216 DETAILED SYNTHESIS. 



319. THE PIVOT-JOINTS OF THE SPINAL COLUMN are 

 the pivots upon which the vertebrae turn ; so that when 

 the column is bent forward, the whole thickness of the 

 intervertebral substance must be compressed (see pre- 

 ceding Fig.), and when the column is bent backward, 

 the whole substance is stretched, though in each case 

 the front part is a little more affected than the back part. 



320. Thus THE VERTEBRAE OF THE SPINAL COLUMN 



are a series of levers with an elastic prop under their 

 front end ; and when all these props yield, the front ends 

 of the levers approach and tend to form a circle ; the 

 length of the column, if measured through the pivot- 

 joints, being the same in each position, but the apparent 

 height of the person becoming so much less as the thick- 

 ness of the props is lessened. 



321. CONTINUED PRESSURE UPON THE INTERVERTE- 

 BRAL SUBSTANCES tends to make them permanently thin- 

 ner, and repose after action tends to restore them. Al- 

 ternate pressure and repose promotes their highest degree 

 of perfection. 



322. Illus. At night a person is not quite as tall as in the morn- 

 ing. By a long horseback-ride the height will be lessened for a tune. A 

 French physiologist says a son lost an inch during one night's dancing. 

 A professor informs the author that, during a hunting excursion from 8 

 till 4, he lost three quarters of an inch, that was not regained till the 

 second morning after. Usually one quarter to one half inch is lost du- 

 ring the day. 



323. Inf. THE ERECTNESS OF FORM MUST USUALLY BE DEPENDENT 

 upon the condition of the intervertebral substances. 



324. THE CAUSES OF CONTINUED DEFORMING PRES- 

 SURE upon the intervertebral substances are, holding one 

 position too long, and tight clothing. The latter is the 

 chief cause. 



325. Inf. AN ERECT POSTURE too long continued is not as b4 as 

 any other position. 



819.Wliatare-? 820. What said of as levers? 321. What said of -? 822. Il- 

 lustrate. 823. On what ? 824. What are ? 825. What said ? 



