lo LABORATORY DIRECTIONS 



vertebra supports the pelvic girdle. The vertebrae articulate 

 with one another by over-lapping processes, a condition charac- 

 teristic of land vertebrates. The centra are amphicoelus as in 

 the shark. 



A sternum, if it may be called such, is represented by a few 

 cartilages on the ventral side. 



The short triangular ribs have two heads, a dorsal tuberculum 

 and a ventral capitulum, for articulation with the transverse 

 processes of the vertebrae. 



Vertebral Column, Sternum and Ribs of the Alligator 



The vertebral column is differentiated into the five regions 

 characteristic of mammals. There are nine cervical, bearing 

 ribs; ten thoracic, bearing long ribs which reach the mid-ventral 

 line; five lumbar; two sacral; and numerous caudal vertebrae. 

 The sacral vertebrae bear short stout sacral ribs for the support 

 of the pelvic girdle. Haemal arches are present on some of the 

 caudal vertebrae. Toward the end of the tail, however, the 

 various processes have become so reduced that practically only 

 the centra remain. Alligator vertebrae are procoelus (centra 

 are concave anteriorly, convex posteriorly). 



A sternum is present between the two halves of the pectoral 

 girdle. It is drawn out posteriorly into long curved cartilages. 



Observe the manner of attachment of the ribs. Are tuber- 

 culum and capitulum present as in Necturus ? Costal (rib) 

 cartilages make their first appearance in reptiles. 



Vertebral Column of the Cat 



In the vertebral column of the cat there are five regions: 

 (i) Cervical vertebrae, seven in number and forming the 

 skeleton of the neck. 



(2) Thoracic or costal vertebrae, thirteen, bearing ribs. 



(3) Lumbar vertebrae, seven, without ribs. 



(4) Sacral vertebrae, three fused into one mass and supporting 

 the pelvic girdle. 



(5) Caudal vertebrae, about twenty-three forming the skele- 

 ton of the tail in ordinary cats, but only four or five in the 

 short-tailed Manx cats. 



Thoracic Vertebrae. — All vertebrae are built on the 

 same general plan, but there are many kinds of modifications. 



