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 vertebre. 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 vertebre 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 vertebre, 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- 
_ tebree in the table already given (§ 627); since it is the attach- 
ment of the ribs that makes the essential difference between 
_ the dorsal vertebre and the cervical or lumbar. The other 
_ extremity of each rib is connected with a cartilage, which is 
_@ 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 
q 1 It is scarcely necessary here to state, that the common notion 
Bee recting the deficiency of a rib on one side of the body of Manisa 
pop error. 
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y 
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