THORACIC LIMBS 



577 



VARIATIONS IN FORM AND DEVELOPMENT. 



The ulna presents few striking variations. The greater sigmoid 

 cavity either is a regular semicircle or, more commonly, the lower 

 part is straighter and slopes slightly downward instead of curving 

 upward. Rarely the lesser sigmoid cavity is flattened. 



The external border of the shaft varies in prominence and the 

 degree of roughness of its upper part. 



The nutrient foramen sometimes is found in the external 

 border. There may be a second foramen near by on the posterior 

 surface. 



The lower extremity appears to vary only in the greater or less 



degree of backward projection of its head. 



37 



