SKELETON 



8l 



fenestra vestibuli (f. ovale) in the lateral wall of the otic capsule, 

 which is occupied by a movable bone, the stapes, of uncertain homo- 

 logies, but probably representing the hyomandibular of the fishes, 

 which otherwise is lacking in all tetrapoda. This view is the more 

 probable since in some vertebrates the stapes is connected develop- 

 mentally with the rest of the hyoid arch. 



In urodeles (fig. 91) and caecilians a slender process extends from 

 the quadrate across the poorly developed tympanic cavity to articu- 

 late with the stapes. In the anurans there is no connexion of quad- 

 rate with stapes, but a slender rod, the columella, extends from the 



P" 



Fig. 79. — Diagram of the middle 

 car of a lizard, after Versluys. a, 

 articulare; c, columella; ec, extra- 

 columella; h, hyoid; ie, inner ear; 

 mpt, pterygoid muscle; 0, oral cavity; 

 Po, parotic process; pt, pterygoid 

 bone; s, stapes; t, tympamc cavity; 

 Im, tympanic membrane. 



Fig. 80. — Diagram of ear bones of embryo 

 pig, the tympanic cavity laid open, g, goniale; 

 i, incus; Ij, lower jaw; tn, malleus; mk, Meckel's 

 cartilage; mm, manubrium of malleus; '5, stapes; 

 sq, squamosal; z, zygomatic. The outlines of 

 the zygomatic arch and the hind.end of the jaw 

 are dotted. 



tympanic membrane to the stapes. This columella arises behind 

 the tympanic cavity, but with growth is included in it, so that in the 

 dult it appears to run directly through the cavity. In the saurop- 

 .-iida (figs. 70, 79), the relations are much as in the anura, but when 

 ossification sets in, the columella may form several elements. In de- 

 velopment, the columella in these forms is directly connected with 

 the hyoid arch. 



In the mammals a chain of three bones carries the sound waves 

 across the tympanic cavity (fig. 80). In the fenestra vestibuli is 

 6 



