3l6 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 imtil 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 distal row 

 of four elements. Other inconstant cartilage may appear and subsequently dis- 

 appear 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. 



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 caudad 

 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 msmbrane 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 number of vertebrae (except cervical) are not infre- 

 quent. The last cervical and first lumbar vertebras occasionally bear ribs, due to the con- 

 tinued 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 Hp and cleft palate have already been mentioned 

 (pp. 146, 149). 



n. The Muscular System 



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

 arches, originate from the myotomes of the mesodermal segments (pp. 51, 292 

 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 i^eason the history of adult muscles 

 formed by fusion, sphtting, or other modifications may be traced with consider- 

 able certainty. 



