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Rh however, as we began to plan a series of observations relating to the question of balance, it became evident that the field was so large that we could hope to cover for the present only some one small phase of the subject, and after a survey of the whole field of inquiry opened by our physiologic research we decided to limit ourselves for this first paper to a study of the very suggestive ques­tion of why such backache was in our clinical experience frequently improved or relieved by the use of corsets or high-heeled shoes. No one realizes more fully than ourselves how very small a portion of the whole field we have here covered, and our present contribution is regarded by us rather as a report of progress, possibly of therapeutic value, than as a final settlement of even the management of static backache. It is our intention to pursue the lines of investigation already undertaken further as opportunity may permit.

For the proper comprehension of the practical application of our experimental work to clinical conditions, it is necessary at this place to introduce certain elemen­tary points in mechanics which are familiar to every one, for the preliminary problem under consideration resolves itself into an analysis of the mechanism of the standing position.

These points are the following:


 * 1) The base of support of the upright human figure consists of a trapezoid formed by the outer borders of the feet and lines connecting the back of the heels and the front of the toes.
 * 2) The center of support lies perpendicularly under the center of gravity and must always lie within this trapezoid.
 * 3) For the purpose of studying the mechanism by which any weight is borne by a solid body in unstable equilibrium, the entire weight may be regarded as concentrated in the center of gravity, and the determination of the relation between the center of gravity and the bearing points determines the lines of stress.

Our observations show that in the erect position the center of gravity of the body lies in front of the ankle-­joints, which are held from dorsal flexion in this posi­tion by the gastrocnemius muscles. The center of gravity lies also in front of the knees, which are similarly held in position by the hamstring and quadriceps exten­sor muscles. The center of gravity lies also anterior to the sacra-iliac joints and most of the vertebral joints. The position of the acetabula cannot be determined in the erect position in the living individual because we have no means of locating them from any available landmark. If we were able to determine the position of the acetabula in the antero-posterior plane it would be po­sible to state definitely, from the relation of the center of gravity to them, whether the trunk in the erect position would tend to fall forward or backward at their level. But from the impossibility of obtaining definite