LOCOMOTION OF ANIMALS. 



LOCOMOTION OF ANIMALS. 







_ m Each bone of the back is bound 10 strongly by ligaments to 



the ooe above and below it, and iU figure U so constructed to lock 

 into the adjoining bones, that no common force will duuuite them : 

 indeed, to firmly an the bone* of the back bound and locked 

 together, that a force sufficient to cnuh the body of the bone will 

 loo* separata them. A fracture of thu kind U, accordingly, one of 

 UM molt serious accident* that can befal a man, and, in tact, death 

 most commonly result* from it. The verte- 

 bral column in not straight, but curved in three 

 place*, a* shown in ></. 8. At a, the ucck, the 

 spinal column lies in front of thu vertical line 

 jr y ; at 6, the back, part of the column sup- 

 porting the shoulders lies behind the same 

 line ; and at c, the loins, the column again lies 

 in front of it Now, it may be imagined by 

 some persons that this curved condition of 

 the back is a defect and a source of weak- 

 ness, but it is, on the contrary, a condition 

 of security and strength; for, the jars inci- 

 dental to walking, leaping, and heavy blows, 

 coming vertically on either extremity, are 

 thus transmitted obliquely through the whole 

 column, so that much of their violence is lost, 

 Moreover, it has been found by Euler that an 

 elastic rod, when slightly curved, will bear 

 upon its end a much greater weight than if 

 straight ; and if the number of small curves 

 be three, the weight it will sustain will be the 

 square of three, with one added, or 4 x 4, that 

 U, 16 times greater than if it were straight. 

 Now, if we may consider it admissible to apply this principle to 

 the human vertebral column in bearing a burden on the head in the 

 erect posture, we shall see that, in this posture, a man U capable of 

 carrying, or rather supporting, such a load as would almost break the 

 back in a prone position. 



We observe that the bones of the back, or vertebral column, are 

 also curved in the lower animals; but, instead of forming several 

 curves in opposite directions, as in man, it forms one large arc, which 

 is convex upwards, when the spine of the animal is directed horizon- 

 tally. This form of the bones of the back in quadrupeds is con- 

 rpicuous in the Rumauintia and Pacliydermata ; for instance, in the 

 Rhinoceros, as seen in fg. 9, the whole of the column lying between 



in which it is found when standing perfectly e,rect : and for this 

 purpose the legs are furnished with three joints, the first of which 

 connects them to the trunk, the second is the hinge-joint of the knee, 

 and the third U the ankle-joint, the structure of which partakes of 

 the hinge-like action of the knee, but has also a small extent of lateral 

 motion. It is owing to the power of flexion and extension of the legs 

 that it is possible to carry the centre of gravity of the body almost in 

 a perfectly horizontal line and with a uniform velocity. The length 

 of the legs in man, if measured from the hip-joint to the ground 

 when standing erect, in found in most pcnons to be greater than tho 

 length of the rest of the body above that point Of the three joints 

 of the leg, the first, or hip-joint, allows it only to move forwards ; 

 the second, or knee-joint, allows it only to move backwarks ; and the 

 third, or ankle-joint, either backwards or forwards. In fg. 10 we 



Fif. . 



' 



* and y forms one Urge arch with its concavity directed downwards ; 

 and, a* rach boo* which enters into the composition of the arch is 

 bound to Uu adjacent bone* by ligament, or fibrous cord of great 

 ttrength, any force or load laid upon the back of the animal, in its 

 natural prone position, by tending to bring that portion of the spine 

 lying between x and y into a straight line, tightens those cords ; and 

 tim UM spin* iUelf is capable of bearing as much, or perhaps more 

 sight than the limbs of the animal could carry in its natural 

 position : but, if UM load were laid on the top of the spine, instead 

 of aero** it, on the supposition that the body of the animal assumed 

 an not posture, UM arch of the back would increase, and the spine 

 would be broken by UM same load which it would easily bear when 

 laid across it Hen, then, we have another illustration of a 

 quadruped requiring a prone position derived from the constitution 

 of its vertebral column. Independently of thin, if we were to 

 examine UM relative positions of the joints, we should find that, 

 when a quadruped is placed in an erect posture, a vertical line passing 

 through the centre of gravity of the head and the axis of the joint 

 which unites it to the trunk, does not, and cannot pas* through the 

 centre of gravity of the animal, (?, and the hip, knre, and ankle 

 Joints at the same time, a* in man ; neither can the horizontal line 



* y be brought to coincide with the vertical line r' y' : and, that 

 therefore, UM erect position of a quadruped in not that of least 

 expenditure of muftcular action, which we maintain is the position 

 natural to every animal in the act of xtanding. 



Tbe human body is supported during progression on one or both 

 legs, and the condition* of it* locomotion diner from those of almcwt 

 all other animal.. We may observe that when locomotion take* place, 

 UM centre of gravity of the body must be lowered from the position 



Fig. 10. 



Fig. U. 



Fig. 12. 



observe the greatest length the leg can assume by the complete 

 extension of all its joints, this length being the distance between a, 

 the hsad of the thigh-bone, and c, the convex surface of the ankle- 

 joint luflj. 11 we have the least length of the leg, produced by the 

 simultaneous flexion of all the joints, but this bending of the leg is 

 evidently much greater than is necessary in the act of quickest walk- 

 ing, during which the bending of the leg is as represented in fg. 12 ; so 

 that the range of motion in the several joints of the legs is greater 

 than is necessary for the common purposes of walking, but is esscutal 

 to some other attitudes. It will be observed that the length of the 

 thigh-bone is nearly equal to that of the leg from the knee to the 

 ankle, and in consequence of the opposite directions in which the 



Fig. 13. 



limbs move upon the joints, we are enabled to perform tho following 

 curious function : If the heel be raised from the ground about 41 , 

 so that the body rests on the ball of the foot only, the centre of 

 gravity may be made to descend in n vertical line by bending all the 

 joints of the legs simultaneously (fig. 13) : a reference to the figure 



