414 



MOTION. 



The number of moveable articulations in a 

 skeleton determines the degree of its mobility 

 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 laminae than those which swim or float, 

 either by means of specific organs of progres- 

 sion, such as the lanthina vulgaris, the Lymnae, 

 and Hyalaea, or upon hydrostatic principles, as 

 in the Nautilus, 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, composed 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- 

 cessary 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 



joints, with ridges and protuberances for the at- 

 tachment of muscles, and with levers adapted 

 to perform all the varied offices of locomotion. 



In quadrupeds, which have four osseous 

 columns to support the superincumbent organs, 

 the pressure of the trunk on each leg is only 

 half that in bipeds ; but owing to the hori- 

 zontal inclination of the trunk and the projec- 

 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 directions 

 of the bones nearer the vertical plane in these 

 than in the posterior : this arrangement is most 

 conspicuous in the larger lluminantia and 

 Pachydermata, especially in the Elephant, 

 Horse, &c. We may, therefore, readily per- 

 ceive why the shafts of the long bones of 

 the legs and arms of most Mammalia are par- 

 tially hollow cylinders; the prismatic outline 

 predominates in the Elephant and Megatherium. 



The weights which cylindrical or prismatic 

 flexible columns will support perpendicularly 

 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, 5, 

 they will support weights without flexion rela- 

 latively in the proportions 1, \, l g , J g , ? > 5 , so that 

 whilst the lengths increase in an arithmetical 

 progression, the weights will decrease in a 

 geometrical progression ; 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 obvious. 



According to Galileo, the power of a beam 

 or bar to resist a fracture by a force acting late- 

 rally, is as the section ofthe beam, where theforce 

 is applied, multiplied into the distance of the 

 centre of gravity of the section from the pointer 

 line where the fracture will end. By applying 

 this principle to the case of bones, we deduce 

 the following propositions, which must, however, 

 be regarded only as approximations to the truth. 



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, d e (fig. 217, A, B,) be the sec- 

 tion of the two bones : then the strength of the 

 tube d e is to that of the solid a b as d e to a b. 



Fie. 217. 



* De cun 'is elasticis, No. 37. 



