176 The DeveloiDinent of tlie Arm in Man 



The Skeletal System. — The vertebral column. The intervertebral 

 discs are reduced in thickness, while the bodies of the vertebra have 

 increased and occnpy about four-fifths of each segment. The neural and 

 transverse processes are larger and for the most part of cartilage. At 

 the tip of the neural processes, which reach about one-half way around 

 the spinal cord, is a small mass of condensed tissue at what juay be 

 considered the growing point. These processes arise entirely from the 

 body and not from the disc. So the body has probably grown at the 

 expense of the disc. The perichondrium, wliich surrounds the body and 

 its processes, is thickened along the ventral side of the bodies into the 

 anterior common ligament. 



The rihs are of cartilage surrounded by thick perichondrium, wliicli 

 is continuous with the condensed tissue anlage of the one-half the 

 sternum. The distance between the two halves of the sternum is not 

 as great as in the preceding stage and at the anterior end they ai'e just 

 beginning to come in contact with each other. There are no joint 

 cavities between the ribs and vertebrae. 



The clavicle is composed of cartilage somewhat different in appear 

 ance from that in the other bones. It is continuous with the acromion 

 and sternum by an area of condensed tissue. It is surrounded by a 

 typical perichondrium. There are distinct coraco-clavicular, costo- 

 clavicular, and interclavicular ligaments. 



The cartilaginous scapula is very much larger than in the preceding- 

 stage and contains no large areas of condensed tissue. It has moved 

 posteriorly and lies in the region from the last cervical to the fifth 

 thoracic vertebrae. Its dorsal border also extends farther dorsal than in 

 any of the preceding stages. The acromion and coracoid processes are 

 large and of cartilage with only the ordinary thickness of perichondrium 

 which is continuous with that surrounding the rest of the scapula. The 

 spine has not yet appeared but the thickened anterior border from which 

 the supraspinatus muscle arises probably represents by its lateral lip 

 the spine and by its median lip the future anterior border. The acro- 

 mion arise partially from the lateral side of the anterior border. The 

 head seems to have enlarged at the expense of part of the base of the 

 coracoid process as the long head of the biceps now arises from the 

 junction of the coracoid and the head, and the head of the humerus 

 does not rest against such a large proportional area of the coracoid. 

 There is a distinct suprascapular and a coraco-acromial ligament. At 

 the posterior angle of the scapula there is small mass of condensed tissue 

 which gives attachment to a portion of the serratus, latissimus, and teres 

 major muscles. 



