i HI ORO INfl 01 mi: IVM.K ,i .KM LAI I:K. 



289 



From the Join-tit toteit OJ PfMMftJ ' rches 



begin to be ///*////<,/ /'// / /////<>// fr> MM intli*-r, mrin,/ (<> a 'more m/'i-f 



yi-fWth of thf jirxt twti than of thr t'u'l<>,ri ,i<J mtftt (li.LT. I They 



glide over one another." MS II is remarks. " lik- tin- tubrsof a telescope, 

 in Mich ;i way that. Viewed from the out -idf, III-M tin- fourth arch i< 

 surrounded and covered in by the third, and this in turn by the 

 Mil. whereas on the iniirr siirt'aiv, tliat which i-> tui'iird toward the 

 j)haiynx. the fourth arch 

 OV.T the third, the 

 third over the second." As 

 a result the length of the 

 oro-pharyngeal cavity is 

 relatively 1 ss in the older 

 than in the younger em- 

 bryos. In consequence of 

 t his inn (jiial growth, which 

 moreover takes place in an 

 entirely similar way in the 

 embryos of Birds and Mam 

 inals, then- is formed a deep 

 depression of the surt'a. 

 the posterior margin of the 

 cephalo-cervical region, the 

 neck-sinus, sinus cermcalis 

 (RABL) or sinus prcecervi- 

 cu/t'x (His) (figs. 156 and 

 1 5 s ///,). Inthedepti 

 this depression and on its 

 front wall lie the third 

 and fourth vise-era 1 arch* s. 

 which are now no longer 

 visible from without. The 

 entrance to the sinu> is bounded in front by the second visceral, or 

 the hyoid, arch (zb). The latter gradually de\ h.j.s a .MUM 11 j>: 

 backward, wliich covers over the cervical Minis and ha> been juMlv 

 compared by RATJIKK with the op/ivuluui of Fishes and Amphibia. 

 The opercular process at last fuses with the lateral wall of the /><></,/. 

 Thereby the /////.% wri'tcali*. n-hich r<//-/v>/'//'/.v tu th< //.///// 



the o/ierculnm which in /'/*// r.s n,,<l A ii<fltil><" / th* real y it l- 



arches, is closed up. 



One easily gets an accurate conception of the>- important processes 



19 



Fig. 156. Frontal reconstruction of the oro pharyngeal 

 cavity of a human embryo (Ry of His) 11 -5 mm. long, 

 neck measurement. Kn-ia II I, Mfn-.-lilu-ln- Km- 



.MaKiiiticd 1. tlianietere. 



The upper jaw ia seen in perspective th>> !.. \\.-r jaw \\\ 

 section. The last visceral arches are no longer 

 . externally, since they have moved int tin- 

 il.'jiths of the cervical sinus. 



