THE HEAD AND NECK 1273 



arch ; its nerve is the seventh cranial, or facial, nerve, with the chorda tympani 

 and the auditory nerves, and its cartilaginous bar is called the hyoid bar, or 

 cartilage of Reichert. This bar ventrally is connected with its fellow of the 

 opposite side ana with the thyro-hyoid bar of the same side by a transverse 

 piece of cartilage, called the copula, which lies between the tuberculum impar 

 and the loop of the furcula. 



The second arch becomes transformed into the following structures : 



1 . The upper or dorsal segment of the hyoid bar gives rise to the head, neck, 

 and crura of the stapes, the foot-piece of that ossicle being developed from 

 the cartilaginous capsule of the labyrinth, within the fenestra ovalis. 



2. The succeeding segment of the hyoid bar gives rise to (a) the tympano- 

 hyal. and {b) the stylo-hyal, which collectively constitute the styloid process 

 of the temporal bone. 



3. The next portion of the hyoid bar becomes converted into fibrous 

 tissue, and forms the stylo-hyoid ligament, which sometimes exisfts as an 

 epi-hyal bone. 



4. The lower or ventral segment of the hyoid bar gives rise to the cerato- 

 hyal, or small cornu of the hyoid bone. 



The second arch also gives rise superiorly to the antihelix, antitragus, and 

 lobule of the pinna ; and inferiorly, along with the third arch, to one-half of 

 the posterior third of the tongue. 



Third, or Thyro-Hyoid, Arch. — This arch is situated between the second 

 and third pharyngeal pouches. Its artery is the third primitive aortic arch ; 

 its nerve is the ninth cranial, or glosso-pharyngeal nerve ; and its cartilaginous 

 bar is known as the thyro-hyoid bar. This bar is connected ventrally with 

 its fellow, and with the hyoid bar of the same side, by the copula already 

 referred to. 



A large portion of the thyro-hyoid bar disappears, but its lower or ventral 

 segment gives rise to the thyro-hyal or great cornu of the hyoid bone. The 

 basi-hyal or body of the hyoid bone is developed from the copula. 



Fourth and Filth Visceral Arches. — The artery of the fourth arch is the 

 fourth primitive aortic arch, and its nerve is the superior laryngeal nerve, 

 wMch is a branch of the pneumogastric. The artery of the fifth arch is the 

 fifth primitive aortic arch, and its nerve is the inferior, or recurrent, laryngeal 

 hranch of the pneumogastric. The greater portions of these two arches 

 disappear; but the lower or ventral ends of their cartilaginous bars are usually 

 regarded as giving rise to the corresponding ala of the thyroid cartilage. 



Sixth Visceral Arch. — The artery of this arch is the sixth primitive aortic 

 arch. The sixth arch itself, being undifferentiated, leaves no traces behind it. 

 The metamorphoses of the pharyngeal pouches have been already described 

 in connection with these pouches. 



The first external furrow, corresponding to the iirst internal pharyngeal 

 pouch, gives rise, as stated, to the external auditory meatus, and the walls 

 of this furrow become differentiated into the component parts of the pinna. 



Sinus Cervicalis and Cervical Fistula. — The first or mandibular and second 

 or hyoid arches increase more rapidly in all directions than the succeeding 

 branchial arches proper. The third and fourth branchial arches therefore 

 become overlapped by the second or hyoid arch, and now lie at the bottom 

 of a depression. This depression is called the sinus cervicalis. The lining 

 membrane of the sinus is formed by the ectodermic constituents of the over- 

 lapped branchial arches. The sinus is, as a rule, transitory, being soon 

 obliterated as the second visceral arch grows backwards towards the fifth 

 arch, the Ups of the sinus fusing. In rare cases, however, the sinus cer\-icalis 

 may persist as an ectodermal cervical cyst. In other rare cases the ectoderm 

 of the sinus may become ruptured, and then a communication might be 

 established with a mucous canal — namely, the pharyngeal portion of the 

 fore-gut, thus giving rise to the condition known as cervical fistula. 



Morphology of the Visceral Arches and Clefts. — In aquatic animals — e.g.. 

 Fishes, and Amphibia at an early stage, but only in Perennibranchiata per- 

 manently — these are called the branchial or gill-arches and clefts. The clefts 

 range in number from five to eight, and they differ from those of Mammals 

 and Birds inasmuch as they are complete clefts, the closing membrane being 



