.-.Ill 



LOCOMOTION OF ANIMAL i 



LOCOMOTION OF ANIMALS. 



602 



will show how this is accomplished. The entire range of the kuee- 

 joint being, in round numbers, 144, the least angle that can be formed 

 between the leg and thigh is 36 ; and since the distance from the 

 hip-joint to the knee when bent is equal to tha distance from the 

 knee to the ankle, the hip-joint a will always be found in the vertical 

 line x y, and when the knee-joint is fully bent, that is, when the leg 

 and thigh are inclined at an angle of 36, each of the angles at the 

 base of the equilateral triangle ale will be 72 ; now, the utmost 

 forward range of the leg upon the ankle is only 31, therefore the 

 heel must be raised 41 in order to throw the leg sufficiently forward 

 to make the angle between itself and x y, the vertical line 72, by 

 which means the point a will have descended to the lowest possible 

 position in that line. This is a property peculiar to the structure of 

 the human frame. In the act of standing both legs concur to the 

 support of the trunk, and for this purpose alone there is a considerable 

 expenditure of force employed to keep the centre of gravity of the 

 body above the ground ; and we all know that in standing for a con- 

 siderable length of time great weariness is experienced ; and although 

 the force exerted in standing is less than in walking and running, it is 

 nevertheless estimated to be equal to the height of the centre of gravity 

 multiplied by the weight of the body. 



In walking, as well as in running, we may divide the body into two 

 portion a : first, the portion which constitutes the burden to be borne, 

 consisting of the trunk, head, neck, and arms : and, secondly, that 

 which supports the burden, and propels it along, consisting of the 

 legs. In walking, the trunk, fl<th its appendages, is carried forward 

 on the heads of f he thigh-bones, like a rod poised lengthwise on the 

 tip of the finger ; and we know that if the finger be moved onwards 

 when the rod is poised vertically, it will fall backwards, and that it 

 must be inclined forwards to preserve its position on the finger, indeed 

 its inclination must be very nicely adjusted to prevent it falling as 

 the finger moves forwards, and this inclination must be greater as the 

 movement of the finger is increased ; and if the wind moves in an 

 opposite direction, a still greater inclination of the rod becomes 

 necessary ; and those who have made the experiment must be aware 

 of the difficulty, unless after much practice, to keep the rod thus 

 balanced from falling. Now, the human body is balanced on each leg 

 alternately, in a manner precisely similar to aud subject to the same 

 conditions of equilibrium as the rod ; the trunk must therefore be 

 inclined forwards during progression, and its inclination must be 

 greater or less, according to the velocity of the movement and the 

 resistance of the air. It is for this reason that we see persons inclin- 

 ing then- bodies very much forward in walking against a strong wind, 

 and almost erect when walking in the name direction as the wind. 

 Bearing, then, these principles in mind, we can easily understand why 

 it is that every movement of the arms, head, neck, and trunk, and 

 every position of the legs, require a simultaneous movement of all 

 the rest of the body to keep the whole poised on the heads of the 

 thigh-bones during walking. 



The measures of the inclinations of the trunk due to different 

 velocities have been taken by Messrs. W. and E. Weber. The arms, 

 being attached to the upper part of the trunk, and considerably above 

 the axis of motion, or line through the hip-joints, and being more- 

 over free to move in almost every direction, except in that which 

 would tear them from the body, contribute a great deal towards keep- 

 ing the trunk in a state of equilibrium on the legs, and thus dispense 

 with that large amount of muscular force which would otherwise be 

 required for that purpose : hence in walking we observe the arms in 

 constant motion, oscillating backwards and forwards at every step, and 

 we find, on more closely inspecting the order in which they move, 

 with respect to the legs, that whilst the right leg swings forwards, the 

 trunk is turned round horizontally on the head of the left thigh-bone ; 

 which tends to advance the right shoulder before the left ; but this is 

 counteracted by the right arm, which swings backwards, and by the 

 left, which at the same time swings forwards, and the combined effect 

 of these two motions is to neutralise the twisting of the upper part of 

 the trunk on the legs during each step. A corresponding effect takes 

 place when the left leg swings forwards; so that a good walker can 

 move without any sensible twisting of the body, which is however 

 generally apparent in the female sex, arising from the greater distance 

 between the heads of the two thigh-bones in them than in males. The 

 swinging of the arms then is attended with considerable advantage ; 

 for not only do they lessen the amount of muscular exertion, but 

 give a greater freedom to the attitudes assumed by the trunk in walk- 

 ing and a more easy style of movement, and a graceful gait results 

 from their well-regulated oscillation. When the arms however are 

 made to describe very large curves externally to the vertical plane in 

 which they swing naturally, a trick which youths are prone to, under 

 the erroneous idea of thereby adding to the manliness of their appear- 

 ance the effect is in reality extreme awkwardness, for the shoulders 

 are thrown alternately forwards, and much muscular strength is 

 wasted, as may be verified by any one who tries to walk in this manner 

 for a long time and at a rapid pace. 



