Adipose tissue is a form of connective tissue, and is, to a considerable extent, interchangeable with loose connective tissue. Adipose tissue is formed by the infiltration of fat into the connective tissue cells, where it is laid down in store, and this store fat may at different times be variously abundant.

In Obesity an excess of fat is present in the body, and the fat is laid down in store chiefly in the subcutaneous connective tissue and the omentum, but also in other situations where loose connective tissue is present.

Around or in disused or atrophied organs it is common to find a fatty infiltration. A most typical example of this is afforded by muscles which have become fixed at their ends by stiffening of joints. The muscle can no longer produce any movement, arid its fibres gradually atrophy as we have already seen. At the same time, in the connective tissue around the muscle and in that which supports it, there is a great infiltration of fat, so that adipose tissue appears between and around the fibres. Then, again, in pseudohypertrophic paralysis - a disease chiefly of children - there is a similar process. The muscular tissue atrophies, but there is at the same time an excessive transformation of the connective tissue into adipose tissue, so that the wasting of the muscle is more than counterbalanced by the excess of adipose tissue, and there is thus a pseudo-hypertrophy. (See Fig. 46).

Fig. 46. - Fatty infiltration of muscle. Pseudo-hypertrophic paralysis. The muscular fibres are narrowed and adipose tissue appears between them.

A similar fatty infiltration occurs in the heart, and may seriously incommode it in its action (see further on).

Fat is often deposited in excessive quantity around diseased and useless glands, such as the kidney, pancreas, etc. In the contracted kidney of chronic nephritis there is often an excess of fat at the hilus, which may make the kidney appear much less reduced in bulk than it really is. In hydronephrosis there is often an enormous increase of the fat which normally surrounds the kidney.