312 



THE VERTEBRATE BODY 



have been modified in this change, but actually the same bones can be 

 identified in most instances. We have only one bone in our lower jaw as 

 compared to three pairs of bones in the jaw of the frog, and we have 

 three ear bones in each ear as compared to one in the frog. Careful 

 studies have shown that the two extra ear bones in man have been de- 

 rived from two bones in the corners of the jaws of the frog. The bones 

 are still there ; they have merely changed their functions and positions. 

 Alan has seven cervical vertebrae in the neck instead of one as in the 

 frog. The frog has seven trunk vertebrae with long transverse proc- 



Episternum 



Omosternum 

 Supra scapula 



Scapula 

 Clavicle 



Coracoid 



Epicoracoid 



Mesosternum 

 Xiphisternum 



Fig. 22.4. Ventral view of the sternum and pectoral girdle from the frog skeleton. 



esses. In man the trunk vertebrae are divided into two kinds, the 

 thoracic vertebrae which bear ribs, and the lumbar vertebrae which have 

 long transverse processes as in the frog. The twelve pairs of ribs of 

 man encircle the thorax, and most of them are attached to the sternum, 

 creating the thoracic basket which holds the lungs and heart. In the 

 pelvic region there are five vertebrae fused to form the sacrum instead 

 of a single sacral vertebra as in the frog. Man also has a vestige of 

 the tail called the coccyx which corresponds to the urostyle in the frog. 

 The pectoral girdle in man lacks a coracoid bone, but a vestige of 

 this bone is present in a child. It eventually fuses to the scapula, form- 

 ing the coracoid process. The radius and ulna are separate bones in 

 man allowing us to turn the hand freely, a movement which is indis- 



