246 COMPARATIVE ANATOMY 



Eusthenopteron help to bridge over the structural gulf separating the 

 ichthyopterygium and the cheiropterygium. 



The emergence of animals from the water to the land was accompanied 

 by momentous changes in their skeletons. Somehow or other fins were 

 changed into fingered appendages capable of supporting the weight of the 

 body. This transition was probably accomplished by amphibians, which 

 because they had lungs and could breathe air, were able to meet this 

 crisis in animal life. Legs and not tails became the locomotor organs. 

 Even the vertebral column was affected by these developments. 

 The pelvic girdle became attached to the vertebral column and sacral 

 vertebrae were differentiated. Speed was of life-saving value and legs 

 elongated. All organs, — heart, kidneys, brains, and all the others — were 

 improved. 



Profound changes in the visceral skeleton also occurred. With the 

 replacement of gills by lungs as organs of respiration, the skeletal supports 

 of the gills were set free for other uses. Fishes had already demonstrated 

 that a cartilaginous gill arch could be used for seizing food. Land animals 

 turned the remaining arches to other divergent uses. The tongue became 

 attached to the hyoid arch. The hyomandibular element in amphibia 

 became a sound-conducting apparatus. Some of the arches were used to 

 support the vocal cords and the voice box. 



Amidst all the many adaptive changes which affect the skeleton of the 

 appendages in land animals, both pectoral and pelvic extremities retain 

 their fundamental similarity to one another. Even the differentiation 

 of hand and foot does not obscure this fact. The diversities of form and 

 function of the tetrapod appendage do not concern us in this discussion. 

 It is, however, interesting to note that notwithstanding the high degree 

 of specialization of the extremities of man they differ little in fundamental 

 structure from those of amphibia. 



To assist their hind legs, amphibia connected the pelvic girdle with the 

 sacrum; to assist their hearing, they converted the suspensorium of the 

 jaw into an earbone, the columella. Tripartite girdles made their appear- 

 ance in amphibia. The sternum also is a novelty in this group. In the 

 theromorph reptiles, changes in the articulation of the jaw began to take 

 place, so that when mammals appeared, the old hinge between articulare 

 and quadrate had been lost, and a new hinge between dentary and 

 squamosal had taken its place. Thus were articulare and quadrate set 

 free to become malleus and incus of the ear. The beginnings of hands 

 and feet appear in arboreal mammals in adaptation to life in the trees. 

 In man, the backbone becomes vertical and the skull is balanced on the 

 occipital condyles. The facial angle, in correlation with the great enlarge- 

 ment of the brain, increases to a right angle; and a sigmoid flexure makes 

 its appearance in man's spine. 



