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I think it useful to insist, more than has been done hitherto, on the me- 

 chanism by which the human body is supported in the erect posture; for 

 a knowledge of that mechanism facilitates the explanation of the motions 

 of progression. To walk, or to run, the body must be upright; now, 

 when it is known by what power the centre of gravity of the body is main- 

 tained perpendicular on the plane which supports it, it will be easy to 

 understand the different ways in which it changes its place, in the course 

 of locomotion. 



Let us first inquire into the question so long agitated, whether man la 

 intended to support himself and to walk on his four limbs, in the early 

 period of his existence after birth ? 



CLXXXI. An upright position would be to man a state of rest, if his 

 head were in a perfect equilibrum on the vertebral column, and if the 

 latter^ forming the axis of the body and supporting equally, in every di- 

 rection, the weight of the abdominal and thoracic viscera, fell perpen- 

 dicularly on the pelvis placed horizontally, and, in short, if the bones of 

 the lower extremities formed columns set perpendicularly under their 

 superincumbent weight; but not one of these circumstances is to be ob- 

 served in the human body: the articulation of the head docs not corres- 

 pond to its centre of gravity; the weight of the thoracic and abdominal 

 viscera, and of the parietes of the cavities in which they are contained, 

 rests, almost entirely, on the anterior part of the vertebral column. The 

 vertebral column is supported on an inclined base, and the bones of the 

 inferior extremities, which are connected to each other by convex and 

 slippery surfaces, are, more or less, inclined towards one another. It is 

 therefore necessary, that an active power* watch incessantly, to prevent 

 the fall which would be the natural consequences of their weight and 

 direction. 



This power resides in the extensor muscles which keep the parts of 

 OUP body in a state of extension, the more perfect, and which render our 

 erect posture the firmer, as they are endowed with a more considerable 

 power of antagonism, and as our parts arenaturally less disposed to flexion; 

 and, besides, as we have seen (CLXVI.) these powers are not sufficient 

 to balance those whose action is directly opposed to theirs. 



The relative weakness of the extensor muscles is not the only obstacle 

 which renders impossible an erect posture, at an early period of life. 

 Other causes, into which we are about to enter, concur in unfitting the 

 new born child for the exercise of that faculty. 



The articulation of the head to the vertebral column being nearer the 



* An upright posture is not, in all animals, as it is in man, the consequence of an 

 effort. This is proved by the following fact, observed by M. Dumeril. The sea fowl, 

 and especially the waders (Grallae, Lin.) as the herons and storks, forced to live in the 

 midst of marshes and muddy waters, in which they find the fishes and reptiles, on which 

 they feed, have long since afforded matter of surprise to Naturalists, by the length of 

 time they can remain motionless in an erect posture. This singular power, so necessary 

 to animals obliged to expect their prey, mure from change than from industry, they owe 

 to a peculiar conformation of the arliculation between the leg- and the thigh. The arti- 

 culating surface of the thigh bone, as M. Dumeril had an opportunity of observing- in a 

 stork (Ardea ciconia, Linn.) contains, in its centre, a depression, into which there is re- 

 ceived a projection of the tibia. To enable the animal to bend its leg, that projection 

 must be disengaged from the depression into which it is lodged, and this is resisted by 

 several ligaments which keep the leg extended in standing, in flying, and other progres- 

 sive motions, without the assistance of the extensor muscles. Copland. 



