112 PHILOSOPHY OF ZOOLOGY. 
united with the bone, and can scarcely be regarded as dif- 
ferent from an elongation of the cartilaginous basis. Where 
it occurs at a jomt with considerable motion, it is termed 
articular or obducent cartilage. In other cases, it occurs 
as a connecting medium between bones which have no ar- 
ticular surfaces, but where a variable degree of motion is 
requisite. The ribs are united to the breast-bone in this 
manner. Between the different vertebrae, there are inter- 
posed layers of cartilage, by which the motions of the spine 
are greatly facilitated. As these connecting cartilages are 
compressible and elastic, the spine is shortened when the 
body remains long in a vertical position, owing to the su- 
perincumbent pressure. Hence it is that the height of man 
is always less in the evening than in the morning. Al] these 
cartilages are more or less prone to ossification, in conse- 
quence of the deposition of earthy matter in the interstices. 
To this circumstance may be referred, in a great measure, 
the stiffness of age, the elasticity of the cartilages decreas- 
ing with the progress of ossification. 
Cartilage occurs even unconnected with bone, in parts of 
the system where strength and elasticity are required. 
If. ArTIcuULATIONS OF THE BoNEs. 
The manner in which the bones of the skeleton are unit- 
ed to one another, exhibits such remarkable differences, in 
respect to surface, connection, and motion, that anatomists 
have found it difficult to give to each kind of articulation an 
appropriate name, and a distinguishing character. With- 
out attempting to enter into the details of the subject, it 
will be sufficient for our, purpose, to enumerate the more 
obvious kinds of articulation, and the motions each is des- 
tined to perform ; not confining our attention to the bones 
of the vertebral animals, but including the modes of junc- 
tion between the hard parts which supply the place of 
bones in the lower animals. 
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