PIGMENT-CELLS. 39 



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, stearin, 

 and palmitin. 



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 re- 

 quires, 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 preventing 

 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, tj form a soft and yielding yet elastic mate- 

 rial 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 men- 

 tioned 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 

 coat of the eye, at the back of the iris, in the skin, etc. 



