256 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



the important corresponding parts. We draw special 

 attention to the following points: 



1. Hip Girdle. — This is particularly strong in all bi- 

 peds, and therefore in man, in birds, in kangaroos, and 

 in the extinct biped reptiles (dinosaurs). The hip 

 girdles are not represented in the figure. 



2. Position of the Knee. — In man, in monkeys, and 

 bears among mammals, and in all reptiles and amphibians, 

 the whole hind limb is free of the body and the knee is 

 halfway down the limb. This is undoubtedly the origi- 

 nal and normal condition in land vertebrates ; but in 

 the more specialized mammals, such as carnivores and 

 herbivores, especially the ungulates or hoofed animals, 

 the knee is high up on the side of the body, in the mid- 

 dle of the so-called thigh, and the limb is free of the body 

 only from the knee down. 



3. Position of the Heel. — In man, in monkeys, in the 

 bear, and several other mammals, and in all reptiles and 

 amphibians, the tread is on the whole foot — i. e., heel 

 down. This is undoubtedly the original and normal 

 tread of the primal land vertebrate. But in all the more 

 specialized mammals the heel is lifted high in the air — 

 in the horse fifteen to eighteen inches above ground — 

 and the tread is on the toes only. Therefore land verte- 

 brates, as to their tread, may be divided into two 

 groups, Viz., plantigrade and digitigrade. Man, monkeys, 

 bears, and some other mammals, and all reptiles and 

 amphibians, are plantigrade, but all the more specialized 

 and swifter mammals and all birds are digitigrade — 

 tread only on their toes. 



Again, in mammals there are two degrees of digiti- 

 gradeness. The carnivores and all other clawed digiti- 

 grades and all birds tread on the whole length of the 

 toes to the ball, while the hoofed mammals (ungulates) 

 tread only on the tip of the last joint of the toes. The 



