THOKACIC LIMBS 531 



slightly convex anterior surface faces upward, forward, and outward 

 and its concave posterior surface faces downward, backward, and 

 inward. The spine is not throughout of the same height ; that is, its 

 free edge is not at every point equally distant from the outer surface 

 of the bone. 



The free edge begins at the vertebral border in a small triangular 

 flattened area which is smooth and covered by the trapezius muscle. 

 From this point the free edge slopes outward, as well as forward and 

 downward, as far as the middle of the bone, whence it is continued 

 downward and forward and only slightly outward until the spine has 

 attained its maximum height above the surface. The edge is thin 

 and rounded ; it is slightly swollen near the middle. The maximum 

 height is reached at some little distance above the lower end, where 

 the edge gives off, quite abruptly, a thin quadrate process which is 

 directed backward, downward, and outward. This process is the met- 

 acromion. 1 The lower end of the spine, beyond the metacromion, is 

 continued downward as a pointed process, the acromion. 



The acromion 2 (Figs. 418, 421) is a thin triangular process, the 

 apex of which points downward and forward. The outer edge is 

 continuous with the outer edge of the spine. The inner edge runs 

 inward and upward as the lower edge of the spine, and joins the outer 

 surface where the neck joins the body. The acromion thus lies on 

 the outer side of the head and neck of the scapula, and in some cases 

 projects below the head. It is convex from within outward on its 

 anterior surface, and slightly concave in the same direction on the 

 posterior surface. Its tip is connected by fascia with the rudimen- 

 tary clavicle. The spine, the metacromion, and the acromion afford 

 attachment to a number of important muscles. The posterior part 

 of the supraspiuatus muscle arises from the convex anterior surface 

 of the spine and of the acromion. The anterior part of the infra- 

 spinatus muscle arises from the greater part of the concave posterior 

 surface of the spine and from the inner surface of the metacromion. 

 The posterior border of the metacromion and the adjoining part of 

 its outer surface receive the insertion of the so-called levator clavicula3 

 muscle. The line of insertion of the trapezius begins below, at the 

 centre of the outer surface of the metacromion, and is continued 



1 From (Gr.) meta, beyond, and acromion, the acromion. 



2 From (Gr.) akron, a summit, and omos, the shoulder. 



