OSSIFICATION OF SCAPULA AND CLAVICLE. 



89 



limb being developed on the superior margin. The upper limb appears before the 

 lower, and sooner exhibits a separation of digits. The division into arm and forearm, 

 thigh and leg, is observable about the eighth week (Kolliker, loc. cit.). The nerves 

 are seen extending directly into the limbs soon after their first appearance ; but it is 

 not determined whether the bones and muscles are derived from extensions of the 

 dorsal plates, or have an independent origin in the ventral plates. 



OSSIFICATION OP THE UPPER LIMB. With the exception of the clavicle, all the bones 

 of the upper limb begin to ossify from cartilage. The Scapula is developed in the 

 greater part of its extent from a single osseous nucleus, but possesses also supplementary 

 nuclei in the coracoid process and acromion, and along the base. The nucleus of the 

 coracoid process is especially worthy of attention, both because it appears in the first 

 year, while the other supplementary nuclei are formed only after puberty, and because, 

 although reduced to a mere epiphysis in mammals, it forms a distinct and sometimes 

 large bone in other vertebrate animals. The acromion is ossified from two or more 

 nuclei. Along the base of the young scapula a strip of cartilage extends, corre- 

 sponding with a much more largely developed permanent cartilage found in many 

 animals ; and in this there first appears a nucleus of bone at the inferior angle, then 

 a prolonged ossification throughout its length. Occasionally a separate epiphysial 

 lamina occurs, in the border of the glenoid cavity. 



Fig. 80. OSSIFICATION OF THE CLAVICLE. Fig. 80. 



a, the clavicle of a foetus at birth, osseous in 

 the shaft, 1, and cartilaginous at both ends. 



b, clavicle of a man of about twenty-three 

 years of age ; the shaft, 1, fully ossified to tbe 

 acromial end ; the sternal epiphysis, 2, is repre- 

 sented rather thicker than natural. 



The Clavicle begins to ossify before any other bone in the body. Its ossification 

 commences before the deposition of cartilage in connection with it, but afterwards 

 progresses in cartilage as well as in fibrous substance. It is formed from one prin- 

 cipal piece, and has a thin epiphysis at its sternal end. 



Fig. 81. OSSIFICATION OF THE HUMERTTS. 



A, humerus of a full-grown foetus; B, humerus at two years of age ; C, in the third 

 year ; D, at the beginning of the fifth year ; E, at about the twelfth year ; F, at the age 

 of puberty. 



1, the primary piece for the shaft ; 2; nucleus for the articular head ; 3, that for the 

 tuberosity ; 4, for the radial condyle and adjacent part of the trochlea ; 5, for the inner 

 er trochlear eminence ; 6, for the inner part of the trochlea ; 7, for the external or 

 eapitellar eminence. 



