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 joint with considerable motion, it is termed 

 articular or obduccnt 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. All 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. 



II. ARTICULATIONS 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 Ixitwecn the hard parts which supply the place of 

 bones in the lower animals 



