THE ENDOSKELETON: GIRDLES, THE STERNUM, AND APPENDAGES 93 



muscles are dissected. The scapulae or shoulder blades are the large, flat triangu- 

 lar bones situated dorsal to the anterior ribs. The mammalian scapula has certain 

 characteristics which differentiate it readily from the scapulae of other verte- 

 brates. It is triangular in form, the apex of the triangle articulating with the 

 humerus. Its outer surface bears a prominent ridge, the spine of the scapula. 

 The ventral end of the spine terminates ventrally in a pointed projection, the aero- 

 mion process; just above this and projecting laterally and posteriorly is the meta- 

 cromion process, very long in the rabbit. The apex of the scapula is concavely 

 curved, forming the glenoid fossa. From the anterior side of the rim of the fossa 

 a small beaklike process projects medially; this is the coracoid process, the vestige 

 of the coracoid bone. In mammalian embryos, this process is a separate bone. 

 For facilitating the description of muscle attachments the various surfaces and 

 borders of the scapula are named as follows : the part of the external surface ante- 

 rior to the spine is the supraspinous fossa; the part posterior to the spine, the 

 infraspinous fossa; the whole of the internal surface is the subscapular fossa; 

 the dorsal border is the vertebral border; the anterior margin, the anterior border; 

 the posterior margin, the axillary border. Draw the scapula from the outer surface. 



The sternum consists of a longitudinal series of pieces, the sternebrae, eight 

 in the cat, six in the rabbit. The first sternebra is called the manubrium 

 and articulates with the first thoracic rib at its center. The next six (cat) or 

 four (rabbit) sternebrae constitute the body of the sternum. The last piece is 

 called the xiphisternum and terminates in a xiphoid or ensiform cartilage. Note 

 points of articulation of the sternebrae with the ribs. 



The fore limb is fairly typical. The humerus has a large rounded head fitting 

 into the glenoid fossa and greater and lesser tuberosities at the sides of the head. 

 The anterior surface of the humerus below the tuberosities is slightly elevated 

 into ridges or crests (two in the cat, one in the rabbit) which serve as points of 

 muscle attachment. The lower end of the humerus is rounded for articulation 

 with the bones of the forearm and is divided into two portions, an outer mass, 

 the capitulum, and a medial mass, the trochlea. Above the capitulum is a pro- 

 jecting ridge, the lateral epicondyle; and a similar medial epicondyle is situated 

 above the trochlea. Near the medial epicondyle the bone is pierced by an 

 opening, the supracondyloid foramen, absent in the rabbit. 



The forearm consists of radius and ulna, the latter the larger. The proximal 

 end of the ulna forms a prominent projection, the elbow or olecranon. Distal to 

 this is a deep semicircular concavity, the semilunar notch, which articulates with 

 the trochlea of the humerus. The distal border of the notch forms another 

 projection, the coronoid process. Observe that the proximal end of the radius is 

 situated lateral to or in front of the proximal end of the ulna, while its distal 

 end is medial to the distal end of the ulna. In other words, the radius crosses 

 obliquely in front of the ulna. The cause of this crossing was explained in 

 Section III. 



