236 COMPARATIVE ZOOLOGY 



wanting in the teeth of most fishes, snakes, sloths, 

 armadillos, sperm whales, etc. 



True dental tissue is confined to vertebrates. 



(6) Adipose Tissue. Certain cells become greatly en- 

 larged and filled with fat, so that the original protoplasm 

 occupies a very small part of the space within the cell 

 membrane. These cells are united into masses by con- 

 nective tissue, in the skin (as in the " blubber " of whales), 

 between the muscles (as in " streaky " meat), or in 



FIG. 207. Fat Cells embedded in Subcutaneous Areolar Tissue. _/, fat cells; . nucleus; 

 c, connective-tissue corpuscles; w, migratory cells; e, elastic fibers; b, capillary 

 blood vessel. 



the abdominal cavity, in the omentum, mesentery, or 

 about the kidneys. The marrow of bones is an example. 

 Globules of fat occur in many mollusks and insects ; but 

 true adipose tissue is found only in backboned animals, 

 particularly in the herbivorous. In the average man, 

 it constitutes about $ part of his weight, and a single 

 whale has yielded 120 tons of oil. The fat of animals 

 has the different names of oil, lard, tallow, suet, sperma- 

 ceti, etc. It is a reserve of nutriment in excess of con- 

 sumption, serving also as a packing material, and as a 

 protection against cold. 



(7) Muscular Tissue. If we examine a piece of lean 

 meat, we find it is made up of a number of fasciculi, or 

 bundles of fibers, placed side by side, and bound together 



