497 



LOCOMOTION OP ANIMALS. 



LOCOMOTION OF ANIMALS. 



498 



to oscillate on each side of the rope, and the eye of the performer is 

 fixed on one end of it, by which means he is enabled to keep his body 

 steady, and moreover, when his centre of gravity falls on the left side 

 of the rope, a sufficient portion of the pole is shifted to the right side 

 in order to restore the equilibrium. A long rod is then a very good 

 auxiliary for keeping the body steady in positions of difficult equili- 

 brium, such as walking over narrow parapets, or wooden bridges 

 thrown across rivulets without a hand-rail, which may often be met 

 with in country districts. When a porter carries a burden, the 

 Fig. 3. 



attitude of the body must accommodate itself to the position of the 

 common centre of gravity of himself and his load. Thus, in the 

 above figures it will be observed that when the man stands upright 

 the centre of gravity of the man G falls within the base of support, 

 and if his load L falls without the base, as does likewise y, the com- 

 mon centre of gravity of the man and load, the consequence would 

 be that he would fall backwards ; but this is prevented, or which is 

 the same thing, the point y is brought within the base by the man 

 bending his body forward. The reverse happens when the load is 

 carried in front ; as, for instance, by the laundress, whose basket is 

 carried in front, as in fig. 4. In this case, instead of bending forward, 



Fig. 4. 



Fig. 5. 





as in the former case, the body is thrown back, in order that the 

 centre of gravity common to the woman and the basket may be 

 brought within the base of support. In these and similar cases the 

 person will be prevented from falling when the line G g multiplied by 

 the weight of the man is equal to the line y L multiplied by the 

 weight of the load. When a person stoops to place a load, such as a 

 pail of water, on the ground, the hips are thrown backward so as to 

 bring the point y within the base of support. Just as the pail reaches 

 the ground the common centre is for a moment however beyond the 

 base, and there is great danger of the body falling forwards, which is 

 sometimes prevented by the person instinctively clinging to the edge 

 of the pail. Corpulent persons are observed to walk very erect, in 

 order to throw the centre of gravity immediately over the hip-joints, 

 upon which the body rotates, as in fig. 5. They are also observed to 

 take very short steps, and walk more slowly than others. 



The erect attitude, which contributes, amongst many other things, 

 to distinguish man from the lower animals, is chiefly owing to his 

 organisation, and not the effect of mere mental development. It is 

 found that animals instinctively adopt that position of the body which 

 enables them to bear their own weight with the least expenditure of 

 muscular labour, and that in all other positions the support of the 

 body becomes irksome. Let us now turn our attention to what takes 

 place in the erect posture in man. The position in which the muscular 

 force is least expended is that in which the bones of the skeleton, and 

 all other parts, are poised upon each other at the joints about which 

 they move : the muscles in this case have merely to perform the office 

 of keeping the superincumbent parts exactly balanced. When a man 

 stands erect, with the eyes directed horizontally forwards, or slightly 

 inclined upwards, the centre of gravity of the head lies vertically 

 over the centre of the joint that connects the head with the neck : 

 also, the weight of the body and head is equally supported on the 



AI. HIT. DIV. VOL. lit 



hip-joints; these again, with the thighs, rest on the centres of the 

 knee-joints, and these on the elastic arches formed by the bones of 



the foot which are placed on the ground. 

 Fig. 6. 



Thus, in fiy. 6 it will be 



Fig. 7. 



seen that the vertical line x, y, passes through all the points just enu- 

 merated ; and, whilst the body is erect, there is required only suffi- 

 cient muscular power to keep it balanced on the joints, and the joints 

 from bending. Now, to show that the several parts are most easily 

 supported in the erect position, let us see what happens when the 

 body is horizontal. Take the head, for example. The centre of 

 gravity of the head Q (fig. 7), instead of lying vertically above the 

 joint of the neck y, as when erect, is distant from it by the length of 

 the line Gy, so that the muscles which keep the head in the horizontal 

 position have to support it with a mechanical disadvantage at the end 

 of a lever G y ; and it will be found that it cannot be sustained in 

 that position, even for a short period, without great fatigue, which is 

 not felt in the erect posture. It will also be observed that the eyes 

 would be directed towards the ground, so that a man could not look 

 horizontally forwards without difficulty, nor vertically upwards at all, 

 both of which actions are essentially necessary to the pursuits and 

 habits of the human race. In the lower animals the case is quite 

 different ; for, if we take the horse as an example, we find that the 

 centre of gravity of the head is brought nearer the joint uniting the 

 head with the neck in the horizontal than in the erect position. 

 From the position of the eyes in the head, if the animal were 

 standing erect on its hind legs, it would look directly upwards, 

 and could not direct them to the ground. These considerations, 

 alone are sufficient to enable us to come to the conclusion that 

 man is not organised to move as a quadruped, nor quadrupeds as 

 man ; and moreover, in the posture of the quadruped, the whole sole 

 of the human foot would not reach the ground, but the toes only ; 

 and he would become a digitigrade, instead of being, as he is now, a 

 plantigrade animal. In the position of a quadruped, also, if the hands 

 and arms were employed to contribute to the support of the head and 

 trunk, ihey would no longer be free to perform all those various offices 

 to which these organs are manifestly adapted. 



There is a considerable difference in the figure of the vertebral 

 column of man and of the lower animals ; in the former it is con- 

 structed to bear the greatest weight in the erect, but in the latter in 

 the horizontal position. In man the vertebral column (or, as it is 

 more commonly called, the back-bone) is divided, from the head to 

 the lower part of the loins, into 24 joints. Between each bone of 

 the back there is a very elastic substance interposed, of such a nature 

 as to bear the pressure of the bones without injury, to enable them 

 to be twisted and bent upon each other without coming in contact, 

 and thus to avoid the injury that would result from friction. It 

 might be supposed at first sight that a chain of bones, piled upon 

 each other, separated by so many joints, and capable of moving in 

 various directions, would be a weak and imperfect organ, when 

 considered as a pillar to support the head and body : this, however, 

 is by no means the case ; for, independently of its office in upholding 

 the head and trunk, each bone is hollow, so that the vertebral column 

 forms a tube for the passage of the spinal marrow or cord, which it 

 encases, and protects from external violence, in all the varied occu- 

 pations of life. Destined, then, as the bones of the back are to 

 protect so important an appendage of the brain, and to support the 

 weight of the head and trunk, special care has been taken by the 

 all-wise Creator to construct each bone with sufficient strength for 

 these purposes ; we consequently find the bones of the neck, which 

 have the least weight to bear, the smallest, and that they gradually 

 increase in size down the back to the loins, where they are the 



2 K 



