THE CLAVICLE 139 



inches) transversely and 6.2 centimetres (2^ inches) in its sagittal axis. It is bounded by the 



first thoracic vertebra behind, the upper margin of the manubrium sterni in front, and the first 



rib on each side. As the upper margin of the manubrium sterni is oftenest on a level with 



jthe disc between the second and third thoracic vertebrae, it follows that the plane of the open- 



[ing is directed obliquely upward and forward. The angle of the sternum {angulus Ludovici) is 



riusually opposite the body of the fifth thoracic vertebra and the xiphi-sternal junction corre- 



Ijlsponds to the disc between the ninth and tenth thoracic vertebrae. The lower aperture of the 



thorax is very irregular, and is formed by the twelfth thoracic vertebra behind, the twelfth ribs 



laterally, and in front by two curved lines, ascending one on either side from the last rib, along 



the costal margin to the lower border of the gladiolus. The two borders form the costal arch, 



which in the median line below the sternum forms the infrasternal angle. From this angle the 



xiphoid process projects downward. The intervals between the ribs are the intercostal spaces, 



and are eleven in number on each side. 



The ratio of the sagittal and the transverse diameter of the thorax forms the thoracic index, 

 which is higher in the female and in children, in whom the thorax is more rounded. In the 

 embryo, the index is very much higher, the sagittal diameter being greater than the transverse. 

 In the early embryo, the index is nearly 200; at birth it is about 90. In the adults it varies 

 from 70 to 75, averaging 2 or 3 per cent, lower in the male than in the female. It is also 

 lower in the negro than in the white race. (Rodes, Zeitschr. f. Morph. u. Anthrop., Bd. 9.) 



//. THE APPENDICULAR SKELETON 

 A. BONES OF THE UPPER EXTREMITY 



The bones of the upper extremity may be arranged in four groups correspond- 

 ing to the division of the Hmb into four segments. In the shoulder are the 

 clavicle and the scapula, which together constitute the pectoral or shoulder girdle; 

 in the arm is the humerus; in the forearm are the radius and ulna; and in the hand 

 the carpus, the metacarpus, and the phalanges. 



THE CLAVICLE 



The clavicle [clavicula] or collar bone (figs. 168, 169) is situated immediately 

 above the first rib and extends from the upper border of the manubrium sterni, 

 laterally and backward to the acromion process of the scapula. It connects the 

 upper limb with the trunk, and is so arranged that whilst the medial end rests on 

 the sternum and first costal cartilage, the lateral end is associated with the scapula 

 in all its movements, supporting it firmly in its various positions and preventing it 

 from falling inward on the thorax. 



The clavicle is a long bone, and when viewed from the front presents a double 

 curvature, so that it somewhat resembles in shape the italic letter /. The medial 

 curve, convex forward, extends over two-thirds of the length of the bone; the 

 lateral, concave forward, is smaller and confined to the lateral part. For descrip- 

 tive purposes the clavicle may be divided into a medial prismatic portion, a 

 lateral flattened portion, and two extremities. 



Prismatic portion. — The medial two-thirds of the bone, extending from the 

 sternal extremity to a point opposite the coracoid process of the scapula, has the 

 form of a triangular prism. This portion, however, is subject to considerable 

 variations of form, being more cylindrical in ill-developed specimens and be- 

 coming almost quadrangular when associated wdth great muscular development. 

 In a typical specimen it is marked by three borders separating three surfaces. 

 Of these, the anterior surface is convex and chvided near the sternal end by a 

 prominent ridge into two parts, a lower, giving origin to the clavicular portion of 

 the pedoralis major; an upper, for the clavicular portion of the sterno-cleido- 

 mastoid. Near the middle of the shaft the ridge disappears, the surface is smooth, 

 and is covered by the platysma myoides. Occasionally this surface is pierced by a 

 small canal, transmitting a cutaneous nerve from the cervical plexus. The 

 posterior surface is concave, forming an arch over the brachial plexus and the 

 subclavian artery, broadest medially and smooth in its whole extent. _ It 

 gives origin near the sternal extremity to a part of the sterno-hyoid and occasion- 

 ally to a few fibres of the sterno-thyreoid. Somewhere near the middle of this 

 surface is a small foramen, directed laterally, for the chief nutrient artery of the 

 bone, derived from the transverse scapular (suprascapular) artery. Sometimes the 



