In each of these situations it varies in quan tity and physical properties. In the least cor pulent persons a portion of fat is deposited in the adipose membrane of the cheeks, orbits, palms of the hands, soles of the feet, pulp of the fingers and toes, flexures of the joints, round the kidney, beneath the cardiac serous membrane, and between the layers of the me sentery and omentum. In the more corpulent, and chiefly in females, it is found not merely in these situations, but extended in a layer of some thickness, almost uniformly over the whole person; but is very abundant in the neck, breasts, belly, mons Veneris, and flexures of the joints.
It has been long observed that the subcu taneous adipose layer presents considerable differences from the adipose matter found be tween the folds of the serous membranes ; and the older anatomists, aware of these differences, distinguished the former by the name of pin guedo, and the latter by that of sebum. The subcutaneous adipose membrane is, when viewed as a whole, more elastic, softer, and less granular than the omental fat, and evi dently presents the arrangement of vesicular bags much more distinctly than the omental. It is in the subcutaneous adipose membrane indeed, almost exclusively, that the vesicular arrangement can be recognized. The subcu taneous cellular fat also contains a greater quantity of oil than the omental, which abounds chiefly in firm, brittle, granular fat.
The situation where the vesicular structure of the adipose membrane is most easily de monstrated is in the hips between the skin and the gluteal muscles, and at the flexures of the joints generally. In the former situation especially, the constituent fibres of the vesi .cular bags are tough, firm, and ligamentous, and the bags themselves are large and distinct.
It is a remarkable anatomical character of the sebaceous or tallow-like fat that its distri bution is confined chiefly to the external or commutual surfaces of several of the serous membranes ; and this arrangement presents a series of interesting anatomical analogies. Thus sebaceous fat is found on the external surface of the pleura costalis, between it and the inter costal muscles, and between the layers at the posterior and anterior mediastinum. It is also found between the cardiac pericardium and the muscular substance of the heart, especially around the vessels of the organ. In some of the large mammalia even this circumstance is connected with peculiar anatomical appear ances. Thus, in the heart of the dolphin ( del phinus tursio ) we find the cardiac pericardium formed into broad prominent fringes, consisting each of two folds of the membrane, between which is interposed a considerable quantity of sebaceous fat. In the same manner the several
omenta, or peritoneal duplicatures in the abdo men, may be recognized as analogous fringes containing more or less sebaceous fat; and the omental appendages (appendices epiploice e ) of the colon must be regarded as examples of the same arrangement. Lastly, in the interior of the articular capsules we find the synovial membranes forming large prominent fringes, which, if immersed in water, show to what extent they are made to recede from the cap sule and bone, and forming cavities of dupli cation in which sebaceous matter is contained.
It thus appears that none of the serous mem branes is exactly applied either to the parietes of cavities or to the surface of the contained organs, but that they form intervals on their outer or attached surfaces, on which various quantities of sebaceous fat are deposited. In all these substances we do not recognize the same distinct arrangement of an appropriatd organ, but simply masses of adipose, or rather sebaceous matter, interposed between the at tached surface of the serous membranes and the adjoining or the contained organs.
Fat occurs in a third modification in the marrow of bones. The adipose granules, which are soft, whitish-yellow, and oleaginous, are here contained in a peculiar membrano-cellular web, forming numerous vesicles, which may be regarded as an ultra-osseous adipose tissue. It is a remarkable proof of the influence of the vital principle that during life the substance of the bones is never tinged with this animal oil, but the moment life is extinct, the marrow begins to transude and impart to the bones a yellow tint and a greasy aspect.
Fat, though chiefly observed to occur in the bodies of animals, is nevertheless not confined solely to these bodies. Thus not only do va rious kinds of oil and consistent oleaginous matter occur in certain vegetables, but sub stances similar even to tallow are found in some vegetable productions. A sort of tallow is obtained from the vateria Indica, a forest-tree of the camphor family, indigenous in the Indian Archipelago. In a species of croton indigenous in China, namely, the croton sebiferum of Linnaeus, the stillingia of Mi chaux, or tallow-tree, the seeds are covered with a quantity of fat, bearing so close a re semblance in all its properties to tallow, that it is used by the Chinese in the manufacture of candles ; and the fruits of the aleurites triloba, a native of the Sandwich Islands, of the same natural family with the croton, are the candle nuts of the inhabitants of these remote regions.