144 HUMAN ANATOMY. 



ones. It has been shown (Wullstein) that injury to the dura and cord and even 

 fracture of the arches and processes are possible concomitants of forcible rectifica- 

 tion of kyphosis. 



If the curve forward of the lumbar spine is exaggerated, constituting lordosis, 

 it is usually compensatory, and is acquired in an effort to maintain the erect* 

 position, as in cases of high caries, great obesity, pregnancy, ascites, abdominal 

 tumors, etc. 



Scoliosis or lateral curvature commonly results from faulty positions in young, 

 undeveloped persons with weak muscles, as school-girls, who sit or stand in such atti- 

 tudes that the muscles are relieved and the strain is borne by insensitive structures, 

 like ligaments and fasciae. This results in a deflection of one part of the column — 

 generally the thoracic — to one side, usually the right, and the formation of a compen- 

 satory curve below, and occasionally of one above also. The bodies of the affected 

 vertebrae are at the same time rotated, partly by the action of the slips of the longis- 

 simus dorsi which are attached to the ribs near the angles and to the tips of the trans- 

 verse processes (Fig. 520), so that in advanced cases the tips of the spinous pro- 

 cesses of the affected segments turn towards the concavity of the curves, while the 

 transverse processes of the vertebrae involved tend to lie in an antero-posterior plane 

 and can often be felt projecting backward. 



A further explanation of the causes of the rotation may be found in the behavior 

 of a straight flexible rod under similar conditions. Torsion results from any motion 

 in which all particles of a straight flexible rod do not move in parallel columns. 

 Therefore, if it be bent in two planes at the same time torsion must inevitably occur. 

 The vertebral column being bent in the antero-posterior plane by a series of gentle 

 curves, lateral bending must, therefore, inevitably lead to torsion, since it means 

 bending in two planes. 



A little consideration of the relations of the spine to the ribs, scapula, and pelvis 

 will show that lateral flexion and rotation cannot take place without causing (a) sep- 

 aration of the ribs on the convex side ; ((5) change in the costal angles, making 

 the ribs more horizontal on the convex and more oblique on the opposite side ; (^) 

 undue prominence of their angles on the convex side, the scapula being carried upon 

 them so that it also is more prominent ; {d^ diminution of the ilio-costal space on 

 the concave side ; (^) elevation of the shoulder on the convex side ; (y") flatten- 

 ing of the chest in front on the convex and undue prominence of the chest on the 

 opposite side ; (^) projection of the ilium on the concave side. Lateral curvafure 

 with these secondary deformities may also be produced by unequal length of the 

 lower limbs, one-sided muscular atrophy, hypertrophy, or spasm, sacro-iliac disease, 

 empyema, and asymmetry of either the pelvis or the head. 



The latter factor is especially interesting from an anatomical stand-point. From 

 what has been said (page 142) of the position of greatest stability of the joints be- 

 tween the head and the atlas and the latter and the axis, it is evident that the position 

 of greatest ease is with the head slightly turned to one side, the condyles of the 

 occiput not being in their best contact with the superior articular surfaces of the 

 atlas when the head is held straight, but rather when the head is slightly twisted 

 (Dwight). The effects of this are far-reaching. First, there is an instinctive effort 

 to get the eyes on the same plane in looking forward, which is presumably the 

 primary cause of the asymmetry of the face that is usually found. It is also easier 

 to support the weight in standing chiefly on one leg, hence the other side of the 

 pelvis is allowed to fall so that the lumbar region slants away from the supporting leg. 

 This must be corrected by a lateral motion of the spine above it, and as this is not 

 pure but mixed with rotation, there occurs a twist in the spine ; one shoulder is higher 

 than the other as well as farther forward. In healthy persons such positions, if not 

 maintained too long, do little harm ; but there is likely to be some spinal asymmetry 

 in all, and there is the danger that it may become pronounced and fixed in the weak. 

 Sprains of the spine are most common in the cervical and lumbar regions : in 

 the former because of the greater mobility of the articulation with the cranium, and 

 in both because of their own mobility, the greatest degree of bending in an antero- 

 posterior direction being possible in those two segments of the spine. The thoracic 

 and pelvic curves are primary, form part of the walls of the thorax and pelvis. 



