LECTURE III. 115 



zontal or turning motion of the head upon 

 the atlas, and admits only of a slight yield- 

 ing forwards and backwards. The condyles 

 are placed so exactly parallel to the center 

 of gravity, that when we sit upright, and 

 go to sleep in that posture, the weight of 

 the head has a tendency to preponderate 

 equally in every direction, upon the slight- 

 est inclination in the line of gravity ; as 

 we see in those who are dozing in a car- 

 riage. Nay, their heads sometimes revolve 

 in a circle, like the head of Harlequin on 

 the stage. The head and atlas turn round 

 upon the second vertebra; the upper articu- 

 lar processes of which, as well as the lower 

 ones of the atlas, are formed into nearly ho- 

 rizontal planes, a form well calculated to 

 admit of this motion. They are not, in- 

 deed, perfectly horizontal, but shelve a 

 little to either side, so as to admit of a part 

 of the lateral motion formerly mentioned. 

 The second vertebra has growing up from 

 its body a tooth-shaped process, and it is 

 therefore, usually called the vertebra den- 

 tata. This process is tied to the head by 

 ligaments of surprizing strength, which 



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