LOCOMOTION IN ANIMALS. 



LOCOMOTION IN ANIMALS. 



HI 



by Galileo that the strength of bodies U augmented in a 

 thousand ways without increasing their weight ; and that if a wheat- 

 straw, which support* the ear which is heavier than the whole stalk, 

 were made of the same quantity of matter, but solid, it would break 

 or brad with far greater ease than is now found to be the case. The 

 feathers of birds present, as well as the bones of animals, similar pro- 

 vision for the combination of strength, lightness, and economy of 

 material It must however be borne in mind that neither the strength 

 of bodies nor the size of animals can be increased with the same 

 quantity of matter without limit ; because when the diameter of the 

 tuba exceeds certain dimension* it becomes so thiu and fragile as to 

 break without offering any sensible resistance. 



The bones, ligaments, and joints of animals are denominated the 

 passive organs, and the muscles the active organs of motion. The bones 

 serve the purpose of levers, which are acted upon after the principle 

 of what is called the first, second, and third orders of lever, so named 

 according to the relative positions of the prop or fulcrum, the power, 

 and the resistance or weight. 



The power of the muscles which move the limbs is so great that if 

 toe resistance to their action be sufficiently strong, they are often 

 known to break the bones upon which they act It is in consequence 

 of the great force which the muscles are capable of exerting that they 

 are generally attached to the bones very near the axis of the joint 

 about which the bone moves. The advantage of this disposition is, 

 that a greater power of the muscle U made equivalent to a greater 

 velocity in the movement of the limb. The action of the biceps 

 muscle affords a good example of this principle. In the annexed 

 figure (1) we observe that this muscle p is attached to the bone of the 

 fore arm at a, very near the axis of the elbow-joint /, the effect of 

 which is that in raising the weight in the band b to b', the muscle 

 moves the point a only to c ; but then, as the distance between 6 and b' 

 is much greater than the distance from a to c, it is manifest that a 

 very small contraction of the muscle ia sufficient to produce a very 

 great range in the movement of the extremity of tbe limb. 



Fig. 1 



In this case tbe muscle is acting under a mechanical disadvantage 

 owing to the obliquity of it* direction, and its proximity to the joint : 

 but, as it is endowed with great power, these circumstances do not 

 constitute defects. On the contrary, not only can the limbs be moved 

 upon each other with much more velocity, but the symmetry and 

 beauty of proportion in tbe human figure are preserved. It is mom- 

 over said that a muscle cannot contract upon itself beyond one-fourth 

 of its entire length ; so that its work must of necessity be done by 

 acting in some such manner as we find in the case already mentioned. 



A* the muscles have the power of contracting throughout nearly 

 their whole length, they are capable of drawing the moveable parts 

 attached to them at both their ends ; for example, we can, with the 

 same set of muscles, either bend the thigh upon the body, or body 

 upon tbe thigh. If the muscles had not been endowed with the power 

 to more the limb* under a mechanical disadvantage with tbe velocity 

 they do, the sledge and the hammer, now so daily in use, would have 

 been instruments of no value to the smith, nor the axe and hammer 

 to the carpenter ; and tbe numberless uses to which rapid motions of 

 the limbs are applied must have been effected by some other mean*. 

 Toe figure of the arm affords an illustration of the principles of the 

 third order of lever; that i, the power of the muscle is applied 

 betsresn the weight 6 and the fulcrum / ; and the power of the muscle 

 i> is to tbe weight 6, as the length of the line bf to tbe length of the 

 line ft : from this proportion we can readily find the power of the 

 miwcle when tbe weight is known. 



If we select four animal* of tbe same order, having similar figures, 

 whose dimensions of one kind are as 1, 2, S, 4, the weuchta of the 

 animal* and of all their corresponding parts will be an the cubes of 

 these number* ; that is, as 1, 8, 27, 64 ; but since it is found that the 

 faro* of a muscle depend* on the number of iU fibre*, this force must 

 luui'ssss in tbe ratio of iu transverse section, that is, as the square of 

 one of the dimension* of the animal, or as 1, 4, 9, 16. The contractile 

 force of the muscles in a healthy man, according to Dr. Young, is 

 equivalent to about 600 Ibs. for each square inch of the surface pre- 

 sented by their transverse sections. We msy then easily understand 

 from tbe preceding remarks why it is that tbe most powerful men have 

 their muscles most developed ; and why the largest mutcles are placed 

 in those part* of the body where they are subjected to tbe greatest 

 quantity of work. For example, tbe force of the muscles that close 

 tbe lower jaw in man is estimated at 500 Ibs. ; indeed the force of 

 nnscles is well known to be sufficient to crash the soundent 



teeth when a hard substance is placed between them. Some indi- 

 viduals are capable of holding between the teeth and of supporting 

 by the lower jaw 240 Iks. and upwards. In many of the lower animals, 

 such as the C'arnivora, the muscles which close the lower jaw are 

 much stronger and more developed thin in man, as is exemplified in 

 the ease with which the lion ana tiger lacerate their prey. 



