FAT 



1045 



swim freely about in water. If the thin fluid that is copiously dis- 

 charged from the nose in the first stage of an ordinary ' cold in the 

 head ' be subjected to microscopic examination, it will commonly 

 be found to contain a great number of ciliated epithelium-cells, 

 which have been thrown off from the lining membrane of the nasal 

 passages. 



Fat. One of the best examples which the bodies of higher 

 animals afford, of a tissue composed of an aggregation of cells, is 

 presented by fat, the cells of which are distinguished by their power 

 of drawing into themselves oleaginous matter from the blood. Fat- 

 cells are sometimes dispersed in the interspaces of areolar tissue ; 

 whilst in other cases they are aggregated in distinct masses, con- 

 stituting the propei- adipose substance. The individual fat-cells 

 always present a nearly spherical or spheroidal form ; sometimes, 

 however, when they are closely pressed together, they become some- 

 what polyhedral, from the flattening of their 

 walls against each other (fig. 780). Their 

 intervals are traversed by a minute network 

 of blood-vessels (fig. 795), from which they 

 derive their secretion ; and it is probably 

 by the constant moistening of their walls 

 with a watery fluid, that their contents are 

 retained without the least transudation, 

 although these are quite fluid at the tem- 

 perature of the living body. Fat-cells, when 

 tilled with their characteristic contents, have 

 the peculiar appearance which has been 

 already described as appertaining to oil- 

 globules, being very bright in their centre. 



and very dark towards their margin, in FIG. 780. Areolar and adi- 

 consequence of their high refractive power; 

 but if, as often happens in preparations that 

 have been long mounted, the oily contents 



should have escaped, they then look like any oilier cells of the same 

 form. Although the fatty matter which fills these cells (consisting 

 of a solution of stearine or margarine in oleine) is liquid at the 

 ordinary temperature of the body of a warm-blooded animal, yet its 

 harder portion sometimes crystallises on cooling, the crystals shoot- 

 ing from a ceiiti-e, so 'as to form a star-shaped cluster. Osmic acid 

 has been found by Dr. B. Solger to separate a more fluid central 

 portion from a firmer peripheral part. In examining the structure 

 of adipose tissue it is desirable, where practicable, to have recourse 

 to some specimen in which the fat-cells lie in single layers, and in 

 which they can be observed without disturbing or laying them 

 open ; such a condition is found, for example, in the mesentery of 

 the mouse ; and it is also occasionally met with in the fat-deposits 

 which present themselves at intervals in the connective tissiies of the 

 muscles, joints, &c. Small collections of fat-cells exist in the deeper 

 layers of the true skin, and are brought into view by vertical 

 sections of it (fig. 775, /). And the structure of large masses of fat 

 may be examined by thin sections, these being placed under water 



pose tissue: a a, t';i! fells; 

 l> b, tibres of areolar 

 tissue. 



