390 



THE PERIPHERAL NERVOUS SYSTEM 



Fuchs (1905), studying rabbit embryos, on the contrary, concludes:- (i) the stapes is 

 derived from the capsule of the labyrinth; (2) the malleus and incus arise independently 

 of the first branchial arch. 



The External Ear. The external ear is developed from the first 

 ectodermal branchial groove and its adjoining tissue. The auricle arises 

 from six elevations, which appear, three on the mandibular, and three on 

 the hyoid arch (Fig. 388). Modern accounts of the transformation of 

 these hillocks into the adult auricle agree in the main. 



FIG. 388. Stages in the development of the auricle. (Adapted in part after His.) A, 

 II mm.; B, 13.6 mm.; C, 15 mm.; D, adult. I, 2, 3, elevations on the mandibular arch; 4, 5, 6, 

 elevations on the hyoid arch; of, auricular fold; ov, otic vesicle; I, tragus; 2, 3, helix; 4, 5, anti- 

 helix; 6, antitragus. 



Caudal to the hyoid anlages a fold of the hyoid integument is formed, 

 t he auricular fold, or hyoid helix. A similar fold forms later, dorsal to the 

 first branchial groove, and unites with the auricular fold to form with it 

 the free margin of the auricle. The point of fusion of these two folds 

 marks the position of the satyr tubercle, according to Schwalbe. Darwin's 

 tubercle appears at about the middle of the margin of the free auricular 

 fold, and corresponds to the apex of the auricle in lower mammals. The 

 tragus is derived from mandibular hillock i ; the helix from mandibular 

 hillocks 2 and 3 ; the antihelix from hyoid hillocks 4 and 5 ; the antitragus 

 from hyoid hillock 6. The lobule represents the lower end of the auricular 

 fold. 



