THE SMALL BOXES AND THEIR MUSCLES. 749 



the two processes (crus breve) of the incus projects nearly horizontally 

 backwards from the upper part of the body of the bone, and is connected 

 by ligainentous fibres with the posterior wall of the tympanum near the 

 entrance of the rnastoid cells. The long process (crus longum) tapers 

 rather more gradually, and descends nearly vertically behind the handle of 

 the malleus : at its extremity it is bent inwards, and is suddenly narrowed 

 into a short neck ; and upon this is set a flattened rounded tubercle (pro- 

 cessus lenticularis), tipped with cartilage. This tubercle, which articulates 

 with the head of the stapes, was formerly, under the name of os orbiculare 

 sen lenticulare, described as a separate bone, which indeed it originally is in 

 childhood. 



The stapes, the third and innermost bone of the ear, is in shape remark- 

 ably like a stirrup, and is composed of a head, a base, and two crura. The 

 head is directed outwards, and has on its end a slight depression, covered 

 with cartilage, which articulates with the lenticular process of the incus. 

 The base is a plate of bone placed in the fenestra ovalis, to the margin 

 of which it is fixed by ligameutous fibres. The form of the base is 

 irregularly oral, the upper margin being curved, while the lower is nearly 

 straight. The crura of the stapes diverge from a constricted part (neck) 

 of the bone, situated close to the head, and are attached to the outer 

 surface of the base near its extremities. The anterior cms is the shorter 

 and straighter of the two. The crura, with the base of the stapes, enclose 

 a small triangular or arched space, which in the recent state is occupied by 

 a thin membrane stretched across. A shallow groove runs round the opposed 

 surfaces of the bone, and into this the membrane is received. 



LIGAMENTS AND MUSCLES OF THE TYMPANUM. 



Ligaments. In the articulations of the small bones of the ear with each 

 other, the connection is strengthened ly ligainentous fibres which cover the 

 synovial membranes. 



The attachment of the bones of the ear to tho walls of the tympanum is 

 effected partly by the reflections of the mucous membrane lining that cavity, 

 but chiefly by muscles and by the following ligaments. 



The suspensory ligament of the malleus consists of a small bundle of fibres, 

 which descends perpendicularly from the roof of the tympanum to the head 

 of the malleus. / 



The incus is likewise suspended by a small ligament (the posterior liga- 

 ment of the incus), which extends from near the point of the short crus 

 directly backwards towards the posterior wall of the tympanum, where it is 

 attached near the entrance to the mastoid cells. 



Arnold describes an upper ligament which attaches the incus, near its articulation 

 with the malleus, to the roof of the tympanum. It lies close behind the suspensory 

 ligament of the malleus. 



The annular or orbicular ligament of the stapes connects the base of the 

 bone to the margin of the fenestra ovalis, in which it is lodged. 



Muscles. There are three well-determined muscles of the tympanum. 

 Sornmerring describes four, and some authors a larger number ; but the 

 descriptions of these last muscles are not confirmed by later research. Of 

 the three muscles generally recognised, two are attached to the malleus, and 

 one to the stapes. 



The tensor tympani (musculus internus mallei) is the largest of these 

 muscles. It consists of a tapering fleshy part, about half an inch in length, 

 and a slender tendon. The muscular fibres arise from the cartilaginous 



