i86 



CHORDATE ANATOMY 



squamosal had taken its place. Thus were articular 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 Hfe 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. 



The Appendicular Skeleton in Man 



In man, as in other mammals, each limb is divided into four regions. 

 These are, in the upper limb, the pectoral girdle, upper arm, forearm and 



DERMATOME 



Fig. 176. — Stages in the development of the extremities in mammals. (Redrawn 

 after Bardeen, Lewis and Corning.) ^ is a cross section of a monkey embryo showing 

 an arm bud. (Redrawn from Arey after Kollman.) B is a seven-week human embryo 

 in left lateral aspect. (Redrawn after Corning.) C is an eight-week human embryo in 

 left lateral aspect. (Redrawn after Corning.) £> is a nine-week human embryo in left 

 lateral view. 



hand. In the lower hmb they are hip, thigh, leg or shank, and foot. A 

 comparison of the human skeleton (Figs. 144 and 145) with that of the 

 gorilla (Fig. 175) shows that, bone for bone, the two correspond. The dif- 



