

THE EXTERNAL EAR 1033 



middle ear and auditory tube are developed from the first pharyngeal pouch. 

 The entodermal lining of the dorsal end of this pouch is in contact with the ecto- 

 derm of the corresponding pharyngeal groove; by the extension of the mesoderm 

 between these two layers the tympanic membrane is formed. During the sixth or 

 seventh month the tympanic antrum appears as an upward and backward expan- 

 sion of the tympanic cavity. With regard to the exact mode of development 

 of the ossicles of the middle ear there is some difference of opinion. The view 

 generally held is that the malleus is developed from the proximal end of the 

 mandibular (Meckel's) cartilage (Fig. 43), the incus in the proximal end of the 

 mandibular arch, and that the stapes is formed from the proximal end of the hyoid 

 arch. The malleus, with the exception of its anterior process is ossified from a single 

 center which appears near the neck of the bone; the anterior process is ossified 

 separately in membrane and joins the main part of the bone about the sixth month 

 of fetal life. The incus is ossified from one center which appears in the upper 

 part of its long crus and ultimately extends into its lenticular process. The 

 stapes first appears as a ring (annuhts stapedius) encircling a small vessel, the stape- 

 dial artery, which subsequently undergoes atrophy; it is ossified from a single 

 center which appears in its base. 



The external acoustic meatus is developed from the first branchial groove. The 

 lower part of this groove extends inward as a funnel-shaped tube (primary meatus) 

 from which the cartilaginous portion and a small part of the roof of the osseous 

 portion of the meatus are developed. From the lower part of the funnel-shaped 

 tube an epithelial lamina extends downward and inward along the inferior wall of 

 the primitive tympanic cavity; by the splitting of this lamina the inner part of 

 the meatus (secondary meatus) is produced, while the inner portion of the lamina 

 forms the cutaneous stratum of the tympanic membrane. The auricula or pinna 

 is developed by the gradual differentiation of tubercles which appear around the 

 margin of the first branchial groove. The rudiment of the acoustic nerve appears 

 about the end of the third week as a group of ganglion cells closely applied to the 

 cephalic edge of the auditory vesicle. Whether these cells are derived from the 

 ectoderm adjoining the auditory vesicle, or have migrated from the wall of the 

 neural tube, is as yet uncertain. The ganglion gradually splits into two parts, 

 the vestibular ganglion and the spiral ganglion. The peripheral branches of the 

 vestibular ganglion pass in two divisions, the pars superior giving rami to the 

 superior ampulla of the superior semicircular duct, to the lateral ampulla and to 

 the utricle; and the pars inferior giving rami to the saccule and the posterior 

 ampulla. The proximal fibers of the vestibular ganglion form the vestibular nerve; 

 the proximal fibers of the spiral ganglion form the cochlear nerve. 



The External Ear. 



The external ear consists of the expanded portion named the auricula or pinna, 

 and the external acoustic meatus. The former projects from the side of the head 

 and serves to collect the vibrations of the air by which sound is produced ; the latter 

 leads inward from the bottom of the auricula and conducts the vibrations to the 

 tympanic cavity. 



The Auricula or Pinna (Fig. 904) is of an ovoid form, with its larger end directed 

 upward. Its lateral surface is irregularly concave, directed slightly forward, and 

 presents numerous eminences and depressions to which names have been assigned. 

 The prominent rim of the auricula is called the helix ; where the helix turns down- 

 ward behind, a small tubercle, the auricular tubercle of Darwin, is frequently seen ; this 

 tubercle is very evident about the sixth month of fetal life when the whole auric- 

 ula has a close resemblance to that of some of the adult monkeys. Another 

 curved prominence, parallel with and in front of the helix, is called the antihelix ; 



