THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 



203 



discussed in the chapter on the muscular system (Chap. XI). It has been 

 seen that the greater part of the axial skeleton is derived from the sclerotomes, 

 is preformed in cartilage, and maintains its segmental character throughout life. 

 It has also been seen that the head skeleton is in part preformed in cartilage, is in 

 part of intramembranous origin, and shows but a trace of segmental character, 

 and that only in the occipital region at a very early stage. The appendicular 

 skeleton is derived wholly from the embryonic connective tissue which forms the 

 cores of the developing extremities, and shows no trace of a segmental character. 

 Here also, as in the axial skeleton, three stages may be recognized — a blastemal, 

 a cartilaginous (Fig. 181), and a final osseous. 



Acromion Coracoid process 



Scapula 



Humerus 



Radius 



Metacarpal I 



Large multangular 

 (trapezium) 



Navicular (scaphoid) 



Lunate (semilunar) 



Small multangular 

 (trapezoid) 



Metacarpal IV 



Capitate (os magnum) 



Triquetral (cuneiform) 



Hamatate (unciform) 



Fig. 181. — Cartilages of left upper extremity of a human embryo of 17 mm. Hagen. 



In the region of the shoulder girdle a plate of cartilage appears in the em- 

 bryonic connective tissue which lies among the developing muscles dorso-lateral 

 to the thorax. This plate of cartilage is the forerunner of the scapula, and in 

 general resembles it in shape. During the eighth week of foetal life a single 

 center of ossification appears and gives rise to the body and spine of the scapula. 

 After birth certain accessory centers appear and produce the coracoid process, the 

 supraglenoidal tuberosity, the acromion process, and the inferior angle and verte- 

 bral margin (Fig. 182). Later the supraglenoidal fuses with the coracoid and 

 forms part of the wall of the glenoid cavity. About the seventeenth year the 

 single center formed by the union of these two fuses with the rest of the scapula. 



