 

 consists of a thickened part, with two processes. The thickened 

 part, or body, has a concavo-convex articular surface, which unites 

 with a corresponding surface on the body of the malleus, both sur- 

 faces being provided with cartilage united by a synovial membrane. 



One process, the crus breve, projects backwards, and is attached 

 by ligament to the hinder wall of the tympanum. The other 

 process, the crus longum, which is longer, passes downwards behind 

 the manubrium, while its end is bent suddenly inwards and articu- 

 lates with a third bone, which is a minute rounded ossicle called the 

 os orbicularc, and which is generally anchylosed to the end of the 

 crus longum as the so-called processus lenticularis. The crus longum 

 articulates by the intervention of the os orbiculare with the fourth 

 auditory ossicle or stapes (Fig. 138, D), so called from its resemblance 

 to a stirrup. That portion of it which resembles the part of a 

 stirrup on which the foot rests is called its base, and the opposite end 

 its head, while these are connected by the two crura which diverge 

 from the neck. The base is fixed by ligamentous fibres to the 

 margin of the fenestra ovalis. The space enclosed between the crura 

 and base of the stapes, is naturally closed by a thin membrane. A 

 very small muscle, called the stapedius, (to be shortly described) is 

 inserted into the neck of the stapes. 



