Hi THE ANATOMY OF VEETEBEATED ANIMALS. 



wall of the tympanum is the tympanic memhrane, its inner 

 wall is the periotic mass with its .fenestrce ; and, in all 

 Vertebrata below Mammals, the outer end of the stupes is 

 either free, or, more commonly, is fixed to the tympanic 

 membrane, and thus the latter and the membrane of the 

 fenestra ovcdis become mechanically connected. In all these 

 animals the mandible is connected with the skull by the 

 intermediation of an os quadratwm. 



But, in the Mammalia, the mandible is articulated dii'ectly 

 with the squamosal, and the quadratmn is converted into 

 one of the so-called ossicula auditus, and named the malleus. 

 The malleus becomes attached to the membrana tympani, 

 by a special process ; while its other extremity, which was 

 continuous with Meckel's cartilage in the embryo, is con- 

 verted into the processus gracilis, or Folianus, and lies be- 

 tween the tympanic, the squamosal, and the periotic bones. 



In the singular lizard Sphenodon (A, Fig. 24), the anterior 

 cornu of the hyoid is continuous with the distal end of the 

 stajjcs, and the latter sends a cartilaginous process upwards, 

 which passes into the wall of the periotic capsule, just be- 

 hind the proximal end of the os quadratum. Thus the stapes 

 stands ou.t at right angles to the hyoid cornu, and the latter 

 becpmes divisible into a supra-stapedial part, and a part 

 which lies below the stapes, and answers to the styloid 

 process, or stylohyal, of the Mammalia. The supra-stape- 

 dial part is represented by cartilage, or ligament, in other 

 Sa^|.ropsida, but seems not to ossify. In the Mammalia 

 (B, Fig. 24) the supra-stapedial part ossifies, becomes the 

 incus, and its proximal end is usually articulated by a 

 synovial joint with the malleus ( = quadratum). A distinct 

 ossification, the os m-bicidare, usually arises at that part 

 of the hyoidean cartilage in which the stapes and the inc^ls 

 unite. That part of the hyoidean cartilage which is converted 

 into the styloid process is generally connected with the orbicu- 

 lare by muscular fibres, which constitute the stapedius muscle. 

 On the other hand, the posteiior, or short process of the incus^ 

 is connected by ligament with that part of the periotic mass 

 into which the styloid process is directly continued, and it 



