88 



ward, by a wedge-shaped fissure, and giving to 

 each of the parts thus separated an ovoidal form. 

 The articulation is thus divided into two separate 

 working pieces, one used when the head-ball 

 pressure is from the left, the other when it is from 

 the right, and in the same way for the sockets. 



Bearing in mind that the head joint always 

 opens in front, we may represent this new form of 

 the articulation by supposing the two halves, which 

 Avould be obtained by the longitudinal division of 

 a pear, about one inch long, to be soldered by 

 their flat surfaces to the base of the skull, in such 

 a manner that the points converge in front, while 

 the globular ends diverge in rear, at an angle va- 

 rying in different animals, according to the mode 

 of action to be accommodated. 



These ovoidal pieces are called the head-con- 

 dyles, and have each a separate socket of corre- 

 sponding shape on either side of the upper surface 

 of the first vertebra of the spine. 



§ 90. In the snake a couple of vertebrae, freed 

 from the ground, are all that is required in order 

 to adjust the eccentric movements of the anterior 

 part of the spine with a steady position of the 

 head ; but, as we have seen in man and quadru- 

 peds, there is, after what may be called the ophid- 



