Fig. 217. 
7 e 
if 
‘Upon the principle of this proposition, it was 
long since observed by Galileo that nature 
‘greatly augments, in a thousand ways, the 
ngth of bodies without mcereasing their 
t, and that if a wheat straw which sup- 
is the ear, that is heavier than the whole 
k, were made of the same quantity of mat- 
but solid, it would bend or break with 
greater ease than it uow does. The feathers 
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 
t possibly be inferred from the preceding 
roposition, be increased with the same quan- 
s of matter indefinitely, because when the 
neter of the tube exceeds certain dimensions 
will become so thin and fragile as to break 
most without offering any resistance. 
The lateral strengths of prismatic bones of 
he same materials are as the areas of their 
ons and the distances of their centres of 
vity directly; and their lengths and weights 
Po the deductions which may be drawn 
the preceding proposition Galileo very 
y concludes that “ there are limits set 
the magnitudes of the works of nature 
md art, and that the size of ships, palaces, 
ind 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 
ny 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- 
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 
a = of oak, whilst its specific gravity is only to 
Fork of the latter as 1656 to 1170, or as 92 to 
a 
, 
MOTION. 
A415 
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 ares of circles, of 
which the centres of motion are the axes of the 
heads of the bones or the tubercles on which 
they revolve. 
eae: * 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. 
