ORGAN OF HEARING. 



547 



der Amboss. This has been compared, also, to 

 a bicuspid molar tooth. It is divided into a 

 body, and two processes, or crura. 



The body, corpus, presents a concave articu- 

 lar surface, by which it is joined to the malleus : 

 around this surface is a groove, (particularly 

 deep and broad on the side towards the laby- 

 rinth, that is, the side on which the lenticular 

 process projects,) for the insertion of the articu- 

 lar capsule. 



The shorter of the two processes, cms s. pro- 

 cessus superior s. brevis, is blunt at its apex, 

 and compressed from one side to another. 



The longer process, crus s. processus inferior 

 s. longus, is more slender, and becomes gradually 

 thinner towards its extremity, where it is slightly 

 curved, and where it presents, supported on a 

 short bony pedicle, given off at a right angle 

 from its side, the lenticular process, processus 

 lenticularis incudis;* a small oval plate so 

 situated, that a line drawn through the long 

 diameter of it would intersect obliquely a line 

 corresponding to the long crus of the incus. 

 The free surface of the lenticular process is 

 convex, and is destined to articulate with the 

 corresponding concave surface on the head of 

 the stapes. The lenticular process has been, 

 and is still, often described as a separate bone, 

 under the name of os lenticulare. 



The stirrup bone, (stapes). Fr. L'etrier. 

 Germ. Der Steigbugel. Exactly like a stirrup, 

 this bone presents a base, two crura, and a head, 

 where the crura unite. 



The base, basis, the essential part of the bone, 

 has precisely the same shape as the vestibular 

 fenestra to which it is applied, only a little 

 smaller. The arched margin of the base cor- 

 responds to the upper edge of the fenestra, and 

 the indented margin to the lower edge. 



The surface of the base corresponding to the 

 vestibular fenestra is slightly convex. The other 

 surface is grooved ; but the groove is subdivided 

 by a ridge, which extends obliquely along it 

 lengthways, and which is continuous at its ex- 

 tremities with the upper margin of the groove 

 on the inner surface of one crus, and the lower 

 margin of the groove of the opposite crus. 

 The margin of the base projects like a ledge 

 beyond the insertion of the crura. 



Of the two crura, one is shorter and straighter 

 than the other ; both are grooved on the surfaces 

 regarding each other, and the grooves are con- 

 tinued into that just described in the base, in 

 such a way that the groove of one crus is con- 

 tinued into one of the divisions, and the groove 

 of the other crus into the other. 



The head, cupitulum, somewhat oblong and 

 flat, presents a superficial depression on its top, 

 oblique from above downwards, and from with- 

 out inwards, for receiving the convex articular 

 surface of the lenticular process of the incus. 

 There is sometimes an appearance of a neck 

 supporting the head. 



Position, connexions, and articulations of the 

 small bones of the tympanum. The handle of 



* Blumenbach, Geschichte, und Beschreibung 

 der menschl. Knoehen, s. 50, p. 145. 



Fig. 250. 



Small bones of the tympanum of the left side, mag- 

 nified considerably more than twice. (From Soem- 

 merring.) 



A is the malleus seen from the side correspond- 

 ing to the membrana tympani : a. head ; b. articu- 

 lar surface ; c. neck j d. handle ; e. short process ; 

 f. long process. 



B. The incus seen from its outer surface also : 

 a. body ; b. articular surface ; c. short crus ; d. 

 long crus ; e e. lenticular process. 



C. The stapes : a. head ; b. neck ; c. anterior 

 and less bent crus ; d. posterior and more bent crus ; 

 e. base. 



D. A fore-shortened view of the stapes : a. an- 

 terior and less curved crus ; b. posterior crus, the 

 two are seen uniting at the head, the articular sur- 

 face of which is seen; c. base. 



the malleus is fixed to the membrana tympani. 

 The articular surface on the head of the malleus, 

 to the corresponding surface on the body of the 

 incus, and the long process of the incus, is 

 through the medium of its lenticular process 

 articulated with the stapes. These two joints 

 are furnished with small articular capsules. 



The head of the malleus lies in the upper 

 space of the tympanum, above the upper margin 

 of the membrana tympani. Its articular sur- 

 face is directed obliquely backwards and in- 

 wards. The surface of the neck, corresponding 

 to the prominence of the angle which it forms 

 with the manubrium, is hitched like a shoulder 

 under the upper part of the circumference of 

 the inner extremity of the auditory passage. 

 The handle of the malleus, it has been said, 

 is compressed from one side to another, so that 

 it presents two flat surfaces and two edges or 

 ridges. That edge or ridge which is continued 

 down from the short process is turned outwards, 

 and corresponds to the membrana tympani ; 

 into it, indeed, along its whole extent, the cen- 

 tral extremities of the radiating fibres of that 

 membrane are inserted. The extremity of the 

 handle of the malleus, which is curved forwards 

 and outwards, is compressed, but in a direction 

 contrary to the rest of the handle ; so that one 

 of the flat surfaces, that corresponding to the 

 outer ridge of the rest of the handle, is con- 

 nected with the membrana tympani at a point 

 below its centre, and nearer its anterior edge. 

 It is at this point that the bottom of the con- 

 cavity is which the membrana tympani presents 

 externally. At its upper part the membrana 

 tympani is pushed outwards by the short pro- 

 cess of the malleus, which projects towards the 

 auditory passage. 



2 o 2 



