366 THE CORRESPONDENCE BETWEEN THE [CHAP. 



the middle segment is bent downwards, so that its ventral 

 surface faces inwards and its dorsal surface outwards ; and 

 the joints between these segments (elbow and knee) form 

 prominent angular projections. The third segment being 

 bent to a greater or less degree, in the opposite direction 

 to the middle one, retains much of its primitive position, 

 the dorsal surface being directed upwards and the ends of 

 the digits pointing outwards. The relations of the pre-axial 

 and post-axial borders of the limb are unchanged. No 

 Mammal habitually carries its limbs in this position, although 

 the climbing Galeopithecus and the Sloths are not far from 

 it. It is, however, very nearly the normal position of 

 some Reptiles, especially the Tortoises, though it is ill 

 adapted for anything but a very slow and clumsy mode of 

 progression. 



Trie next change, and one which takes place at a very 

 early period in embryonic life, and which is one of the most 

 essential in giving the characteristic conformation of the 

 extremities of the higher Vertebrates, is a rotation of the 

 whole limb from the proximal end, though in opposite 

 directions in each case. 



The anterior extremity (see Fig. E.) is rotated from the 

 shoulder, through nearly a quarter of a circle, backwards^ so 

 that the humerus, instead of being at a right angle to the 

 axis of the trunk, is nearly parallel with it, the elbow points 

 backwards, the pre-axial side is outwards, and the post-axial 

 side towards the middle line of the body, and as long as the 

 radius and ulna retain their primitive parallel position, the 

 manus is placed with the ends of the digits directed back- 

 wards, the pre-axial side being external. 



The hind limb (see Fig. F.) is, at the same time, rotated 

 from the hip to the same extent forwards, so that the femur 

 is also nearly parallel to the axis of the body, but with the 



