GREGORY, QUADRUPEDAL LOCOMOTION 293 



GRAV1P0KTAL AND CURSORIAL TYPES OP FORE LIMB 



Much of what has been said above regarding the mechanical advan- 

 tages of short hind feet in moving a heavy load or of long feet in throw- 

 ing forward a light load applies also to the fore feet. The table of ratios 

 (Plate XXXIV), however, shows that in both graviportal and cursorial 

 types, the third metacarpal is usually longer in proportion to the radius 

 than is the third metatarsal to the femur. The radius also is usually 

 much longer in proportion to the humerus than is the tibia to the femur 

 and the radiohumeral ratio responds less clearly to changes in mode of 

 locomotion than does the tibiofemoral ratio. The adaptive reasons for 

 these facts are not altogether clear, but they must be conditioned partly 

 upon the marked differences in form and musculature between the shoul- 

 der girdle and the pelvis and upon the different situations of the fore 

 and hind limbs with respect to the center of gravity of the animal. The 

 relatively long metacarpals and long radius even of graviportal animals 

 gives a very long reach to the fore arm, as shown in Fig. 1, I. In 

 some graviportal animals, as in the Proboscidea, the humerus is much 

 longer than the scapula, the elbow is widely exserted from the body and 

 is turned outward in the back stroke, thus bringing the wrists well in. 

 Interference is avoided by the sharp pronation of the radius, so that 

 during flexion of the wrist the palm of the manus is turned partly out- 

 ward. In this type of scapula, the backward extension of the posterior 

 angle of the blade, as well as the length of the humerus, gives space for 

 a very heavy caput longus of the triceps, which thus also secures a more 

 direct upward pull on the olecranon. The postspinous fossa is located 

 almost directly above the posterior part of the great tuberosity of the 

 humerus, so that the pull of the infraspinatus muscle is nearly vertical. 

 The tuber spinas of the scapula is greatly enlarged, its upper border for 

 the trapezius, its lower for the heavy deltoideus. In some other gravi- 

 portal types (e. g., Toxodon, Rhinoceros), the scapula instead of being 

 broad and short is narrow and high, with vertically extended pre- and 

 postspinous fossae. In some cursorial types (e. g., Equus), the scapula 

 is also long, with rather narrow pre- and postspinous fossae; in others 

 (e. g., Pecora), the scapula is fan-shaped, truncate at top, with pre- 

 spinous fossa reduced and postspinous fossa much developed. 



Eegarding the relative power of the fore and hind limbs, many people 

 think that in the horse, the hind limbs furnish much more than half of 

 the locomotive power, but Stillman arrays cogent evidence 43 tending to 



43 Op. cit., pp. 69, 79, 89. 



