30 ANATOMY OF THE RAT 



creases in size, and air enters the lungs, when the ribs are 

 pulled anteriorly in breathing. 



Exercise VII. Sketch one of the longer ribs. 



THE ANTERIOR APPENDAGES 



The pectoral girdle includes the scapula (shoulder 

 blade), and the clavicle (collar bone). The former is a 

 flattened triangular bone with its apex pointing cranioven- 

 trad and its broad inner surface facing the vertebral 

 column. A cartilaginous disc connects one end of the clavi- 

 cle with the acromion process of the scapula. A similar 

 disc unites the other end with the anterior end of the 

 sternum. The clavicle is the only bony connection between 

 the scapula and axial skeleton. 



The concave thickened superior margin of the scapula 

 is directed cephalad. The convex vertebral margin is dor- 

 sal. The third margin, the axillary^ faces ventrocaudally, 

 and is nearly straight. The anterior, posterior, and ventral 

 corners of the bone are designated the medial, inferior, and 

 lateral angles respectively. At the lateral angle is the 

 pear-shaped glenoid cavity for articulation with the hu- 

 merus bone of the upper arm. The short coracoid process 

 (homologous to the bone of the same name in the lower 

 animals) projects medially from the cranial end of the 

 glenoid cavity. The scapular spine is a thin, prominent 

 plate (forming nearly a right angle with the scapula) 

 which divides the lateral surface of the scapula into the 

 supraspinous fossa (anterior) and the concave infraspin- 

 ous fossa (posterior). The spine tapers dorsally to a point 

 at the vertebral border; it ends ventraJly in a free pro- 

 jection, the acromiion process or acromion. The inconspicu- 

 ous projection on the caudal surface of the latter is prob- 

 ably the metacromion process. There are two longitu- 

 dinal ridges on the inner surface of the scapula. 



