28 LABORATORY DIRECTIONS 



Pectoral and Pelvic Girdles of the Alligator 



The pectoral girdle consists of a dorsal scapula; a ventral 

 coracoid; and a slender interclavicle bone between the two 

 coracoids. There are no clavicles. 



The pelvic girdle is wholly ossified and contains the typical 

 three regions. The ilium extends dorsally and forms an articu- 

 lation with the vertebral column; the pubis extends ventrally 

 and anterior; the ischium ventrally and posterior. The ischia 

 of the two sides meet and form the ischial symphysis. 



Pectoral Girdle and Fore Limb of the Cat 



1. Scapula. — This is the shoulder blade. At the ventral 

 end is the glenoid fossa for reception of the humerus bone of the 

 arm. A short spine, the coracoid process, projects medially from 

 the anterior end of this fossa. This is a remnant of the coracoid 

 bone found in certain lower vertebrates. The prominent keel 

 on the outer surface of the scapula is the spine. This has two 

 projections; the distal one is the acromion, the other the 

 metacromion. 



2. Clavicle. — In some mammals, as in man, this is a well 

 developed bar of bone, extending from the scapula to the 

 sternum. In the cat, however, it is greatly reduced, being repre- 

 sented on each side by a short, slender rod of bone entirely 

 embedded in the muscles, and not articulated with any other 

 bones. It is usually absent on mounted skeletons. 



3. Humerus. — This is the single bone of the upper arm. At 

 its proximal end is the head, a rounded swelling with a smooth 

 face for articulation with the glenoid fossa. The prominent 

 ridge extending from the lateral border of the head is the greater 

 tuberosity; the less conspicuous ridge on the medial side of the 

 head is the lesser tuberosity. The depression between the two 

 is the bicipital groove. From the ventral end of the greater 

 tuberosity the pectoral ridge extends down the anterior face of the 

 shaft. Not quite halfway down, it is joined on the lateral side 

 by the deltoid ridge, which starts at the posterior end of the 

 greater tuberosity. The distal end of the humerus presents an 

 eminence on either side, the outer and inner condyles. Above the 

 inner condyle the shaft is pierced by the supracondyloid foramen, 



