THE SKELETON 73 



mation of this ancestral history of the horse is found by an 

 examination of the early embryonic stages. According to 

 Ewart, a horse embryo 35 cm. long possess quite well-devel- 

 oped second and fourth metacarpals terminating with phalan- 

 geal structures. The ulna and radius of an embryo 50 mm. long 

 are strikingly similar to these same bones in Mesohippus. 

 In a still younger embryo the ulna is complete and corre- 

 spondingly as large as in Orohippus. 



As to the method of walking, mammals are spoken of as 

 plantigrade, digitigrade, ungtdigrade, and rectigrade. The first 

 mode of progression is exemplified by the bear, which places 

 its metacarpals and phalanges flat on the ground in walking. 

 The cat is digitigrade, walking on its toes. The horse and cow 

 are unguligrade, as they walk upon the hoof. The elephant 

 is rectigrade, the entire weight resting on a large pad, and the 

 foot immobile. In some cases, as in the Cetacea, the forefoot 

 is adapted for swimming instead of walking, and the number of 

 phalanges to each digit is more than three. 



LABORATORY STUDIES AND SUGGESTIONS 



1. What bones compose the shoulder girdle? 



2. How is the thoracic limb attached to the trunk? 



3. What does the condition of the clavicle in the cat indicate as to its past 

 history? 



4. Draw the medial aspect of the scapula and label all parts. 



5. What is the significance of the coracoid process? 



6. Which processes of the scapula derive their names from their location and 

 which from their shape? 



7. How do you distinguish the right from the left scapula? 



8. Draw the caudal aspect of the humerus and label all features. 



9. Describe what is seen in looking directly at the distal articulating surfaces 

 of the humerus. 



10. How do you distinguish the right from the left humerus? 



11. What features of the humerus derive their names from their shape? 



12. Draw the medial or inner aspect of the ulna and label all features. 



13. What features of the ulna derive their names from their form? 



14. How do you distinguish the right from the left ulna? 



15. Describe the articulations of the radius. 



16. Draw lateral aspect of radius and label all features. 



