SITTING. 291 



931. The sitting posture, even without the support of 

 the back, can be maintained much longer than that of 

 standing, because the centre of gravity is thrown nearer 

 the base ; and because the weight is diminished, and con- 

 sequently the muscular power required to support the 

 erect posture, is lessened. But this position, without the 

 support of the back, after a time becomes too painful to 

 be endured with patience. 



932. The sad consequences of long confinement in the 

 sitting posture, without any support for the back, have 

 already been described at some length under another 

 head, but the more we have thought upon and examined 

 this subject, the more important it appears, and we can- 

 not therefore, in duty to the youth of our country, dismiss 

 it without some further considerations and remarks. 



933. Causes of Spinal Curvatures. It is proposed 

 here to trace the effects of the causes to which we have 

 already referred with respect to curvatures of the spine, 

 and to show why certain positions will make this defor- 

 mity permanent. 



934. The spine itself, detached from all other parts, is 

 figured and described at p. 80, Fig. 63, where the light- 

 colored transverse lines between each two vertebra show 

 the cartilages of the spine. These cartilages are in the 

 subject, about the fourth of an inch in thickness, and are 

 compressible and elastic like pieces of India-rubber. 

 Were it not so the spine would be rigid, and unyielding 

 as though it was formed of one continued piece of bone. 

 Its motions, therefore, are in consequence of the elas- 

 ticity of these cartilages, so that when the spine is bent, 

 one of their sides, or edges, is compressed more than 

 the other. 



935. In the night, when we take the recumbent pos- 

 ture, and there is no pressure on the spinal column, these 

 elastic plates swell and become thickened, but their thick- 

 ness is again reduced by the weight of the body during 

 the day, and especially in laborers who carry weights 

 on their heads. The diurnal difference in the thickness 

 of each cartilage, from these causes, it is true, is very 

 slight, but their number is such as to make an apprecia- 



