608 



VERTEBRATED ANIMALS. 



although these are quite fluid at the temperature of the living 

 body. Fat-cells, when filled with their characteristic contents, 

 have the peculiar appearance which has been 

 FIG. 3-22. already described as appertaining to oil- 



globules ( 99), being very bright in their 

 centre, and very dark towards their margin, 

 in consequence of their high refractive 

 power ; but if, as often happens in prepara- 

 tions that have been long mounted, the oily 

 contents should have escaped, they then 

 look like any other cells of the same form. 

 Although the fatty matter which fills these 

 cells (consisting of a mixture of stearine or 

 of margarine with oleine) is liquid at the 

 ordinary temperature of the body of a warm- 

 blooded animal, yet its harder portion some- 

 times crystallizes on cooling; the crystals 

 shooting from a centre, so as to form a star- 

 ?ireoiar tissue. shaped cluster. 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 with- 

 out disturbing or laying them open ; such a condition is found, 

 for example, in the mesentery of the mouse, and it is also occa- 

 sionally met with in the fat deposits which present themselves at 

 intervals in the connective tissues of the muscles, joints, &c. 

 Small collections of fat-cells are found in the deeper layers of the 

 true skin, and may be brought into view by vertical sections ot 

 it. And the structure of large masses of fat may be examined 

 by thin sections, these being placed under water in thin cells, so 

 as to take off the pressure of the thin glass from their surface, 

 which would cause the escape of the oil particles. No method 

 of mounting (so far as the Author is aware) is successful in 

 causing these cells permanently to retain their contents. 



423. Cartilage. In the ordinary forms of Cartilage, also, we 

 have an example of a tissue ssentially composed of cells ; but 

 these are commonly separated from each other by an intercel- 

 lular substance, the thickness of which differs greatly in different 

 kinds of cartilage, and even in different stages of the growth ot 

 any one. Thus in the cartilage of the external ear of a Bat or 

 Mouse (Fig. 323), the cells are packed as closely together as are 

 those of an ordinary vegetable parenchyma (Fig. 150, A); and 

 this seems to be the early condition of most cartilages that are 

 afterwards to present a different aspect. In the ordinary carti-. 

 lages, however, that cover the extremities of the bones, so as to 

 form smooth surfaces for the working of the joints, the amount 

 of intercellular substance is usually considerable ; and the carti- 

 lage-cells are commonly found imbedded in this, in clusters of 

 two, three, or four (Fig. 324), which are evidently formed by 



