MOTION. 



415 



Fig. 217. 

 B 



Upon the principle of this proposition, it was 

 long since observed by Galileo that nature 

 greatly augments, in a thousand ways, the 

 strength of bodies without increasing their 

 weight, and that if a wheat straw which sup- 

 ports the ear, that is heavier than the whole 

 stalk, were made of the same quantity of mat- 

 ter, but solid, it would bend or break with 

 far greater ease than it now does. The feathers 

 of birds as well as the bones of animals pre- 

 sent similar provision for the combination of 

 strength, lightness, and economy of material. 

 The strength of bones, however, cannot, as 

 might possibly be inferred from the preceding 

 proposition, be increased with the same quan- 

 tities of matter indefinitely, because when the 

 diameter of the tube exceeds certain dimensions 

 it will become so thin and fragile as to break 

 almost without offering any resistance. 



The lateral strengths of prismatic bones of 

 the same materials are as the areas of their 

 sections and the distances of their centres of 

 gravity directly; and their lengths and weights 

 inversely. 



From the deductions which may be drawn 

 from the preceding proposition Galileo very 

 justly concludes that " there are limits set 

 to the magnitudes of the works of nature 

 and art, and that the size of ships, palaces, 

 and temples, trees and animals, cannot surpass 

 certain dimensions ; and he observes that large 

 animals have neither the strength nor speed 

 proportionate to their bulk, and if there were 

 any terrestrial animals much larger than those 

 we know, they could hardly move, and would 

 be much more subject to the most serious acci- 

 dents : and also that it is impossible for nature 

 to give bones to men, horses, or other animals, 

 if they were enlarged to immense heights so as 

 to perform their offices proportionally unless 

 the structure of bones were composed of ma- 

 terials much more firm and resisting, or that 

 they were made of a thickness out of all 

 proportion, which would render the figure and 

 appearance of animals monstrous." Mr. Banks 

 has found that an oak rod one inch thick, sup- 

 ported at each end, will break by its own 

 weight at the length of 57.45 feet, and a similar 

 one of iron at 38.223 feet ; Emerson also found 

 that the cohesive strength of bone is double 

 that of oak, whilst its specific gravity is only to 

 that of the latter as 1656 to 1170, or as 92 to 



65. In the Megatherium and Elephant the 

 length of the bones of the legs is small com- 

 pared with their diameters, and consequently 

 they possess greater comparative strength as 

 columns in supporting their ponderous super- 

 structures. 



In the thigh bones of most animals an angle 

 is formed by the head and neck of the bone 

 with the axis of the body, which prevents the 

 weight of the superstructure coming vertically 

 upon the shaft, converts the bone into an elastic 

 arch, and renders it capable of supporting the 

 weight of the body in standing, leaping, and 

 in falling from considerable altitudes. 



Joints. Where the limbs are designed to 

 move to and fro simply in one plane, the gin- 

 glymoid or hinge-joint is applied ; and where 

 more extensive motions of the limbs are requi- 

 site, the enarthrodial, or ball and socket joint, is 

 introduced. These two kinds of joints predo- 

 minate in the locomotive organs of the animal 

 kingdom. Though the ginglymoid joint re- 

 stricts the movements of the limbs to one 

 plane, yet it secures that precision of direction 

 and firmness of step recognized in their motions, 

 as well as the steadiness with which they sup- 

 port the trunk. The elbow, knee, and ankle- 

 joints, in man more especially, though gingly- 

 moid, are differently constituted and possess 

 different degrees of mobility. The limbs re- 

 volving upon the elbow-joint move in concen- 

 tric planes, whilst those articulated at the knee 

 allow, according to Weber, a rocking motion 

 upon each other; the ankle has the greatest 

 latitude of motion of these three ginglymoid 

 joints. 



The enarthrodial joint has by far the most 

 extensive power of motion, and is therefore 

 selected for uniting the limbs to the trunk, 

 as it is at the enarthrodial joints that the planes 

 in which all the limbs move is determined. 



The enarthrodial joints permit of the several 

 motions of the limbs termed pronation, supi- 

 nation, flexion, extension, abduction, adduc- 

 tion, and revolution upon the axis of the limb 

 or bone about a conical area, whose apex is 

 the axis of the head of the bone, and base cir- 

 cumscribed by the distal extremity of the limb. 



In consequence of the structure of the ankle- 

 joint, the foot may be directed out of the plane 

 of the leg's motion. 



The limbs, in moving upon or about the 

 heads of bones, describe arcs of circles, of 

 which the centres of motion are the axes of the 

 heads of the bones or the tubercles on which 

 they revolve. 



Ligaments. The office of the ligaments 

 with respect to locomotion is to restrict the 

 degree of flexion, extension, and other motions 

 of the limbs within definite limits. 



The strength, form, elasticity, and points of 

 attachment of ligaments are sufficient to effect 

 the objects above mentioned ; they are, how- 

 ever, destined mechanically to limit the motions 

 rather than to expend their forces in supporting 

 the weight of the limbs suspended beneath the 

 joints to which they severally belong. 



The influence of atmospheric pressure in 

 supporting the limbs was first noticed by Dr. 



