316 THE MORPHOGENESIS OF THE SKELETON AND MUSCLES 



The scapula arises as a single plate in which there are two chief centers 

 of ossification. One center early forms the body and spine. The other, 

 after birth, gives rise to the rudimentary coracoid process, which in lower 

 vertebrates extends from the scapula to the sternum. Union between the 

 coracoid process and the body does not occur until about the fifteenth 

 year. 



The humerus, radius, and ulna ossify from single primary centers and 

 two or more epiphyseal centers (Fig. 296 C-F). 



In the cartilaginous carpus there is a proximal row of three, and a dis- 

 tal row of four elements. Other inconstant cartilages may appear, and 

 subsequently disappear or become incorporated in other carpal bones. 

 The pisiform is regarded as a sesamoid bone which develops in the tendon 

 of the flexor carpi ulnaris ; in the same category is the patella which forms in 

 the tendon of the quadriceps extensor cruris. 



The Lower Extremity. The 'cartilaginous plate of the os coxa is at 

 first so placed that its long axis is perpendicular to the vertebral column 

 (Fig. 322). Later, it rotates to a position parallel with the vertebral 

 column, and shifts slightly caudal to come into relation with the first 

 three sacral vertebra;. A retention of the membranous condition in the 

 lower half of each primitive cartilaginous plate accounts for the obturator 

 membrane which closes the foramen of the same name. Three centers of 

 ossification appear, forming the ilium, ischium, and pubis. The three 

 bones do not fuse completely until about puberty. 



The general development of the femur, tibia, fibula, tarsus, metatarsus, 

 and phalanges is quite similar to that of the corresponding bones of the 

 upper extremity. 



Anomalies. Variations in the si?e, shape, and number of skeletal parts are common. 

 Developmental arrest and over-developmelit are the prime causative factors. Variations 

 in the number of vertebrae (except cervical) 'are not infrequent. The last cervical and first 

 lumbar vertebrae occasionally bear ribs, due to the continued development of the primitive 

 costal processes. Cleft sternum or cleft xiphoid process represents an incomplete fusion 

 of the sternal bars. Additional fingers or toes (polydactyly) may occur ; the cause is obscure. 

 Hare lip and cleft palate have already been mentioned (pp. 148, 151). 



II. THE MUSCULAR SYSTEM 



The skeletal muscles, with the exception of those attached to the 

 branchial arches, originate from the myotomes of the mesodermal seg- 

 ments (pp. 53, 291 and Fig. 323). Although the primitive segmental 

 arrangement of the myotomes is, for the most part, soon lost, their original 

 innervation by the segmental spinal nerves is retained throughout life. 

 For this reason, the history of adult muscles formed by fusion, splitting, 

 or other modifications may be traced with considerable certainty. 



