1910.] 



The Incus and Corpus mallei. 



131 



the Mec'keliaii cartilage, just as does the articular in embryonic reptiles and 

 amphibians (Fig. 7C). 



It is scarcely necessary to refer to the objection (Weil, 1899) that because 

 the incus is at first undifferentiated from the malleo-IMeckelian bar, therefore 

 it is morphologically a part of the lower jaw. Kingsley (1900, p. 239) and 

 Broom (1907.3, pp. 9-10) have very fully met this objection by citing the 

 well-known fact that in many cases structures which are known to be mor- 

 phologically distinct are differentiated from a continuous stroma of pro- 

 cartilage. 



Fig. 4. Morphology of the auditory ossicles and adjacent parts in the embryo Lizard {A) 

 and Pig {B). Modified from Kingsley. 



The homologies between the several eleirients in the Lizard and tlie Pig (according to the 

 quadrate = incus theory) are indicated by similar shading. 



Lizard. 

 Qu., quadrate 

 St., stapes 

 Hy., hyoid cornu 

 ex. col. extracolumella 



Pig. 

 = Inc., incus. 

 = St., stapes. 

 = Hy., hyoid cornu. 

 = manubrium mallei. 



ar., articular region of Meckelian cartilage = Mai., corpus mallei. 



Mck., Meckelian cartilage = Mck., Meckelian cartilage. 



VII, facial nerve = VII, facial nerve. 



r. t., chorda tympani = c. t., chorda tympani. 



A well known reason for homologizing the incus and body of the malleus 

 with the quadrate and articulare respectively, lies in the identical relations 

 of these bones in the two classes, not only to the internal ear and the tym- 

 panic cavity but also to the chorda tympani nerve, which is a major branch 

 of the seventh or facial nerve. A comparison of the embryonic conditions 

 in the mammal (Fig. 4, B) and in SpJienodon (Howes and Swinnerton, 1901, 

 })1. V, fig. 12) shows that the incus on the one hand and the quadrate on the 

 other arise externally to the cochlea, dorsally to the stapes and to the " An- 

 lage" of the tympanic cavity, and internally to the outer border of the 



