82 



PRINCIPLES OF ANIMAL BIOLOGY 



Cartilage and bone are likewise specialized forms of sustentative 

 tissue. They are alike in having their cells more or less scattered in a 

 substance, the matrix, which the cells have secreted. In cartilage the 

 cells are entirely separate from one another, though often placed in pairs, 

 trios, or quartets (Fig. 68) resulting from recent divisions of an earlier 

 cell. The matrix is firm or pliable, contains much gelatin, and is used as a 

 buffer to absorb shock or in places requiring flexibility. In bone the 

 cells possess numerous slender projections, some of which, probably, are 

 always in contact with similar projections from other cells (Fig. 69). The 

 hard bony material of the matrix, consisting largely of calcium carbonate 



Cart. 



W-W"'r^ 



vies 





if^'m 



^■<«-* 



B 



Fig. 68. — Sections through cartilage. A, development of cartilage (top) from meson- 

 chyme (bottom); B, hyaUne cartilage. {From Lewis-Stohr, '"Textbook of Histology," 

 The Blakiston Company.) 



and calcium phosphate, is secreted by these cells; consequently there are 

 always spaces in the bone for the cells and their slender processes. 



Fatty or adipose tissue is regarded as sustenfative, but rather because 

 of its original similarity to connective tissue than from any mechanical 

 function which it may serve. The cells are numerous and closely packed, 

 not scattered as in other sustentative tissues. The fat itself is in globules 

 of small or large size contained within the cells. It is reserve food; hence 

 fatty tissue fluctuates greatly in \^olume, depending on the state of 

 nutrition of the organism. Favorite places for the deposit of fat are in 

 the abdominal wall and beneath the skin at many other places. 



In many embryos, and in the adult of certain lower animals, such as 

 the flatworms, there is a tissue known as mesenchyme, which should be 

 included with the sustentative tissue, though chiefly because of its struc- 