It is well known that the quantity of labour which the muscles will 

 endure, and the length of time they will continue to act, increase, 

 within certain limits, in proportion to their daily exercise. If the 

 muscles of the arms or legs, or any others, be called suddenly into 

 action for a longer period than that to which they have been 

 accustomed, they soon communicate to the individual a sense of 

 weariness, and evince a disposition to yield to the action opposed to 

 them, and, unless they are allowed some repose, mischief speedily 

 succeeds. But when their action is very gradually increased day 

 after day, it is observed that the bulk of the muscles, and consequently 

 their power, U augmented. For it has been already stated that the 

 force of muscles is directly proportioned to the square of the surfaces 

 which they present on making transverse sections through them : and 

 if we take the estimate of Dr. Young for the basis of a computation 

 of the additional force they acquire by exercise, we shall find that an 

 increase of a quarter of a square inch in a single muscle is equivalent 

 to a gain in strength of - j", or 125 Ibs. Now, as a great number of 

 muscles act at the same time in moving the limbs, it follows that they 

 must all become enlarged, in proportion as they contribute to produce 

 that motion. It is therefore practicable, by gradually increasing the 

 exercise of the different muscles of the body daily, and thereby 

 augmenting their bulk, to attain a vast increase of animal strength ; 

 and when we consider that health is the result of such exercise, if it 

 ia not carried too far, no stronger argument can be adduced for its 

 adoption. But there is a limit to the amount of exertion which the 

 muscular system will bear : if that limit is passed the muscles lose 

 their vigour, and lassitude and a flaccid state supervene. Experiment 

 teaches us how long the muscles may be continued in action, and the 

 quantity of force which they are capable of expending during that period. 

 We have before stated that the weight of the body is proportional 

 to the cube, and the power of the muscles to the square, of some one 

 of its dimensions : for instance, in two similar-formed men, whose 

 heights are respectively 5 and 6 feet, the muscular power of the 

 former to that of the latter will be as 25 to 36, but their weights will 

 be as 125 to 216, or as 25 to 43 very nearly ; the weight therefore 

 increases much more rapidly than the muscular power, and conse- 

 quently a small man is stronger, in proportion to his size, than a 

 larger one. The same law holds good in all animals of the same kind. 

 In persons predisposed to corpulency, and who neglect to use the 

 proper means of checking it, the quantity of fat continues to increase 

 and the muscles to decrease in volume until they exhibit the deplorable 

 phenomenon of an inability to move even their own weight without 

 pain and difficulty : for it is found on dissection that the quantity of 

 muscular fibre is less, and the fibres themselves more flaccid, in fat 

 persons than in others. 



The attitudes and movements of every animal depend on the 

 position of the centre of gravity* and base of support. When a man 

 stands erect a plumb-line passing through bis ceutre of gravity will 

 fall between bis feet. It is a subject of mathematical investigation, 

 when the feet are equally advanced and equally inclined (as in fig. 2) 



Fif . S. 



to determine the angle ab c, which they must form with the pro- 

 longation of tbe line joining the heels in order to afford the greatest 

 base of support In general, where the leg* are perfectly vertical, it 

 will be found that the line joining the centre of the heela is just equal 

 to the length of the foot, and in this case the required angle will be 

 60* exactly ; as the heels approach each other this angle diminishes, 

 and when they are close together it is rather more than 45. Opera 

 dancers, besides curving the body, employ their arms to bring the 

 ceutre of gravity over the base of support, which is often limited to 

 the ball of the great toe of one foot ; the weight of the raised leg and 

 foot is also concerned in producing the equilibrium of the body in 

 resting on the other foot Rope-dancers use a long heavy pole for the 

 same purpose : the pole is grasped by both hands, and carried nearly 

 at right angles to tbe axis of the rope, as the centre of gravity muftt 

 be continually brought over the very narrow base which the rope 

 afford*, and the rope itself, if slackened, having a motion both lateral 

 and vertical, and being at a considerable height above the ground : 

 the process of rope-dancing requires great muscular activity and 

 precision in tbe attitudes of the body to secure the performer from 

 falling. The celebrated Madame Saqui, as is well known, lost her life 

 by a fall from a rope. The centre of gravity has always a tendency 



The centre of (purity of sny body Is thut point upon which the body, acted 

 on only by the force of gravity, will balance itnelf In ill positions ; consequently, 

 If s line or plane which pawn through the centre or gravity be supported, the 

 body nil) be unpnortrd In nil portions. 



