200 



THE SKELETON. 



Internal or Cylindrical Portion, The cylindrical portion forms the inner two- 

 thirds of the bone. It is curved, so as be convex in front, concave behind, and 

 is marked by three borders separating three surfaces. The anterior border is 

 continuous with the anterior margin of the flat portion. At its commencement 

 it is smooth and corresponds to the interval between the attachment of the Pec- 

 toralis Major and Deltoid muscles; about the centre of the clavicle it divides 

 to inclose an elliptical space for the attachment of the clavicular portion of the 

 Pectoralis Major. This space extends inwards as far as the anterior margin of 

 the sternal extremity. The superior border is continuous with the posterior 

 margin of the flat portion, and separates the anterior from the posterior surface. 

 At its commencement it is smooth and rounded, becomes rough towards the 

 inner third for the attachment of the Sterno-mastoid muscle, and terminates at 

 the upper angle of the sternal extremity. The posterior or subclavian border 

 separates the posterior from the inferior surface, and extends from the conoid 

 tubercle to the rhomboid impression. It forms the posterior boundary of the 

 groove for the Subclavius muscle, and gives attachment to the fascia which 

 incloses that muscle. The anterior surface is included between the superior and 

 anterior borders. It is directed forwards and a little upwards at the sternal 

 end, outwards and still more upwards at the acromial extremity, where it be- 

 comes continuous witli the upper surface of the flat portion. Externally, it is 

 smooth, convex, nearly subcutaneous, being covered only by the Platysma ; 

 but corresponding to the inner half of the bone, it is divided by a more or less 

 prominent line into two parts : an anterior portion, elliptical in form, rough, 

 and slightly convex, for the attachment of the Pectoralis Major; and an upper 

 part, which is rough behind, for the attachment of the Sterno-cleido-mastoid. 

 Between the two muscular impressions is a small subcutaneous interval. The 



Fig. 149. Left Clavicle. Anterior Surface. 



Fig. 150. Left Clavicle. Inferior Surface. 



posterior or cervical surface is smooth, flat, directed vertically, and looks back- 

 wards towards the root of the neck. It is limited, above, by the superior bor- 

 der ; below, by the subclavian border ; internally, by the margin of the sternal 

 extremity ; externally, it is continuous with the posterior border of the flat 

 portion. It is concave from within outwards, and is in relation, by its lower 

 part, with the suprascapular vessels. It gives attachment, near the sternal 



