50 



HUMAN EMBRYOLOGY AND MORPHOLOGY. 



bar (cartilaginous skeleton of maxillary process), while the stapes 

 is an independent formation developed round the stapedial 



(point at which acoustic 

 ganglia arise) 



periotic capsuleW{& 

 ganglia ^fr. 



tymp. 

 Eustachr^fr 



epiblast 

 oto-cgst 



acoustic gang. 

 i 1st cleft recess 

 ^-Meckel's cart. 



t audit, meatus 

 (1st cleft depress. ) 

 tragus 



andib arch 



Fig. 35. —Diagrammatic Section through the Cephalic region of an embryo, showing 

 the origin of the Auditory System. 



artery. It may be derived, in part at least, from the upper end 

 of the cartilage of the hyoid arch (see also page 10). 



In lower fishes the auditory apparatus is composed of elements 

 1 and 2, and are in them of the simplest form. The other 

 elements are added and specialized in the evolution of the 

 higher vertebrates. 



External Auditory Meatus. — The external auditory meatus 

 is derived from the upper part of the first cleft depression. In 

 the adult the meatus is 1^ in. long ; at birth it is ^ of an inch, 

 and is surrounded by fibro-cartilaginous and fibrous tissue. In 



the adult the tympanic ring grows outwards in the fibrous tissue, 

 as we have already seen (page IV) to form the tympanic plate 

 and the inner -| of the meatal floor. The squamous part of the 

 temporal, which is developed over in its roof, also grows outwards 

 and forms a thick, horizontal plate in the inner two-thirds of 



