Mammalia: Nervous System 



695 



reptilian columella with which it is probably, at least in part, homolo- 

 gous. The embryonic origin of the malleus and incus indicates that 

 they correspond, respectively, to the reptilian articular and quad- 

 rate, which do not otherwise appear in the mammalian jaw. The 

 joint between malleus and incus would therefore correspond to the 

 joint between the reptilian lower and upper jaws (Fig. 520). 



The malleus, attached to the tympanic membrane, articulates 

 by its "head" with the incus, which, in turn, articulates with the 

 stapes (Figs. 518, 519). That part of the stapes opposite the incus is 

 an oval flat plate which tits fairly closely into an aperture (fenestra 

 vestibuli, or fenestra ovalis) in the bony wall between the tympanic 



„ HYOMANDIBULAR 



RALATO-PTERYGOID 



•ARTICULAR- ~„ 

 DENTARY 



MANDIBULAR 

 A. ELASMOBRANCH 



2ND 

 MANDIBULAR 



B TELEOST 



STAPES OTOMANDIBULAR) 



HYOMANDIBULAR (COLUMELLA 1 



INCUS CQUADRATE>_ 



MALLEUS (ARTIC ULAR) 



HYOID 



DENTARY- 

 STYLOHYOID LIGAMENT CHYOID) 



C AMPHIBIAN AND REPTILE. 



D. MAMMAL. 



Fig. 520. Diagrams of the first and second visceral arches in (A) elasmobranch, 

 (B) teleost, (C) amphibian and reptile, and (D) mammal, illustrating the trans- 

 formation of the hinge of the jaw of lower vertebrates into the malleus and incus 

 of the mammal. The third ear-bone, the stapes, comes from the hyomandibular. 

 (After Gegenbaur and Stempell. Courtesy, Neal and Rand: "Chordate Anatomy," 

 Philadelphia, The Blakiston Company.) 



