THE MAMMALIA 



291 



processes of the maxillary and squamosal bones. The lower 

 jaw consists of a single piece on each side, is articulated 

 directly with the squamosal, and is not suspended from the 

 skull, as is the case in lower vertebrates, by the intervention of 

 a hyomandibular or a quadrate. The homologues of the 

 latter bones in mammals are to bj found in the chain of 

 minute auditory ossicles which stretch across the middle ear 

 from the tympanum to the fenestra ovalis of the inner ear. 

 There are four of these ossicles, the malleus, incus, orbiculare, 





at.' 



Fig. 72 



The tympanic membrane and auditory ossicles of the human ear, seen 

 from within. //, handle of malleus; z', incus; mall, head of 

 malleus ; nit, membrana tympani ; or, os orbiculare ; pb, processus 

 brevis of incus ; pg, processus gracilis of malleus ; st, stapes. (From 

 Quain's Anatomy, slightly modified.) 



and stapes. The malleus is a small irregularly-shaped bone 

 consisting of a rounded head or capitulum bearing two pro- 

 cesses. The larger process, called the manubrium, is attached 

 to the tympanic membrane, the smaller, processus gracilis, is 

 attached to the walls of the tympanic cavity. The posterior 

 face of the capitulum bears a concave surface for articulation 

 with the incus. The incus has a body and two processes. 

 Anteriorly the body has a saddle-shaped articular surface, 



