APPENDICULAR SKELETON 3 IS 



the maxillm arise directly in membrane. Each palate bone develops from a single 

 center of ossification. According to one view five centers contribute to the for- 

 mation 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 maxiUa. 



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 posi- 

 tion of the adult mandibular foramen. The portion of Meckel's cartilage enclosed 

 in bone disappears, while the cartilage proximal to the mandibular foramen be- 

 comes in order, the spheno-mandibular 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. 



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 conden- 

 sations have formed definite blastemal cores in the primitive limb buds (Fig. 323). 

 Following this blastemal stage the various bones next pass through a cartilagi- 

 nous stage and finally an osseous one. 



Upper Extremity. — The clavicle is the first bone of the skeleton to ossify, 

 centers appearing at each end. Prior to ossification it is compo^sed of a peculiar 

 tissue which makes it difficult to decide whether the bone is intramembranous 

 or intracartilaginous in origin. 



