MECHANICS OF LOCOMOTION 



745 



4. The antero-inferior or ventral triangular surface 

 (fossa iliaca) serves mainly for the attachment of the 

 iliacus muscle, a flexor of the femur which is inserted 

 into the second trochanter (t")- 



Starting with this interpretation of the adaptation 

 of the bony surfaces of the ilium to its muscular func- 

 tions we observe the following simple principles: 

 (1) The expansion of the borders cc and ss is greater 

 or less according to the respective development of 

 weight or of speed; (2) thus in cursorial types such 

 as the Equidae the borders cc and s.s remain rela- 

 tively narrow, while the thin crista iliaca between 

 them remains relatively broad; (3) the expansion of 

 the border ss and of the adjacent suprasacral fossa 

 for the attachment of the gluteus medius muscles 

 is a universally progressive character 

 in mediportal and graviportal ungu- C%N 



lates and is seen in different degrees \| .( 



in Tapirus, Equus, Rhinoceros, Pal- '^Ly 



aeosyops, and other Perissodactyla, 

 but it is relatively far more marked 

 in the graviportal Artiodactyla (such 

 as Hippopotamus) than in the Peris- 

 sodactyla; (4) the expansion of the 

 border cc is also universal in medi- 

 portal and graviportal quadrupeds 

 {Amynodon, Rhinoceros, Brontothe- 

 rium) with a heavy abdomen and 

 large abdominal muscles; (5) thus 

 the primitive concave superior bor- 

 der cs or crista iliaca is contracted 

 {Rhinoceros) and then obliterated by 

 the expansion of the rugose borders 

 cc and ss, which finally become con- 

 fluent and form a continuous arch. 



All the most ancient perissodac- 

 tyls, such as Eohippus, Hyrachyus, 

 and even Amynodon, exhibit a flat or 

 concave crista iliaca between the 

 tuber coxae (cc) and the tuber 

 sacrale (ss). In such cursorial and 

 subcursorial types of ilium with con- 

 cave or interrupted superior border 

 (crista Uiaca) there is an extension 

 of the gluteal muscles forward to 

 unite with the fibers of the longis- 

 simus dorsi, making possible the gal- 

 lop. In primitive artiodactyls also 

 this flat or concave superior border, 

 or crista iliaca, is older than the con- 

 vex superior border, because the 

 subcursorial stage always appears to antedate the 

 graviportal stage. 



The rounding out of the superior border (crista 

 iliaca) of the ilium which accompanies the expansion 

 of the gluteal abdominal muscles is proved to be a 

 progressive or secondary character by the comparison 

 of a great number of mediportal with succeeding 



graviportal types of animals. For example, we may 

 compare the lower Oligocene Caenopus with the upper 

 Miocene Teleoceras (fig. 673, G). Again, we may 

 compare the mediportal upper Eocene Amynodon 

 with the graviportal Oligocene Metamynodon, or the 



A B C 



Figure 675. — Limb structure of perissodact3'ls: Fore limbs of Heptodon and Tapirus 

 A, Heptodon calzkaliM (family Lophiodontidae), one-fourth natural size; B, Tapirus lerreslris, and C, Tapirus 



indicus (family Tapiridae), one-sixth natural size. Showing the contrast between a primitive lower Eocene 



cursorial type {Heptodon) and a persistent modern mediportal type (.Tapirus). 



incipient graviportal Eocene Palaeosyops with the 

 Oligocene Brontotherium. Thus the broad spreading 

 and convex superior border is correlated with the 

 vertically placed pelvis and elongate femur of such 

 graviportal forms as Brontotherium and the rectigrade 

 Elephas; it is correlated with the slower gait and with 

 heavy abdominal muscles. 



