The Skeletal System 109 



associated with the Hfe habits of the animal. These appendages may 

 be suited for running, climbing, burrowing, flying, swimming, grasp- 

 ing, and other activities. 



Pectoral Girdle and Forelimbs. — The three pairs of bones com- 

 posing the pectoral girdle are the clavicle or collar bone, the scapula 

 or shoulder blade, and the coracoid. 



The clavicle varies in importance in different mammals. In an 

 upright form such as man, it is an important bone for muscle attach- 

 ment; while in a quadruped such as the cat, it is reduced to a tiny 

 bone imbedded in the chest muscle. In birds, it forms the large 

 "wishbone." 



The large blade-shaped scapula is important in all mammals as 

 an attachment point for the heavy muscles of the arm and shoulder. 

 Numerous special processes on the scapula accommodate these muscles. 

 The flattened lateral surface is divided into halves by one of these 

 processes, the spine. At the ventral tip of the spine is the acromion 

 process and to one side in some species is the metacromion process. 

 Ventrally a cup-shaped indentation, the glenoid fossa, provides a place 

 for the articulation of the head of the proximal bone of the arm. The 

 coracoid is much reduced in mammals, and is not present as a separate 

 bone. It is but a process on the scapula. In birds, it is a large structure 

 which helps to support the wings. 



The forelinib is composed of a series of bones, the most proximal 

 of which is the humerus. Its proximal portion, the head, articulates 

 in the glenoid fossa of the scapula. In this same proximal area are 

 roughened enlargements, the greater and lesser tuberosities, which pro- 

 vide attachment surfaces for arm muscles. At the distal portion of 

 this bone are the rounded condyles which form articulating surfaces for 

 the bones of the forearm. Here the epicondylcs, ridges above the con- 

 dyles, provide additional surfaces for muscular attachment. The fore- 

 arm is formed of two bones. The smaller of these, the radius, is able 

 to turn and thus provides free movements for the forearm. The 

 larger bone, the tdna, has a proximal semilunar notch which articu- 

 lates with the condyles of the humerus. Above this notch is the olec- 

 ranon process or elbow. Distally these two bones articulate with the 

 small cube-shaped bones, the carpals. These also allow for consider- 

 able freedom of movement. Distally the forearm terminates in the 

 long metacarpals and finally in the phalanges. In the human being 

 the carpals form the wrist, the metacarpals the hand, and the pha- 

 langes the fingers. 



