=Posterior Curvature of the Spine.=--The spine (see Figs. 28, 31) has two backward curves (opposite chest and hips) and two forward curves (at loins and neck). The deformity called posterior curvature is chiefly an exaggeration of the upper posterior curve. Round shoulders is the slightest, and hunchback the most marked, degree of this deformity. Causes: 1, _bending over the work_ while either standing or sitting; 2, _slipping down in the seat_, as in Figure 51; 3, working habitually with the _work low in front_, as reading and writing at too low a desk (Fig. 49), or bending over while hoeing, sitting on the floor (Japanese and Chinese); 4, _weak muscles_ in the back; 5, wearing shoes with _high heels_; 6, binding the ribs down with _tight clothing_; 7, walking with the _head drooped forward_ or the chest flat; 8, wearing suspenders without a pulley, or lever, at the back; 9, carrying the hands in the pockets. (Swing the arms to keep the hands out of the pockets and break the habit); 10, wearing a coat or vest that is tight at the back of the neck. This deformity is brought about by _stretching the ligaments_ at the back side of the spine, and by _compressing the cartilages until they become wedge-shaped_, with the thin part of the wedge in front. The flexibility of the spine is a great advantage, but it increases the risk of deformity. One of the most serious evils of posterior curvature is a flat chest and restricted breathing. [Illustration: FIG. 37.--INCORRECT POSTURE.] [Illustration: FIG. 38.--CORRECT POSTURE, but strained and stiff.] =Lateral Curvature of the Spine.=--A perfect spine curves to neither side (Fig. 47), but is perfectly erect. The least habitual lateral curvature is deformity. Causes: 1, writing at a _desk that is too high_; 2, habitually carrying a book, satchel, or other _weight in the same hand_; 3, carrying the _head on one side_ (Fig. 46); 4, habitually standing with the weight on the same foot; 5, a certain defect of vision (astigmatism, Chap. IX). =To overcome Spinal Deformities.=--The work, or the manner of doing the work, should be so changed as _to give extra labor to the neglected muscles_. Avoid the habits mentioned above as causing deformity. Sit and stand in the manner described in the next paragraph. Sleeping on the back upon a hard mattress without a pillow tends to cure posterior curvature and flat chest. =The correct position= in standing is: _chest forward, chin in, hips back_ (Figs. 38, 39). To sit correctly, _sit far back in the chair_ (Figs. 49, 50, 51) with the body erect and balanced. In youth the bones are soft and growing; they will readily grow into perfect shape, and will almost as readily grow deformed.