Chap. 7.] Motion of the Head. Hf 



nium. By means of this joint we move the. head 

 backwards and forwards on the fpine, or perform the 

 action of nodding. As it is neceffary, however, for 

 the head to have alfo a rotatory motion, we here 

 find a peculiar-icy of ftructure to which there is no- 

 thing fimilar in any other part of the body. In the 

 upper furface of the fecond vertebra of the neck there 

 is a long tooth -like procefs or projection, which is 

 received into a perforation of the firft vertebra. 

 This procefs is rendered fmooth by a covering of 

 cartilage ; it pafies quite through the vertebra above 

 it, and is connected to this as well as to the cra- 

 nium by ftrong ligaments, which give ftrength to 

 the connection, and guard againft the effects of a too 

 extenfive motion. The rotatory motions of the head, 

 therefore, are not performed on the firft vertebra of 

 the neck, but on the fecond ; the firft vertebra, with 

 the head, moving on the tooth-like procefs of the 

 fecond vertebra, as a wheel .moves on its nave. 



The fpine, however, though it forms a column, 

 does not form by any means an upright column. 

 The fpine, viewed tideways, if the os facrum is confi- 

 dered as a continuation of it, is bent very much in the 

 form of the letter/. In the neck it projects fome- 

 what forwards, lower down it takes a curved direc- 

 tion backwards, to make room for the heart and lungs, 

 la the loins it advances again forwards under the center 

 of gravity, io as to fupport the abdominal vifcera; and 

 in the pelvis it recedes backv/ards, fo as confiderably to 

 enlarge that cavity. 



Each vertebra is divided into a body and feven 

 projections, apophyfes or proceffes. The body is 

 placed before, it is fmooth, of a roundiih form, and 

 a remarkably fpongy texture. The procefles are of 

 a much firmer texture, and project backwards. TWO 



of 



