ARTICULATION OF HEAD WITH SPINAL COLUMN. RIBS. 475 



dentatus (or tooth-like process), occupies the place of the body 

 of the atlas ; and by the rotation of the atlas around it, the 

 movements of the head from side to side are accomplished. 

 Wherever great freedom of motion is permitted, displacement 

 or dislocation is necessarily more easy ; and accordingly we 

 find that the atlas and axis can be more easily separated from 

 each other, than can any other two vertebrae. This dislocation 

 may be produced by violence of different kinds ; thus if the 

 head be suddenly forced forwards while the neck is held back, 

 the tooth of the axis may be caused to press against the 

 spinal cord, and thus to interrupt or completely check its 

 functions. Or, again, if the weight of the body be suspended 

 from the head, and especially if it be thrown upon it with a 

 jerk, the two vertebras are liable to be dragged asunder, and 

 the spinal cord to be stretched or broken. This is sometimes 

 the immediate cause of death in hanging ; and it has not 

 unfrequently occurred when children have been held in the 

 air by the hands applied to the head, a thing often done in 

 play, but of which the extreme danger should prevent its 

 ever being practised. Any serious injury of the spinal cord 

 in this region must be immediately fatal, for the reason for- 

 merly stated ( 470), that it causes the suspension of the 

 motions of respiration. 



633. The number of the ribs which are attached to the 

 bodies and transverse processes of the dorsal vertebra, is, in 

 the Human species, twelve on each side. 1 The number in 

 different animals may be judged-of by that of the dorsal ver- 

 tebras in the table already given ( 627) ; since it is the attach- 

 ment of the ribs that makes the essential difference between 

 the dorsal vertebras and the cervical or lumbar. The other 

 extremity of each rib is connected with a cartilage, which is 

 a sort of continuation of it ; in Birds, the cartilages of the 

 ribs are ossified or converted into bone. The cartilages of 

 the first seven ribs (in Man), which are termed the true ribs, 

 are united to the sternum or breast-bone, which forms the 

 front wall of the thorax (fig. 163). The cartilages of the five 

 lower ribs are not directly connected with this, and they are 

 hence called false ribs ; those of three of them, however, are 



1 It is scarcely necessary here to state, that the common notion 

 respecting the deficiency of a rib on one side of the body of Man is a 

 popular error. 



