198 



INTRODUCTION TO EVOLUTION 



The automatic springing action of the ligaments is augmented by the 

 contraction of the flexor muscles of the leg, the tendons of which are also 

 attached to the phalanges (in the diagram these tendons are not shown, 

 though the cut ends at the attachments of one of the principal ones. 

 Flexor digitalis profundus, are indicated). Near the upper ends of these 

 tendons check ligaments connect to adjacent bones. Thus when tension 

 is placed on these tendons, as for example by impact of the foot on hard 

 ground, these check ligaments may be stretched, reinforcing the action of 



Cannon bone 



Interosseus medius muscle 



Interosseus tendon 



Pad 



Sesamoid bone 



ToesV -IV -111 



HYRACOTHERIUM 



Fetlock 

 joint 



Sesamoid ligaments 



Tendon of flexor 



digitalis profundus 



muscle (cut) 



Toe 111 



EQUUS 



FIG. 10.2. Pad-supported forefoot of Hyracofherium compared with the spring- 

 ing mechanism in the foot of the modern horse. The pod in Hyracofherium is 

 shown in black, as are the tips of digits IV and V. Tendons and ligaments are 

 shown in white; bones are dotted. (Modified from Camp and Smith, "Phylogeny 

 and functions of the digital ligaments of the horse," Memoirs, University of 

 California, Vol. 13, 1942.) 



the ligaments shown in Fig. 10.2 and preventing injury to the latter by over- 

 stretching (sprain). 



Horses obtain their food by grazing — feeding on such vegetation as 

 grass covering the surface of the ground. If a long-legged animal is to do 

 this, some means must be provided for getting the mouth down to the 

 ground. Lengthening the neck would accomplish the result, and to a 

 moderate degree the neck of the horse has been lengthened. But a horse 

 has a large and heavy head; there would be obvious mechanical dis- 

 advantage in perching it on the end of a really long neck. To a considerable 

 extent the problem has been solved for the horse by lengthening the 



