By putting on her bedroom slippers any one can easily convince herself of the greater grip the foot has on the floor when so clad, and of the greater ease and sureness of the foot in walking. For outdoor athletics a short skirt, coming just below the knees, may be worn over the bloomer costume. Tennis shoes should always be worn. All rooms used for exercise, gymnasiums, and ball-rooms must be thoroughly ventilated before the assemblage of the people. The air must be kept cool, between 50° and 60° F., and proper arrangements must be made to keep the room well ventilated while in use without causing direct drafts. Well-waxed, hard-wood floors are the best, because they can be kept freest from dust. Students should never be allowed to enter the gymnasium with their street shoes on, as they carry with them much dust that will be thrown in motion and inhaled during the performance of the various exercises, and there follows not only the irritation caused by the inhalation of the particles of dust, but also the danger of inspiring all kinds of germs of disease with which the air is laden. [Illustration: Fig. 26.—Upper surface, bones of foot (Allen).] =The Configuration of the Foot.=—No study of the correct attitude of the body at rest or in motion would be complete without some knowledge of the structural arrangement of the foot. The feet form the base of support for the entire body, and at every step are subjected to a pressure of from one hundred to two hundred pounds. This base is in the form of two arches, a transverse and an anteroposterior. The latter is the most important, and has been subdivided into two by an imaginary line, drawn between the third and fourth metatarsal bones. The inner portion of this arch is much more curved than the outer, and forms the instep. The arch is supported by two piers. The posterior pier is formed by the os calcis, or heel bone, and the posterior part of the astragulus. It is shorter, has but one joint, is more curved, and is, at the same time, more solid than the anterior pier, and receives the greater part of the weight of the body. The anterior pier includes all the bones in front of the astragulus to the junction of the three metatarsal bones with the toes. It is much the longer, is less curved, and has many joints, giving it greater elasticity, and also enabling it to diminish the force of shocks transmitted to the arch. The summit of the arch is the ankle. It is evident that the superincumbent pressure, by flattening the arches, both lengthens and broadens the foot. The anteroposterior arch is further lengthened by a turning upward of the toes, which form a hinge-joint with the instep.