In the fcetus, and for some time after birth, the fat is confined to the surface of the body, and is only found in a stratum under the skin. It begins, however, gra dually to be deposited in the intervals of the muscles, and on the surface of some viscera. In old subjects, however thin they may seem on an external view, there is always much fat, penetrating even the substance of the muscles : the hones are greasy throughout ; the heart is more or less lOaded, as are also the parts in the ab domen.
There is a considerable difference in the guantity of fat in different individuals : and in some there is a propensity or disposi tion to its accumulation ; a sedentary life, copious food, and tranquil state of the mind, are particularly favourable to the in crease of fat, which sometimes proceeds to ch a pitch, from the continuance of these causes, that it must be considered as a dis ease, and is attended with the greatest in convenience to the individual..., General diseases of the frame are commonly at tended with an absorption of the fat from the cellular substance : acute disorders cause a very rapid emaciation. In no case is the adipous substance more completely removed from the whole body than in anasarca where its place is supplied by a serous duid.
The uses of the fat seem to he, in part, common to it with the cellular substance: it connects contiguous parts, and at the same time prevents their coalition. It ad mita of their moving on each other with Ire &loin and facility. Its deposition under the integuments gii•es a roundness and convexity to the sw-face, on which the beauty of the human form principally de pends. Indeed, its accumulation in par ticular situations immediately influences the outline of the part; as in the orbit, the cheek, and the buttocks. The ef fects of its loss is most disagreeably mani fested in the lank cheek and hollow eye of an emaciated patient.
It has been supposed that the fat ab sorbed under certain circumstances is applied to the nutrition of die body ; as in hybernating animals.
Jfembranes.—In the foregoing observa tions on cellular substance, we have stated that membranes are formed by a conden sation of that substance. They consist of thin sheets of compacted and close cellu lar texture. This is proved by long ma ceration in water. The fluid gradually penetrates the interstices, and resolves the membrane into a loose and flocculent substance. They are found in every va riety of density and softness.
A grand use of membranes is, to line what anatomists call the circumscribed ca vities of the body. These are hollow
spaces, containing the different viscera, and in every instance completely and ac curately tilled by such viscera ; so that the term cavity, when used by anato mists, does not, as in common language, denote a void or empty space.
Membranes have a smooth internal po lished surface, turned towards the con tained viscera. This is constantly moist ened by a lubricating fluid exhaled by the minute arteries of the part, and bestows on the surface of the membrane the greatest softness and smoothness. Hence the motions of the viscera arc performed with perfect facility, and they are pre vented from adhering to each other, or to the sides of the containing cavity. The ex tent of such cavities is bounded and de fined by the lining membranes, and hence arises the epithet circumscribed. To in crease the facility of motion, the surface of the contained viscera is covered by a continuation of the same membrane, and always therefore possesses the same smoothness and polish with the sides of the cavity. The membrane lining a cir cumscribed cavity is a complete and en tire sac, which is reflected over all the vis cera contained in tire cavity. In the carcase of an animal just 'Slaughtered, the lubrica ting secretion flies off in the form of a fine vapour when the cavity of the belly or chest is laid open. It is nothing more than an increase of this natural secretion, combined perhaps with a deficient ab sorption, that gives rise to dropsies of the different cavities.
The opposite or external surface of the membrane is rough and cellular ; and ad heres to the various parts which form the sides of the cavity.
Another use of membranes is, to form blood-vessels or tubes for conveying the nutritious fluid to all parts of the body. The bore or hollow of the tube is per fectly smooth and polished, so that the blood experiences no obstruction in its course ; and die external surface is rough, to connect it with the surrounding parts. In a similar mariner are formed the sta. mach and intestines, which receive the food; the urinary bladder, which holds the urine, &c.
It must be obvious, that for all the pur poses which we have enumerated, whe ther for lining circumscribed cavities, for conveying the blood, for receiving the food, or holding any other liquors, it is essentially necessary that membranes should be impermeable to fluids in the hying state.