XVI. 



ON THE EFFECT OF IMPACTS AND STEAINS ON 

 THE FEET OF MAMMALIA.* 



The principal specializations in the structure of the feet of the 

 Mammalia may be summarized as follows : 



I. The reduction of the number of the toes to one in the Peris- 

 sodadyla (horses, etc.), and two in the Artiodadyla (cloven feet). 



II. The second hinge-joint in the tarsus of the Artiodactyla. 



III. The trochlear ridges and keels at the various movable 

 articulations of the limbs. These are as follows : 



1. Looking downward — 



a, Intei'trochlear crests of the humerus. 

 p. On the carpal end of the radius. 

 7. Metacarpals, distal ends. 



6. Tibia distally. 



e. Metatarsals distally. 



2. Looking upward — 



a. Kadius distally. 

 p. Astragalus, edges. 



7. Astragalus distally {Artiodadyla). 

 6. Phalanges (very weak). 



The following observations may be made respecting the struct- 

 ures included under Division III : The trochlear keels which look 

 downward are much the most prominent and important. Those 

 enumerated as looking upward are weak and insignificant, or of a 

 different character from the down-looking ones. The latter are 

 all projections from the middles of the ends of the respective ele- 

 ments. The up-looking are generally projections of the edges of 

 bones. Such are the lateral crests of the astragalus, and the ad- 

 jacent edges of the cuboid and navicular bones which cause the 

 distal emargination of the astragalus in the Artiodactyla. The 



* Read before the National Academy of Sciences, April, 1881. Abstract. Some 

 of the points of tliis paper have already been discussed in the preceding article, but 

 the present abstract contains additional matter. 



