gS6 DEVELOPMENT OF THE OSSEOUS SYSTEM. 



as the remains of a fetal cleft between the middle and lateral portions of the 

 forming lower lip. 



From the posterior portion of the first visceral arch there develop the incus, 

 the malleus (which undergo ossification in the fourth month), and the long 

 cartilaginous process of Meckel, which arises from the malleus behind the tym- 

 panic ring, and passes forward, and which extends on the inner side of the lower jaw 

 almost to its median union. This process begins to atrophy at the sixth month. 

 Nevertheless, its posterior portion forms the internal lateral ligament of the 

 temporomaxillary joint. Close to it, at its origin from the malleus, the processus 

 Folli is formed. A portion of its median extremity in ossifying unites with the 

 inferior maxilla. The lower jaw originates in membrane as a protecting bone upon 

 the first visceral arch; the angle and the condyle develop from a cartilaginous 

 deposit. The symphysis of the lower jaws unites in the first year. From the 

 superior maxillary process there develops in addition to the upper jaw also the 

 internal plate of the pterygoid process, as well as the palatine process of the up- 

 per jaw and the palatine bone at the end of the second month; and, finally, the 

 zygomatic bone. 



The second visceral arch, originating from the temporal bone, and running 

 parallel with the first visceral arch, forms successively the stapes (according to 

 Salensky, however, this originates from a cartilaginous mass connected with 

 the first arch), the pyramidal eminence with the stapedius muscle, the styloid 

 process; the (previously cartilaginous) stylohyoid ligament, the lesser cornu of 





22L A 



FIG. 382. Formation of the Face and Developmental Defects of the face: A, First fetal rudiment; /, //, III, IV, 

 the four visceral arches', /, the frontal process; i, internal and, 2, external nasal process; 3, superior maxillary 

 process; , inferior maxillary process; b, c, first and second visceral clefts; a, eye; z, tongue, B, Normal 

 union of the embryonal parts; Z, intermaxillary bone; N l , nasal orifice; O, lacrimal canal ; U, lower jaw: 

 m, abnormal enlargement of the oral cleft, macrostomia. C, Arrested development of the oronasal cleft (hare- 

 lip or wolf's throat). D, Arrested development causing oblique facial cleft, Q. 



the hyoid bone (Landois saw the styloid process transformed into bone down 

 to the lesser cornu on both sides), and finally the glossopalatine arch. 



From the third visceral arch there develop the greater cornu and the body of 

 the hyoid bone and finally the pharyngopalatine arch. 



The fourth visceral arch contains the rudimentary thyroid cartilage. 

 The branchial or visceral arches may in general be regarded as the analogues 

 of the ribs. 



Of the visceral clefts only the first remains as the auditory canal, which 

 is transformed into the tympanic cavity and the Eustachian tube; all of the others 

 .se. It one or another remains open (arrest of development, occasionally heredi- 

 tary in certain families) there then results the congenital complete cervical fistula 

 (mostly arising from the second cleft alone). The passages may persist with 

 sr an inner or an outer opening only; there then result blind passages or diver- 

 :ula, which are designated incomplete cervical fistula. Also branchiogenous 

 imors and cysts take their origin from the visceral formations. Partial duplica- 

 e lower jaw, which is exceedingly rare, is to be attributed to an increase 

 in the number of visceral arches. 



The thymus and thyroid glands are formed as paired diverticula or thicken- 

 ings of the epithelium covering the visceral arches. The thyroid gland results 

 (in swine) from a middle and two lateral rudiments, which subsequently fuse, 

 epithelium of the last two pharyngeal clefts does not atrophy (swine) ; it 



