THE MIDDLE EAR. 857 



vertically behind and parallel to the handle of the malleus, and, bending inward, 

 terminates in a rounded globular projection, the os orbiculare or lenticular process, 

 which is tipped with cartilage, and articulates with the head of the stapes. In the 

 foetus the os orbiculare exists as a separate bone. 



The Stapes, so called from its close resemblance to a stirrup, consists of a head, 

 neck, two crura, and a base. 



The head presents a depression, tipped with cartilage, which articulates with 

 the os orbiculare. 



The neck, the constricted part of the bone succeeding the head, receives the 

 insertion of the Stapedius muscle. 



The two crura diverge from the neck and are connected at their extremities by 

 a flattened, oval-shaped plate (the base), which forms the foot-plate of the stirrup 

 and is fixed to the margin of the fenestra ovalis by ligamentous fibres. Of the 

 two crura, the anterior is shorter and less curved than the posterior. 



Ligaments of the Ossicula. These small bones are connected with each other 

 and with the walls of the tympanum by ligaments, and moved by small muscles. 

 The articular surfaces of the malleus and incus and the orbicular process of the 

 incus and head of the stapes are covered with cartilage, connected together by 

 delicate capsular ligaments and lined by synovial membrane. The ligaments con- 

 necting the ossicula with the walls of the tympanum are five in number three for 

 the malleus, one for the incus, and one for the stapes. 



The anterior ligament of the malleus was formerly described by Sommerring as 

 a muscle (Laxator tympani}. It is now, however, believed by most observers to 

 consist of ligamentous fibres only. It is attached by one extremity to the neck 

 of the malleus, just above the processus gracilis, and by the other to the anterior 

 wall of the tympanum, close to the Glaserian fissure, some of its fibres being pro- 

 longed through the fissure to reach the spine of the sphenoid. 



The superior ligament of the malleus is a delicate, round bundle of fibres 

 which descends perpendicularly from the roof of the epitympanic recess to the 

 head of the malleus. 



The external ligament of the malleus is a triangular plane of fibres passing 

 from the posterior part of the notch in the tympanic ring (incisura Rivini) to the 

 short process of the malleus. 



The posterior ligament of the incus is a short, thick, ligamentous band which 

 connects the extremity of the short process of the incus to the posterior and 

 lower part of the epitympanic recess, near the margin of the opening of the 

 mastoid cells. 



The inner surface and the circumference of the base of the stapes are covered 

 with hyaline cartilage, and the annular ligament of the stapes connects the cir- 

 cumference of the base to the margin of the fenestra ovalis. 



A superior ligament of the incus has been described by Arnold, but it is little 

 more than a fold of mucous membrane. 



The muscles of the tympanum are two : 



Tensor tympani. Stapedius. 



The Tensor tympani, the larger, is contained in the bony canal above the 

 osseous portion of the Eustachian tube, from which it is separated by the processus 

 coculeariformis. It arises from the under surface of the petrous bone, from the 

 cartilaginous portion of the Eustachian tube, and from the osseous canal in which 

 it is contained. Passing backward through the canal, it terminates in a slender 

 tendon which enters the tympanum and makes a sharp bend outward round the 

 extremity of the processus cochleariformis, and is inserted into the handle of the 

 malleus near its root. It is supplied by a branch from the otic ganglion. 



The Stapedius arises from the side of a conical cavity hollowed out of the inte- 

 rior of the pyramid ; its tendon emerges from the orifice at the apex of the pyra- 

 mid, and, passing forward, is inserted into the neck of the stapes. Its surface is 

 aponeurotic, its interior fleshy, and its tendon occasionally contains a slender bony 



