DEVELOPMENT OF THE OSSEOUS SYSTEM. 987 



proliferates, drives the cylindrical processes inward and develops into two epi- 

 thelial vesicles (the paired rudiment of the thyroid gland). These vesicles have 

 a central cleft, which at first still communicates with the pharynx. The paired 

 rudiment of the thyroid gives off from the original cavity buds that are at first 

 solid, but subsequently become hollow; later the paired rudiments fuse. Ac- 

 cording to His, the thyroid (man, fourth week) in the region of the second pair of 

 visceral arches lies in front of the tongue as an epithelial vesicle. Of the epi- 

 thelial portion of the thymus gland (which, according to Born and Fischeles, 

 originates from the third visceral cleft) there persist only the so-called concentric 

 bodies. His describes in man (fourth week) epithelial diverticula from the sides 

 of the fourth and fifth aortic arches as the rudimentary thymus. The carotid 

 gland also is of epithelial origin, a variety of the thyroid. 



The Extremities. The course and the origin of the nerves of the brachial 

 plexus indicate that the upper extremity has had a position more toward the 

 cranium on the vertebral column (last cervical and first dorsal vertebrae) . The 

 rudiment of the lower extremity corresponds to the region between the last lumbar 

 and the third or fourth sacral vertebra. 



The clavicle, preformed not in connective tissue, but in cartilage, like the 

 furcula of birds, exhibits marked growth, so that in the second month it is four times 

 as large as the thigh. It ossifies, the first of all of the bones, in the seventh week. 

 At the time of puberty a sternal epiphysis is added. Episternal formations must 

 be referred to the clavicle. Ruge regards cartilaginous pieces between the clavicle 

 and the sternum as the analogues of the episternum of animals. The clavicle is 

 wanting in many mammals (hoofed animals, beasts of prey) ; in alipeds it is ex- 

 ceedingly large, in rabbits half membranous. The furcula of birds represents the 

 united clavicles. 



The scapula when first indicated is united with the clavicle; at the end of 

 the second month it exhibits a central nucleus, which soon enlarges. Of the 

 accessory centers of ossification, that in the coracoid process is of interest morpho- 

 logically; the latter forms at the same time the uppermost portion of the articular 

 surface. In birds this rudiment grows as the coracoid bone up to the sternum, 

 while in man only a membranous band passes from the apex of the coracoid 

 process to the sternum. The long basal separate strip of bone corresponds to 

 the suprascapular bone of some animals. Other centers of ossification are as 

 follows : One in the lower angle ; two or three in the acromion ; one in the articular 

 surface; an inconstant one in the spine. Complete consolidation occurs at the 

 time of puberty. 



The humerus undergoes ossification between the eighth and the ninth week 

 in the diaphysis. Other centers of ossification are as follows: One in the upper 

 epiphysis and one in the eminentia capitata (first year) ; one in the greater tuber- 

 osity, and one in the lesser tuberosity (second year) ; two in the condyles (between 

 the fifth and the tenth year); one in the trochlea (twelfth year). The diaphysis 

 unites with the epiphysis between the sixteenth and the twentieth year. 



The radius ossifies in the third month in the diaphysis. In addition, a center 

 occurs in the lower epiphysis (fifth year) ; one in the upper epiphysis (sixth year) ; 

 the centers in the tuberosity and in the styloid process are inconstant. Union 

 occurs at the time of puberty. 



The ulna ossifies in its middle portion in the third month. In addition there 

 is a center in the lower extremity (sixth year) and two in the olecranon (between 

 the eleventh and fourteenth years); a center in the coronoid process and one 

 in the styloid process are inconstant. Consolidation of the bone takes place at 

 puberty. In the flying dog, pteropus, the olecranon persists as a special bone, 

 cubital patella. 



The carpal bones in vertebrates are arranged in two rows. The first row 

 contains three bones side by side, the radial, the intermediate, and the ulnar. 

 These are represented in man by the scaphoid, the semilunar, and the cuneiform 

 (the pisiform is only a sesamoid bone in the tendon of the flexor carpi ulnaris) . 

 The second row contains actually (for example, in the salamanders) as many 

 bones as there are digits; in man the common rudiment of the fourth and fifth 

 fingers corresponds to the unciform bone. 



Morphologically it is noteworthy that between both rows there is at first 

 formed a central bone (corresponding to the central carpal bone of reptiles, am- 

 phibia, and some mammals) , but this atrophies in the third month or fuses with 

 the scaphoid. Only in rare instances does it persist. It persists constantly 



