48 ELEMENTARY TISSUES. 



cell it is not easily or always visible. The ultimate cells 

 are held together by capillary blood-vessels ; while the little 

 clusters thus formed are grouped into small masses, and 

 held so, in most cases, by areolar tissue. The oily matter 

 contained in the cells is composed chiefly of the compounds 

 of fatty acids with glycerin, which are named olein and 

 margarin, and there is also a minute proportion of palmi- 

 tin. The harder kinds of fat in some of the lower animals 

 contain stearin also (p. 19). 



It is doubtful whether lymphatics or nerves are supplied 

 to fat, although both pass through it on their way to other 

 structures. 



Among the uses of fat, these seem to be the chief : 



1 . It serves as a store of combustible matter which may 

 be re-absorbed into the blood when occasion requires, and 

 being burnt, may help to preserve the heat of the body. 



2. That part of the fat which is situate beneath the 

 skin must, by its want of conducting power, assist in pre- 

 venting undue waste of the heat of the body by escape 

 from the surface. 



3. As a packing material, fat serves very admirably to 

 fill up spaces, to form a soft and yielding yet elastic 

 material wherewith to wrap tender and delicate structures, 

 or form a bed with like qualities on which such structures 

 may lie, unendangered by pressure. As good examples 

 of situations in which fat serves such purposes may be 

 mentioned the palms of the hands, and soles of the feet, 

 and the orbits. 



4. In the long bones, fatty tissue, in the form known as 

 marrow, serves to fill up the medullary canal, and to sup- 

 port the small blood-vessels which are distributed from 

 it to the inner part of the substance of the bone. 



Pigment. 



In various parts of the body there exists a considerable 

 quantity of dark pigmentary matter, e.g., in the choroid 



