748 



THE EAB. 



SMALL BONES OF THE EAR. 



Three small bones (ossicula auditus) are contained in the uppr-r part of 

 the tympanum : of these, the outermost (malleus) is attached to the mein- 

 brana tympani j the innermost (stapes) is fixed in the fenestra oveilis ; and 



Fig. 503. 



Fig. 503. BONES OP THE TYMPANUM 

 OF THE RIGHT SIDE (from Arnold), f 



A, malleus; 1, its head; 2, the 

 handle ; 3, long or slender process ; 4, 

 short process ; B, incus ; 1, its body ; 

 2, the long process with the orbicular 

 process ; 3, short or posterior process ; 

 4, articular surface receiving the head 

 of the malleus; C, stapes; 1, hea4 ; 

 2, posterior crus ; 3, anterior crus ; 

 4, base ; C*, base of the stapes ; D, 

 the three bones in their natural con- 

 nection as seen from the outside ; a, 

 malleus ; b, incus ; c, stapes. 



the third (incus), placed between 

 the other two, is connected to 

 both by articular surfaces. The 

 malleus and incus are placed in 

 nearly a vertical, the stapes in a 

 horizontal direction. They form 



together an angular and jointed connecting rod between the membrana 

 tympani and the membrane which closes the fenestra ovalis. 



The malleus, or hammer bone, consists of a central thicker portion, with 

 processes of different lengths. At the upper end of the bone is a rounded 

 head (capitulum), which presents internally and posteriorly an irregularly 

 oval surface covered with cartilage, for articulation with the incus. Below 

 the head is a constricted neck (cervix); and beneath this another slight 

 enlargement of the bone, to which the processes are attached. The handle 

 (manubrium) of the malleus is a tapering and slightly twisted process, com- 

 pressed from before backwards to near its point, where it is flattened in the 

 opposite direction : it descencre with a slight inclination forwards and in- 

 wards, and is received between the middle and inner layers of the membrana 

 tympani, to which it is closely attached. The long process (processus gra- 

 cilis) is a very slender spiculum of bone, which in the adult is usually broken 

 off in its removal from the tympanum, in consequence of its union with the 

 temporal bone ; it projects at nearly a right angle from the front of the 

 neck of the malleus, and extends thence obliquely downwards and forwards 

 to the Glasserian fissure. Its end is flattened and expanded, and is con- 

 nected by ligamentous fibres and by bone to the sides of the fissure. The 

 short process (processus brevis vel obtusus) is a low canical eminence spring- 

 ing from the root of the mauubrium, beneath the cervix, and projecting 

 outwards towards the upper part of the membrana tympani. 



The incus has been compared to an anvil in form ; but it resembles per- 

 haps more nearly a tooth with two fangs widely separated. It consists of 

 a body and two processes. The body presents in front a concavo-convex 

 articular surface, which is directed upwards and forwards, and receives the 

 head of the malleus. The surfaces of the joint thus formed are tipped with 

 articular cartilage and enclosed by a synovial membrane. The shorter of 



