MECHANICS OF LOCOMOTION 



741 



The evolution of the scapula is analogous to that of 

 the ilium, chiefly in the following respects: (1) Length 

 of the neck or peduncle; (2) distance between the areas 

 of origin and insertion of the muscles; (3) breadth or 



contraction of the superior border; (4) support of 

 thorax by scapula as compared with support of viscera 

 by ilium. 



Comparison of mediportal, graviportal, and cursorial types of ungulates 



FORMS OF HUMERUS 



The distal end of the humerus is a highly distinctive 

 region in the ungulates because of the profound 

 changes of function in the passage from the ancestral 

 unguiculate type, in which the manus has powers of 

 pronation and supination, to the specialized ungulate 

 type, in which the manus is restricted to flexion and 

 extension. The progress of this change of function 

 is reflected in the structure of the distal articular 

 surface of the humerus (rotula, capitellum) and of 

 the inner and outer rugosities for the muscular attach- 

 ments, which are loiowij as the entocondylus and 

 ectocondylus. The ancestral ungulate arrangement 

 of these condyles is as follows : 



Region of entocondyle, for the attachment of 

 internal ligament, pronator teres muscle (reduced), 

 flexor carpi radialis muscle, flexor profundus digi- 

 torum muscle, flexor sublimis digitorum muscle, pal- 

 maris longus muscle. 



Region of ectocondyle, for the attachment of exter- 

 nal ligament, supinator longus muscle,^" extensor 

 communis digitorum muscle, extensor minimi digiti 

 muscle, extensor carpi ulnaris muscle. °' 



In the passage from the unguiculate to the ungulate 

 type we observe the following general laws of trans- 

 formation of the ectocondyle, entocondyle, and rotula: 



(1) All primitive ungulates (such as Periptychus, 

 Phenacodus, Meniscotherium, Corypliodon, Uintatlie- 



rium) inherit a large entocondyle and a relatively 

 small ectocondyle. 



(2) In all higher ungulates this condition is exactly 

 reversed: the entocondyle is reduced or wanting, the 

 ectocondyle is greatly enlarged. 



(3) In quadrupeds of intermediate size, or medi- 

 portal types (such as Tapirus), the entocondyle and 

 ectocondyle are equally developed. 



Thus the greatest muscular rugosities of the primi- 

 tive graviportal Corypliodon are entocondylar, or on 

 the inner side of the lower end of the humerus, whereas 

 the greatest muscular rugosities of the progressive 

 graviportal Rhinoceros are ectocondylar. In other 

 words, there is a transfer of the balance of muscular 

 power from the inner pronator and flexor side of the 

 limb to the outer supinator and extensor side, together 

 with a change of function of the extensor carpi ulnaris 

 into a flexor of the forefoot (Windle and Parsons, 

 1901.1, pp. 700, 701). 



In the meantime the distal articular extremity of 

 the humerus, which is primitively composed of the 

 rotula for the ulna and capitellum for the radius, an 

 asymmetric arrangement, becomes a more symmetric 

 joint. This symmetry of the inner and outer sides 

 is attained only in the highly specialized cursorial 

 elbow joint of Equus, which is strictly limited to 

 flexion and extension. 



These four types of humeral adaptation may be 

 summarized and contrasted as follows: 



Types of humeral adaptation 



Intermediate mediportal 

 (e. g., Tapirus) 



Cursorial (e. g., Equus) 



Entocondyle 



Ectocondyle 



Rotula and capitellum 



Medium. 

 Asymmetric. 



Medium. 

 Medium. 

 Asymmetric. 



Greatly reduced. 

 Greatly enlarged. 

 Asymmetric. 



Reduced. 

 Reduced. 

 Symmetric. 



5" Arises from the supinator crest above the ectocondyle. The supinator brevis is absent or vestigial in ungulates. It originally arose partly from the ectocondyle 

 (Windle and Parsons, 1897.1, p. 402; 1901.1, p. 701). 



51 Serves as a flexor of the carpus in ungulates (Windle and Parsons, 1903.1, p. 391). 



