3 88 



THE PERIPHERAL NERVOUS SYSTEM 



about the middle of the margin of the free auricular fold, and corresponds to the tip of the mam- 

 malian auricle. 



The external auditory meatus is formed as an ingrowth of the first branchial 

 groove. In embryos of 12 to 15 mm. the wall of this groove is in contact dorsally 

 with the entoderm of the first pharyngeal pouch. Later, however, this contact 

 is lost, and during the latter part of the second month, according to Hammar, 



Fig. 368. — Six stages in the development of the external ear. 1, 2, 3, elevations on the mandibular 

 arch; 4, 5, 6, elevations on the hyoid arch. 1, tragus; 2, 3, helix; 4, 5, antihelix; 6, antitragus. c, 

 hyoid helix or auricular fold (His from McMurrich's "Human Body"). A, 11 mm.; B, 13.6 mm.; C, 

 15 mm.; D, beginning of third month; E, fetus of 85 mm.; F, fetus at term. 



an ingrowth takes place from the ventral portion of the groove, to form a funnel- 

 shaped canal. 



The lumen of this tube is temporarily closed during the fourth and fifth months, but later 

 re-opens. During the third month a plate of cells at the extremity of the primary auditory 

 meatus grows in and reaches the lower wall of the tympanic cavity. During the seventh month 

 a space is formed by the splitting of this plate, and the secondary portion of the meatus is 

 thus developed. 



The tympanic membrane is formed by a thinning out of the tissue in the region 

 where the wall of the external auditory meatus abuts upon the wall of the tym- 

 panic cavity. 



