414 MOTION. 
The number of moveable articulations ina joints, with ridges and protuberances for th 
skeleton determines the degree of its mobility tachment of muscles, and with levers adap 
within itself; and the kind and number of these 
articulations of the locomotive organs determine 
the number and disposition of the muscles act- 
ing upon them. See ArticuLaTION. 
The strength, density, and elasticity of the 
external skeleton of animals have been but 
very partially investigated or made an ob- 
ject of either physiological or mechanical en- 
quiry, notwithstanding their great importance 
in the animal economy generally, as well as 
their office in locomotion. 
A superficial inspection, however, is suffi- 
cient to detect that the shells of those animals 
which reside constantly at the bottom of the sea, 
as the Astrea triduina, Phombus, &c. are more 
dense, and contain a greater number of calca- 
reous lamine than those which swim or float, 
either by means of specific organs of progres- 
sion, such as the Ianthina vulgaris, the Lymne, 
and Hyalea, or upon hydrostatic principles, as 
in the Neutiios, assisted, it is believed, by the 
siphon. Shells are formed with a design to 
resist the greatest external pressure, consistent 
with the least expenditure of materials, and 
with regard to the habits of the animal. The 
bones of vertebrated animals, especially those 
which are entirely terrestrial, are much more 
elastic, hard, and calculated by their chemical 
elements to bear the shocks and strains incident 
to terrestrial progression than those of the 
aquatic vertebrata; the bones of the latter 
being more fibrous and spongy in their texture, 
the skeleton is more soft and yielding. 
The bones of the higher orders of vertebrata, 
such as the Mammalia, which are designed to 
afford large surfaces for the attachment of their 
powerful muscles of locomotion, are constructed 
in such a manner as to combine lightness with 
strength; therefore their surfaces are convex ex- 
ternally, concave within, and strengthened by 
ridges running across their discs: such are the 
forms of the scapular and iliac bones. 
The long bones of the legs and arms of 
Mammalia are piled on each other endwise, 
forming a series of moveable columns, which in 
the standing position are directed vertically ; 
these are designed to support the head, neck, 
and trunk, with all their contents and appen- 
dages, together with their own weight, and to 
elevate the trunk to some variable height above 
the plane of position. 
It would indeed be a problem of no small 
difficulty, if it were proposed to an artist to 
erect a moveable column, com of a de- 
finite number of rods, so united and inclined 
as to fulfil all the objects, for which the long 
bones of the extremities are designed when 
viewed only mechanically, and adapted to 
support the weight of the superincumbent or- 
gans, to present the lengthened dimensions ne- 
to raise the trunk often far above the 
plane of motion, the strength requisite to bear 
the shocks directed upon them both vertically 
and laterally, the symmetry of form and beauty 
of proportion corresponding to the outline and 
functions of other organs, their extremities being 
furnished with articulating surfaces for the 
to perform all the varied offices of locomotic 
n quadrapeds, which have four osseous 
columns to support the superincumbent orgar 
the pressure of the trunk on each leg is 
half that in bipeds; but owing to the he 
zontal inclination of the trunk and the proje € 
tion of the neck and head, the anterior osseous 
pedestals have to sustain the largest proportion” 
of the weight ; and we consequently find that the - 
angle formed by the bones of the anterior ex-— 
tremities at the joint are less, and the direetic 
of the bones nearer the vertical plane in these 
than in the posterior: this arrangement is most 
» 8 a8 pe in the larger Ruminantia d 
achydermata, especially in the Elephant, 
Horse, &e. We an. shecstdas readily per- 
ceive why the shafts of the long bones of — 
ia are pa “ 
the leas — arms of most Mam: 
tia ollow cylinders; the prismatic outlin 
peolossjualéa in the Elephant and Megatheriu: 
The weights which cylindrical or tic 
flexible columns will support perpendicularl 
when their bases and composition are equal 
is, according to Euler,* in the inverse ratio of — 
the squares of their lengths, therefore if we take 
any bones, of similar materials and thickness, — 
but of which the lengths are as 1, 2, 3, 4, 4 “ 
they will support weights without flexion rela- 
latively in the proportions 1, }, }, t 
whilst the lengths increase in an arithmetical 
progression, the weights will decrease in a 
geometrical | Pha omer= the necessity, there- 
fore, for dividing the columns which sustain the — 
trunk by means of the joints, independently of 
the use of the latter for locomotion, is obviou 
According to Galileo, the power of a beam 
or bar to resist a fracture by a force acting late- 
rally, is as the section of the beam, where the force 
is applied, multiplied into the distance of the — 
centre of gravity of the section from the point or 
line where the fracture will end. By applying — 
this principle to the case of bones, we deduce - 
the following propositions, which must, he 
be regarded only as approximations to the tre 
The lateral strength of two cylindrical bones 
of equal weight and length, of which one is’ 
hollow and the other solid, are to each other 
as the diameters of their transverse sections 
Thus, let a b, de (fig. 217, A, B,) be the: 
tion of the two bones: then the strength of 
tube d ¢ is to that of the solid a b as de to ab. 
: ae 
t , 
» 
oe 
Fig. 217. 3 
x ta 
a 
i 
sf 
y is 
* De curvis elasticis, No. 37. wo 
