MECHANICS (ANIMAL). 



749 



groove, exactly as if a little fixed pulley were 

 there. 



The heel, by projecting so far backwards, is a 

 lever for the strong muscles to act by, which form 

 the calf of the leg, and terminate in the tentlo achillis. 

 These muscles, by drawing at it, lift the body, in the 

 actions of standing on the toes, walking, dancing, 

 &c. In the foot of the negro, the heel is so long as 

 to be ugly in European estimation; and, its great 

 length rendering the effort of smaller muscles suffi- 

 cient for the various purposes, the calf of the leg in 

 the negro is smaller in proportion than in other races 

 of men. 



The arch of the foot is to be noticed as another of 

 the many provisions for saving the body from shocks 

 by the elasticity of the supports. The heels and 

 the balls of the toes are the two extremes of the 

 elastic arch, and the leg rests between them. 



Connected with elasticity, it is interesting to re- 

 mark how imperfectly a wooden leg answers the 

 purpose of a natural leg. With the wooden leg, 

 which always remains of the same length, the centre 

 of the body must describe, at each step, a portion of 

 a circle of which the bottom nob of the leg is the 

 centre, and the body is therefore constantly rising 

 and falling ; while, with the natural legs, which, by 

 gentle flexure at the knee, are made shorter or 

 longer in different parts of the step, as required, the 

 body is carried along in a manner perfectly level. 

 In like manner, a man riding on horseback, if he 

 keep his back upright and still', has his head jolted 

 by every step of the trotting animal ; but the expe- 

 rienced horseman, even without rising in the stir- 

 rups, by letting the back yield a little at each move- 

 ment, as a bent spring yields during the motion of a 

 carriage, can carry his head quite smoothly along. 



In a general review of the skeleton, we have to 

 remark, 1. the nice adaptation of all the parts to 

 each other, and to the strains which they have 

 respectively to bear; as in the s;ze of the spinal 

 vertebrae increasing from above downwards ; the 

 bones of the leg being larger than those of the arm, 

 and so on. 2. The objects of strength and lightness 

 combined ; as by the hollowness of the long bones ; 

 their angular form ; their thickening and flexures in 

 particular places where great strain has to be borne; 

 the enlargement of the extremities to which the 

 muscles are attached, lengthening the lever by winch 

 these act, &c. 3. We have to remark the nature 

 and strength of material in different parts, so admir- 

 ably adapted to the purposes which the parts serve. 

 There is a bone, for instance, in one place, nearly 

 as hard as iron, where, covered with enamel, it has 

 the form of teeth, with the office of chewing and 

 tearing all kinds of matter used as food. In the 

 cranium, again, bone is softer, but tough and resist- 

 ing ; in the middle of long bones, it is compact and 

 little bulky, to leave room for the swelling of the 

 muscles lying there ; while, at either end, it is large 

 and spongy, with the same quantity of matter, to 

 give a broad surface for articulation ; and, in the 

 spine, the bodies of the vertebrae, which rest on an 

 elastic bed of intervertebral substance, are light and 

 spongy, while their articulating surfaces and pro- 

 cesses are very hard. In the joints, we see the 

 tough, elastic, smooth substance, called cartilage, 

 covering the ends of the bones, defending and pad- 

 ding them, and destroying friction. In infants, we 

 find all the bones soft or gristly, and therefore calcu- 

 lated to bear, with impunity, the falls and blows 

 unavoidable at their age ; and we see certain parts 

 remaining cartilage or gristle for life, where their 

 elasticity is necessary or useful, as at the anterior 

 extremities of the ribs. About the joints, we have 

 to remark the ligaments which bind the bones to- 



gether, possessing a tenacity scarcely equalled in any 

 other known substance ; and we see that the muscu- 

 lar fibres, whose contractions move the bones, and 

 thereby the body, because they would have made 

 the limbs clumsy even to deformity had they all 

 passed over the joints to the parts which they have 

 to pull, attach themselves, at convenient distances, 

 to a strong cord called a tendon, by means of which, 

 like a hundred sailors at a rope, they make their 

 effort effective at any distance. The tendons are 

 remarkable for the great strength which resides in 

 their slender forms, and for the lubricated smooth- 

 ness of their surfaces. Many other striking particu- 

 lars might be enumerated; but these may suffice. 

 Such, then, is the skeleton, or general frame-work 

 of the human body less curious and complicated, 

 perhaps, than some other parts of the system, but so 

 perfect and so wonderful, that the mind which can 

 attentively consider it without emotion, is in a state 

 not to be envied. 



The living force of man has been used as a work- 

 ing power in various ways, as in turning a winch, 

 pulling at a rope, walking in the inside of a large 

 wheel to move it, as a squirrel or turn-spit dog moves 

 his little wheel, &c. Each of these has some parti- 

 cular advantage ; but that mode in which, for many 

 purposes, the greatest effect may be produced, is for 

 the man to carry tip to a height his body only, and 

 then to let it work by its weight in descending. A 

 bricklayer's labourer would be able to lift twice as 

 many bricks to the top of a house in the course of a 

 day, by ascending a ladder without a load, and rais- 

 ing bricks of nearly his own weight over a pulley 

 each time in descending, as he can by carrying 

 bricks and himself up together, and descending again 

 without a load, as is still usually done. 



Reflection would naturally anticipate the above 

 result, independently of experiment ; for the load 

 which a man should be best able to carry, is surely 

 that from which he can never free himself the load 

 of his own body. Accordingly, the strength of 

 muscles and disposition of parts are all such as to 

 make his body appear light to him. 



The question which was agitated with sucli 

 warmth some time ago, as to the propriety of making 

 men and women work on the tread-mill, receives an 

 easy decision here. They work by climbing on the 

 outside of a large wheel or cylinder, which is turning 

 by their weight, and on which they must advance 

 just as fast as it turns, to avoid falling from their pro- 

 per situation. There are projections or steps for the 

 feet on the outside of the cylinder, and the action to 

 the workers is exactly that of ascending an acclivity. 

 Now, as nature has fitted the human body for climb- 

 ing hills, as well as for walking on plains, the work 

 of the tread-mill, under proper restrictions as to 

 duration must be as natural and healthful as any 

 other. Its effects have now proved it to be so. 



As animal power is exhausted exactly in propor- 

 tion to the time during which it is acting, as well as 

 in proportion to the intensity of force exerted, there 

 may often be a great saving of it by doing work 

 quickly, although with a little more exertion during the 

 time. Suppose two men of equal weight to ascend the 

 same stair, one of whom takes only a minute to reach 

 the top, and the other takes four minutes ; it will 

 cost the first but a little more than a fourth part <it 

 the fatigue which it costs the second, because the 

 exhaustion has relation to the time during which 

 the muscles are acting. The quick mover may have 

 exerted, perhaps, one twentieth more force in the 

 first instant, to give his body the greater velocity 

 which was afterwards continued ; but the sloth sup 

 ported his load tour times as long. 



A h.ealthy man will run rapidly up a long sta r, 



