ADIPOSE TISSUE is usually associated with Areolar Tissue [AREOLAR TissuE], the two being generally known collectively as Cellular Tissue. It must be distinguished from Fat [FAT], adipose tissue being a membrane of extreme tenuity in the form of closed cells or vesicles, while fat is the material contained within them. The membrane of the adipose vesicle does not exceed the 20,000th of an inch in thickness, and is quite transparent ; it is moistened by watery fluid, for which it has a greater attraction than for the fat it contains. Each vesicle is a perfect little organ, varying, when fully developed, from the 300th to the 800th of a line ; minute capillaries may be observed on their external surface. (Fig. 2). When fat-vesicles are deposited together in large numbers, as is usually the case, they assume a more or less regular polyhedric form from their mutual pressure.
When the first traces of fat appear is not accurately known. In a well-formed five-months' human fcetus, Valentin found in the subcuta neous cellular tissue of the sole of the foot not merely fat cells, such as occur in adults, varying from the ordinary size to the 125th or 100th of a line, within and around which were numerous small vesicles (fig. I, a), but other forms which threw more light on their structure and development. In some the surrounding cell-membrane was much more distinct than as it occurs in adults (b). In others there appeared to be a deposition of fat, not occupying the whole space of the cell (c); the remainder of the cell having ofteu a striped or streaky appearance, and forming a lateral projection ; this is seen in c, and in a more marked degree in d and e. In other fat-cells there were observed to be two vesicles, separated by a septum, wired which they were partially flattened by pressure (g), or merely separated by a coned rictlon in the external walls, as in f. This form leads us to conclude that fat-cells increase by division. The fat-vesicle of the human subject containe Margarin, a solid fat, and Okla, a fluid fat. These sometimes separate spontaneously, presenting a very beautiful microscopic appearance. The margarin collects in a spot on the jailer surface
of the cell-membrane, and presents the appearance of a small star, whilst the olein occupies the remainder of the vesicle, unless when th,g quantity of fat iu it is rather smaller than usual, in which cam we may observe a little aqueous fluid between the olein and the cell-membrane. (Fig. 1, h.) The chemistry of the substances Margarin and Oldn is somewhat complicated, but the function of the adipose tissue cannot be explained without it. Thom two substances, with Stearin, are the most widely distributed fats in the organic kingdom, but they aro not the only ones. They were formerly regarded as salts formed by fatty acids with Gtyceria. Recent investigations have however shown that this view requires a slight modification. Ilerzelius thinks that glycerin does not exist ready formed in the neutral fats, but that it is a product of the formation of soap ; and he considers the base of the neutral fats to be the oxide of a radical (C, II,) which he terms Lipyle. Glycerin is then formed from two equivalents of the oxide of lipyle, with three equi valents of water: 2 C, II, 0 + 3 II 0 =C„ 11, 0. If to this we add one equivalent of water, we obtain the usual formula.
According to this view, which is supported by Redtenbacher, Varrentrop, and Mnhier, the base of every neutral fat yielding glycerin is a compound which is represented by C, H, 0.
The most important of the fatty acids are :— Stearic Acid 0, + H 0 Margiuic Acid . . . C„ 11„ 0, + II 0 Oleic Acid C., 0, + 11 0 These are universally diffused in plants and animals ; and, combined with the oxide of lipyle (C, 11, 0), they form the neutral fats—stearin, margarin, and olein ; and this is the form in which they most commonly occur in the organic kingdom. Sometimes, however, a more powerful base (potash, soda, &c.) removes the oxide of lipyle, and there are then formed compounds of the fatty acids with alkalies.