THE APPENDICULAR SKELETON 



315 



The Branchial Arch Skeleton. The first branchial arch forks into an 

 upper maxillary and a lower mandibular process (Fig. 119). Cartilage 

 fails to appear in the maxillary processes, due to accelerated development, 

 hence the palate bones and the maxilla arise directly in membrane. Each 

 palate bone develops from a single center of ossification. According to 

 one view five centers contribute to the formation of each maxilla; Mall, 

 however, maintains that there are but two centers, one .giving rise to the 

 portion bearing the incisor teeth, the other to the remainder of the maxilla. 



The entire core of the mandibular process becomes a cartilaginous bar, 

 Meckel' s cartilage, which extends proximally into the tympanic cavity of 

 the ear (Fig. 321). Membrane bone, developing distally in the future 

 body, encloses Meckel's cartilage and the inferior alveolar nerve, whereas 

 proximally in the ramus the membrane bone merely lies lateral to these 

 structures hence the position of the adult mandibular foramen. The por- 

 tion of Meckel's cartilage enclosed in bone disappears, while the cartilage 

 proximal to the mandibular foramen becomes in order, the spheno-mandi- 

 bular ligament, the malleus, and the incus (p. 389 and Fig. 387). 



Each second branchial arch comes into relation proximally with the 

 periotic capsule. This upper segment of the cartilage becomes the stapes 

 and the styloid process of the temporal bone (Figs. 321 and 387). The 

 succeeding distal portion is transformed into the stylo-hyoid ligament and 

 connects the styloid process with the distal end of the arch, which also 

 undergoes intracartilaginous ossification to form the lesser horn of the hyoid 

 bone. 



The cartilage of the third branchial arches ossifies and gives origin to 

 the greater horns of the hyoid bone, while a plate connecting the two arches 

 becomes its body. 



The fourth and fifth branchial arches co-operate in the formation of the 

 thyreoid cartilage of the larynx. 



THE APPENDICULAR SKELETON 



Whereas the axial skeleton originates chiefly from the sclerotomes of 

 the mesodermal segments, the appendicular skeleton is apparently derived 

 from the unsegmented somatic mesenchyme. In embryos of 9 mm., 

 mesenchymal condensations have formed definite blastemal cores in the 

 primitive limb buds (Fig. 323). Following this blastemal stage, the vari- 

 ous bones next pass through a cartilaginous stage and finally an osseous 

 one. 



The Upper Extremity. The clavicle is the first bone of the skeleton 

 to ossify, centers appearing at each end. Prior to ossification it is com- 

 posed of a peculiar tissue which makes it difficult to decide whether the 

 bone is intramembranous or intracartilaginous in origin. 



