I40 



HUMAN ANATOMY. 



Fig. 



-I. Cervical 



(atlas) 



'4i 



-I. Thoracic 



{&:,■ 



S^ 



Am 



M 



,,^/if 



-I. Lumbar 



■mm 



^fc?^ 



^^. 



y. 



i 



'W', 



'.m : 



I. Sacral 



)- I. Coccygeal 



»V 



Lateral view of adult spine. 



that age the infant's spine presents in front 

 one general concavity, sHghtly interrupted 

 by the promontory of the sacrum. The liga- 

 mentous spine, containing little bone, is ex- 

 ceedingly flexible in any direction : the atlas 

 can be made to touch the sacrum. It is 

 more accurate to say that the general axis of 

 the spine is a curved one than that any per- 

 manent or fixed curve exists. The cervical 

 curve appears as the infant grows strong 

 enough to hold up its head ; it is never, 

 properly speaking, consolidated (Syming- 

 ton), since it is always obliterated by a 

 change in the position of the head. The 

 lumbar curve appears at from one to two 

 years when the child begins to walk. The 

 mechanism of its production is explained as 

 follows. When an infant lies on its back the 

 thighs are flexed and fall apart. If these be 

 held together and pressed forcibly down, the 

 lumbar region will spring upward, owing to 

 the shortness of the ilio-femoral ligaments, 

 which bend the pelvis and, indirectly, the 

 spine. The psoas muscles, moreover, act 

 directly on the spine. When the child first 

 stands, the body is inclined forward ; when 

 the muscles of the back straighten it, the 

 lumbar curve is produced by the same mech- 

 anism, since it is immaterial whether the legs 

 are extended on the trunk or the trunk on 

 the legs. How or when these curves be- 

 come consolidated is very diflicult to deter- 

 mine. The influence of differences in thick- 

 ness of the front and back of the various 

 bodies and disks is inappreciable in the neck ; 

 in the lower part of the back and in the first, 

 and perhaps the second, lumbar vertebrae the 

 height is greater behind. In the loins the 

 fifth vertebra is much thicker in front and, 

 above it, the fourth and third in a less degree. 

 The intervertebral disks are also much thicker 

 in front. How soon actual difference in 

 the diameters of the vertebrae appears is un- 

 certain. A child of about three shows little 

 of it, except in the last lumbar, and, accord- 

 ing to Symington's plates, there is not much 

 more difference at five or even thirteen years. 

 It is certain that throughout the period of 

 growth the curves can be nearly or quite 

 effaced. The restraining influences are the 

 gradually developing differences in the \erte- 

 brae and the disks, the effect of the sternum 

 and the ribs on the thoracic region, the pull 

 of the elastic ligaments of the arches, and 

 perhaps, above all, muscular tonicity. In 

 the latter part of middle age the curves of 

 the back and loins become consolidated ; 

 this is, however, distinctly a degenerative 

 process. 



