THE ORGANS OF SPECIAL SENSE. 



591 



second branchial arches surrounding the dorsal part of the first outer branchial 

 groove (see Figs, 122, 123, 126, 127). About the end of the fourth week, the 

 caudal border of the first arch exhibits three small elevations or tubercles 

 (Fig. 515, A, 1-3), the cranial border of the second arch the same number (Fig. 

 515, A, 4-6). A groove, extending down the middle of the second arch, marks 

 off a ridge (c) lying caudal to the three tubercles. The ventral tubercle (i) of 

 the first arch gives rise to the tragus. The middle tubercle (5) of the second arch 



\ 





_r 



FIG. 515. Stages in the development of the external ear (auricle). A, Embryo of n mm.; B y of 

 13.6 mm.; C, of 15 mm.; D, foetus at the beginning of the 3d month; E, foetus of 8.5 cm.; 

 F, fcetus at term. For explanation of numerals, see text. His, McMurrich. 



develops into the antitragus. The middle and dorsal tubercles (2 and 3) of 

 the first arch unite with the ridge (c) on the second arch to form the helix. 

 The dorsal tubercle (4) of the second arch gives rise to theanthelix. The 

 ventral tubercle (6) of the second arch produces the lobule. It should be noted 

 that in the third month the dorsal and caudal portions of the helix are bent 

 forward and conceal the anthelix. 



Anomalies. 



Malformations of the nose have been alluded to in connection with hare lip, 

 cleft palate, etc., on page 212, and are also discussed in the chapter on tera- 

 togenesis (XIX) . Malformations affecting the eye (cyclopia, microphthalmia, 

 etc.) and the ear (synotia, etc.) are dealt with in the chapter on teratogenesis. 



