MUSCULAR SYSTEM 1 29 



the radius around the uhia. There also the appendages may form 

 grasping organs, both features being found to a less extent in several 

 lower groups. ]n the bats digits II to V are greatly elongated (either 

 metacarpals or phalanges may be lengthened) to support the wing, 

 the first digit remaining normal. In the whales and sirenians the 

 basal parts of the fore limb are greatly shortened, while there is a 

 multiplication of the phalanges, recalling that of the plesiosaurs. 

 The hind limb is entirely lacking in the sirenians and some of the 

 whales; in other whales there are two vestigial bones (Pfemur and 

 tibia) imbedded in the muscles of the trunk. 



The mammalian humerus is frequently perforated by a (supra- 

 or entepicondylar) foramen passing through the inner lower end, a 

 feature found elsewhere only in some therimorphs. In many un- 

 gulates the ulna is reduced and may be fused with the radius; else- 

 where it is well developed. Even where reduced it always bears 

 on its proximal end a strong olecranon process, extending beyond 

 the elbow-joint for the attachment of the extensor muscles of the fore- 

 arm. The femur bears a varying number (up to three) of prominences 

 or trochanters for the attachment of muscles. The fibula resembles 

 the ulna in its tendency to reduction. The patella (p. 127) at the 

 knee-joint is analogous to the olecranon process, though it never 

 joins the other bones. 



The details of the modification of the feet cannot be described 

 here. The ankle-joint is never intratarsal but always between tar- 

 sal and crural bones. There is considerable variety in the extent to 

 which the bones of the feet rest upon the ground. In the plantigrade 

 foot, as in the bear and man, the sole of the foot includes the meta- 

 podial bones; in the digitigrade forms, like the dog and cat, the sole 

 includes only the distal phalanges, while in tinguligrades (cow, horse) 

 the weight of the body is supported on the hoofs (p. 31) developed on 

 the upper (anterior) surface of the distal phalanges. There is fre- 

 quently a reduction of the digits, reaching its extreme in the horse 

 where only digit III perists in a functional condition. lASA/iy 



THE MUSCULAR SYSTEM J^^-^ 



Practically all motion in vertebrates is caused by muscles which 

 arise from the mesoderm. While other cells may have a certain 

 power of changing shape, the muscle cells possess this in a marked 

 degree, and so that they may cause the greatest amount of motion 

 la the parts to which they are attached, they are very long, stimula- 

 9 



