G6 ANATOMY OF VEKTEBBATES. 



the preparatory movement ; in this the hands are brought near the 

 mesial plane, with the palmar surfaces parallel to each other ; 

 they arc then thrust forward by the extension of the arm, with 

 the points of the fingers in advance to cut the water with the 

 least resistance ; when the hands have nearly reached their greatest 

 distance from the centre of gravity, they are rotated by pronation, 

 so that the palms are directed at an oblique angle outward and 

 downward ; they are then forced backward by the abduction of 

 the Avhole arm through a large arc of a circle, having the shoulder- 

 joint for its centre, and the length of the arm for its radius ; the 

 fore-arm is then flexed, and carried into its former position pre- 

 paratory to making another stroke. During the extension of the 

 arm, the feet are drawn toward the centre of gravity, with their 

 convex surface directed obliquely backward by the extension of 

 the ankle and flexion of the hip and knee joints, and during the ab- 

 duction of the arm the flat surfaces of the feet are driven forcibly 

 backward and downward by the sudden extension of the leg. 

 From the ratio of the areas of the hands and feet, and the ratio 

 of the difference of their velocities in the two strokes, there results 

 such a preponderance of the force in the vertical direction upward 

 and in the horizontal direction forward as is sufficient to keep the 

 respiratory openings above the surface of the water, and to over- 

 come the resistance which the water opposes to the motion of the 

 body, due to its figure and velocity. 



B. Moving on J,and. In mammalian quadrupeds the limbs 

 are usually long, and support the trunk horizontally, uplifted 

 from the ground, as on columns expanded at their base. The 

 uppermost long bone is single, the next two form a pair, side by 

 side, and these rest on more numerous ossicles, transferring the 

 weight upon the base of two, three, four, or five diverging piles : 

 the single hoof of the Horse seems an exception, but it, too, ex- 

 pands to its base. The shafts of the long bones are hollow, 

 agreeably with the principle of combining greatest strength with 

 least weight. According to the lightness and speed of the quad- 

 ruped, the limb-bones are inclined to each other's axes at a 

 greater angle. In the colossal Elephant and Megathere they 

 rest on each other almost vertically, in supporting the trunk. 

 The horizontal trunk and produced head and neck of quadrupeds 

 cause the largest proportion of the weight to fall upon the front 

 pair of supporting columns, of which, accordingly, the angles of 

 the joints are less, and the direction more vertical than in the hind 

 pair, as is well exemplified in the hoofed kinds (vol. ii. figs. 307, 

 309,310). 



