GENERAL STRUCTURE OF ANIMALS. 



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Fig. 6. — Structure of cartilage. 



large quantities (Fig. 5). This is sometimes spoken of 

 as adipose tissue, but it is not properly a tissue at all. 



2. Cartilage. — This will be easily recognized under 

 its popular name, gristle. Its firmness and yet elasticity, 

 its white translucency, its smooth homogeneous surface 

 when cut, are familiar and 

 characteristic properties. If 

 a thin section be placed under 

 the microscope, it is at once 

 seen to consist of innumer- 

 able nucleated cells lying in 

 a structureless, semitranspar- 

 ent, hyaline mass (Fig. 6). The 

 cells are in clusters, evidently 

 formed by cell division. 



Cartilage is the tissue used 

 in the animal body whenever a moderate degree of firm- 

 ness combined with elasticity is required. It therefore 

 caps the ends of the bones at the joints. The anterior 

 portions of the ribs are cartilage, so as to yield to respi- 

 ratory motions. The external ear consists of cartilage, 

 so as to retain its form and yet to be not liable to break. 

 The tip of the nose is of the same substance, and for the 

 same reason. 



Varieties. — In higher animals there are two varieties, 

 viz., permanent cartilage, such as all those already 

 mentioned, and temporary cartilage, which afterward 

 becomes bone. Hence cartilage is very abundant in 

 young animals. But the difference between these varie- 

 ties is too unimportant to detain us. 



3. Bony Tissue. — Bone is the hardest tissue in the 

 body and is used wherever rigidity is required. It is 

 therefore in higher animals the material of the skeleton. 

 It consists of an organic tissue, a kind of connective, 

 hardened by a deposit in it of mineral matter, chiefly 



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