THE HISTORY OF FAT. ADIPOSE TISSUE. 471 



loaded with fat that groups of the cells become practically masses of fat. 

 Connective tissue thus loaded with fat is called adipose tissue ; and masses 

 of adipose tissue of all manner of sizes and of shapes adapted to the several 

 situations are found in various parts of the body. Many of the internal 

 organs, more especially the kidneys, are wrapped in adipose tissue ; but the 

 largest deposit is one lying in the subcutaneous tissue, sometimes called the 

 "panniculus adiposus"; and a "fat" body is distinguished from a "lean" 

 body chiefly, though by no means exclusively, by the amount of subcuta- 

 neous adipose tissue. 



Of all the tissues of the body adipose tissue is the most fluctuating in 

 bulk; within a very short space of time a large amount of 'adipose tissue 

 may disappear, and within an almost equally short time the quantity 

 present in a body may be several times multiplied. When too much or too 

 little food is given it is the subsequent adipose tissue which first and most 

 rapidly increases or decreases in bulk. 



417. A small piece of adipose tissue, examined under a low power, 

 appears to be made up almost entirely of rounded masses of highly refrac- 

 tive material, closely packed together. These rounded masses, which stain 

 an intense black with osmic acid and give other reactions of fat, are 

 arranged in irregular lobules ; between the lobules, and between the indi- 

 vidual rounded masses, may be seen a small amount of fibrillated connec- 

 tive tissue carrying bloodvessels. 



When the tissue has been hardened and stained, and the fat has been 

 removed by solvents, what was previously only visible as a rounded mass 

 of fat is now seen, under higher powers, to be a cell, but a cell nearly the 

 whole of the cell substance of which has become transformed into a single 

 large vacuole. Over the greater part of the circumference of the cell the 

 cell substance is reduced to a mere thin shell or envelope, or cell mem- 

 brane, but at one part a thicker disc-like remnant is seen, and in this is 

 placed a rounded or oval, often flattened nucleus. Between these fat-cells 

 may be seen a few bundles of connective tissue forming a scanty loose net- 

 work, the rounded meshes of which are occupied by the fat-cells, the matrix 

 of the bundles appearing at places continuous with, or adherent to, the 

 envelopes of the cells ; ordinary connective-tissue corpuscles are also here 

 and there present, though rarely visible between the larger, 50/Ji to 130//, 

 fat-cells. In injected specimens it is further seen that the connective-tissue 

 meshwork carries small bloodvessels, which form capillary networks around 

 the groups of fat-cells and even around individual cells. After death, upon 

 cooling, the fat in the fat-cells may solidify in crystals. 



It is obvious that a fat-cell is a cell belonging to connective tissue, in the 

 cell substance of which fat has been collected to such an extent that the cell, 

 which increases largely in bulk during the process, is almost wholly trans- 

 formed into a large vacuole filled with fat, the cell substance being reduced 

 to a thin envelope of the vacuole, thickened at one part where the nucleus, 

 thrust on one side by the gathering fat, is placed. Adipose tissue is a col- 

 lection of such fat-cells held together by a meagre quantity of vascular con- 

 nective tissue. 



By studying the development of adipose tissue in the embryo or else- 

 where, we may trace out the steps of the formation of the fat-cells. In the 

 embryo, in a situation where adipose tissue is about to be formed, the con- 

 nective tissue is seen to contain a number of small nucleated cells, rounded 

 or somewhat irregular in form, the cell substance of which at first presents 

 no special characters, and contains not more than what may be called the 

 ordinary amount of fat globules or spherules. Very soon, however, these 

 minute drops or specks increase in number, the cell substance at the same 



