ANIMAL MECHANICS. CENTRE OF GRAVITY. 161 



of contraction in them, in a given time, will be as four to one. On 

 the other hand, the power of contraction, or the force of a muscle, is 

 in proportion to the number of its fibres ; and hence, in all the most 

 powerful muscles, the fibres are very short, but so arranged on the 

 tendons as to be very numerous. Notwithstanding the perfection of 

 these, and other more mechanical, arrangements in the body, it hap- 

 pens always, in its more important movements, that there is a dispro- 

 portionate expenditure of actual muscular power, in comparison with 

 the useful work accomplished. This is well illustrated in the effort 

 required to maintain the position of standing on one leg. 



Mechanics of the Body. 



Under this head, we may consider, generally, the distribution of 

 weight in the body, or its centre of gravity, its basis of support, and 

 the nature of the levers employed in the movements of its several 

 parts. 



The entire body, being comparable to any other solid mass having 

 certain cubical contents, must have its centre of gravity placed at the 

 point of intersection of its three planes the median-vertical, the 

 antero-posterior vertical, and the horizontal plane, determined by 

 reference to its weight. Of these planes, the median-vertical, of 

 course, corresponds with the median plane of the body, and may be 

 found approximately by a plummet line, when the body is in an erect 

 position; the horizontal plane is found by balancing a man, lying 

 with the arms by his sides, upon a plank, which is supported on a 

 transverse knife-edge; and lastly, the anterior-posterior plane is de- 

 termined by a corresponding method. The point of intersection of 

 these planes, or the normal centre of gravity, when the arms are 

 hanging down by the sides, is generally said to lie in the cavity of the 

 pelvis, a little in front of, and above, the lumbo-sacral articulation; 

 but, by Weoer, it is said to be in the promontory of the sacrum, and, 

 by Meyer, in the vertebral canal, opposite the second sacral vertebra. 

 The slightest displacement of the head, or inclination of the body, a 

 swinging or lateral elevation of either arm, or even stooping, changes, 

 temporarily, the position of what may be called the symmetrical 

 centre of gravity, which of course shifts its situation in the direction 

 of any partial displacement of the movable portions of the frame. 

 The centre of gravity of the head and trunk together, in the sitting 

 position, is midway between the point of the sternum and the vertebral 

 column ; and the centre of gravity of the head alone, is opposite a 

 point, above, and anterior to, the opening of the external ear. Elabo- 

 rate observations have also been made as to the position of the centres 

 of gravity of the upper and lower limbs, and of each separate section 

 of the same. 



The basis of support of the body, necessarily varies in the recum- 

 bent, standing, and sitting posture. As in the case of other solid 

 masses, when the centre of gravity is so placed, that a line, let fall 

 perpendicularly from it, strikes within that base, the equilibrium of 

 the body is easily maintained; but when such a line falls beyond the 



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