In walking, the centre of gravity does not move m a perfectly 

 horizontal line, but is raised and depressed during each step. Weber 

 found, by an apparatus designed for that purpose, that in ordinary 

 walking, when the length of the step taken was 2'39 feet, the mean 

 elevation and depression was I'l inch, and this quantity of vertical 



motion was very nearly the same whether the speed was increased or 

 diminished. In walking on the ball of the foot the mean elevation 

 aud depression of the trunk did not exceed 0'8 inch. 



We have already mentioned what quantity of force is necessary to 

 be expended to support the body in standing; but in walking, each 

 leg has not only to sustain in its turn the whole of the superincum- 

 bent parts, with the additional weight of the swinging leg, but to 

 push the body forwards, and for these purposes the force is very con- 

 siderable. The resistance to the forward movement of the body arises 

 from the friction of the joints, the friction of the sole of the foot upon 

 the ground, and of the air, but the principal resistance is that of the 

 advanced leg when it reaches the ground. 



It has been always supposed, until a very recent period, by those 

 whose business it is to study the structure and operation of the several 

 organs of animal bodies, that the swinging of the legs in walking and 

 running depended on the action of the muscles. It has however been 

 discovered by Messrs. W. and E. Weber that the leg swings after it has 

 been raised from the ground by the force of gravity alone, aud that it 

 obeys the same laws as the pendulum of a clock. In fact they regard 

 the supporting leg as the substitute for the weight of a clock, aud the 

 swinging leg as the substitute for the pendulum, each leg exchanging 

 its office successively. We have here, then, an illustration of the con- 

 nection between the laws which govern the solar system and those 

 which govern the locomotive actions of the organs of human beings. 



Now, as the leg swings according to the laws of the pendulum, and 

 moves forwards without the active interposition of its muscles, we 

 must see how much the economy of the power which is destined to 

 enable us to move upon the earth has been provided for in the forma- 

 tion of the human race ; for were the muscles of the legs employed in 

 this movement, their force would soon be exhausted, and we should 

 be able to move over a very limited space in a long time. 



It is well known that a short pendulum will vibrate much quicker 

 than a long one, and that the time of one vibration of the pendulum 

 of all clocks is regulated upon this principle ; but to be more precise, 

 the times of the periodic oscillations of any two pendulums are 

 respectively as the square roots of their length. By applying these 

 principles to the swinging leg of a man in walking, we can easily under- 

 stand why persons with short legs take more rapid steps than persons 

 with long legs, and why males and females rarely step together, unless 

 the former accommodate the length of their step to the time of the 

 latter. 



Those who have studied the theory of walking have found it conve- 

 nient to divide the time of a step into two portions, namely, that in 

 which one leg and that in which both legs rest on the ground ; at 

 least this arrangement has been adopted by Borelli, Weber, and Bishop. 

 In walking it is necessary that there should be at least one foot always 

 on the ground, and there is no instant in which the body is not sup- 

 ported either by one or both legs. In running the case is different, 

 as we shall hereafter see. 



The period wherein both legs are on the ground is shorter than 

 that in which the trunk is supported by one leg only. During the 

 time the body is supported by one leg the other leg swings from 

 behind forwards ; and, being again placed on the ground, the first 

 interval ends, and the other, namely, that in which the body is sup- 

 ported by both legs, begins, and terminates with the raising oi the 

 other leg. The time that the body is supported by both legs diminishes 

 continually as the velocity is increased, and when it vanishes alto- 

 gether, as in quickest walking, we arrive at the common limit of the 

 quickest pace in walking and the slowest in running. Thus the two 

 states in which the body is supported either by one or both legs 

 alternate in such a manner that one begins at the instant the other 

 terminates; and it is found by experiment that only in very slow 

 walking is the time wherein both legs are on the ground equal to half 

 that in which one only supports the body. 



We will now endeavour to illustrate the preceding remarks by means 

 of a diagram. In jig. 14, which may be conceived to be a horizontal 



Fig. 14. 



I' 



c' 



d" 



plane, let us suppose the upper series of lines to represent the left 

 leg, the lower series the right, the straight lines the leg resting on 

 the ground, the curved the leg swinging, and the letters a, b, &c:, to 

 denote the different periods of movement in walking. During a both 

 legs are resting on the ground, aud at the beginuing of 6 the left leg 

 rises from the ground, and swings forward until c commences, when 

 both legs are again on the ground. During d the right leg in its turn 

 rises and swings from behind forwards, whilst the trunk is supported 

 on the left leg, represented by the upper straight line. At a' both 

 legs are again in contact with the earth ; at V the left leg again rises 

 in its turn, and swings as before; and thus the two legs alternate their 

 offices in succession. We observe that the period a, in which both 

 legs are on the ground, is about half of b, during which the left leg is 



