It is chiefly in the subcutaneous that the organization of the adipose membrane has been investigated. The constituent vesicles or bags consist of firm, tenacious, ligamentous, gray, or whitish-gray coloured substance, mu tually united by means of delicate filamentous tissue. These vesicles or sacs receive arterial and venous branches, the arrangement of which has been described by various authors, from Malpighi, who gave the first accurate account, to Mascagni, to whom we are.indebted for the most recent. According to Malpighi,l' the bloodvessels divide into minute ramifications, to the extremities of which are attached the membranous sacs, containing the globules of fat-so as to bear some resemblance to the leaves attached to the footstalks of trees. These ve sicular or saccular arteries are afterwards di vided into more minute vessels, which then form upon the vesicular sacs a delicate vascular network. According to Mascagni, who repre sents these vessels in accurate delineations, the furrow or space between each packet con tains an artery and vein, which, being subdi vided, penetrates between minute grains or adi pose particles, of which the packet is composed, and furnishes each component granule with a small artery and vein. • The effect of this ar rangement is that each individual grain or adipose particle is supported by its artery and vein as by a footstalk or peduncle, and those of the same packet are kept together not only by contact, but by the community of ramifications from the same vessel. These grains are so closely attached that Mascagni, who examined them with a good lens, compares them to a cluster of fish-spawn. Grutzmacher found much the same arrangement in the grains and vesicles of the marrow of bones.* It has been supposed that the adipose tissue receives nervous filaments; and Mascagni con ceives he has demonstrated its lymphatics. Both points, however, are so problematical, that of neither of these tissues is the distribution known.
The substance contained in these vesicles is entirely inorganic. Always solid in the dead body, it has been represented as being fluid during life, by Winslow, Haller, Portal, Bichat, and most authors on anatomy. The last writer, indeed, states that under the skin it is more consistent, and that in various living animals he never found it so fluid as is represented. The truth is that in the human body, and in most mammiferous animals during life, the fat is neither fluid nor semifluid. It is simply soft, yielding, and compressible, with a slight degree of transparency, or rather translucence. This is easily established by observing it during incisions through the adipose membrane, either in the human body or in the lower animals.
The internal or sebaceous fat, however, espe cially that interposed between the fat of the serous membranes, is much more consistent and solid. The reason of these differences will be understood from what is now to be stated re garding the proximate principles of animal fat.
The microscopical and atomical structure of fat has recently formed the subject of investi gation by M. Raspail.t By placing a portion of lacerated fat upon a sieve, with an earthen vessel below it, and directing upon it a stream of water, numerous amylaceous granules are de tached and pass through the sieve, and after falling to the bottom of the water afterwards rise to the surface, in the form of a crystalline powder, as white as snow. When these par ticles are collected by scumming, and dried, they form a starchy powder, though soft and somewhat oleaginous to the touch, and which does not reflect the light in a manner so cry stalline as an amylaceous deposit does.
According to M. R,aspail, when examined microscopically, these granules present forms and dimensions varying in different animals, in the same animal and even in animals of dif ferent ages, but in all clearly resembling grains of fccula. In the humanl3ody these particles are polyhedral and not susceptible of isolation. As they are more fluid also than in other animals, it is necessary to immerse the portion subjected to examination in nitric acid or liquor potassce, either of which has the effect of consolidating the inclosed or central portion of each granule, and disintegrating the granules by the contraction of chemical agency. The borders of these granules appear by refracted light a little fringed—an effect which M. Ras pail attributes to the corrosive action of the nitric acid.
When magnified to 100 diameters, they ap pear like irregular hexaedral or pentaedral bodies, from two to four lines in diameter, and all accurately fitted or conjoined to each other. The actual diameter of these granules in the adult human subject varies according to Ras pail from .00117 to .00562 of an English inch. In youth and infancy they are stated to be still smaller.
The chief point to bear in remembrance is that the adipose tissue consists of two distinct parts, one a vital organic and secreting part, the other an inorganic and secreted product, which is void of vital principle. The chemical constitution of fat has been investigated by Chevreul, Braconnot, and more recently by M. Raspail. According to the researches of M. Chevreul fat consists essentially of two proximate principles, stearine sebum, sago,) and elaine, (eNatov, oleum.). The former is a solid substance, colourless, tasteless, and almost inodorous, soluble in alcohol, and pre serving its solidity at a temperature of 176° Fahrenheit. Elaine, on the contrary, though colourless, or at most of a yellow tint, and lighter than water, is fluid at a temperature of from 63° to 65° Fahrenheit, and is greatly more soluble in alcohol. To the presence of stearine in a large proportion, the intra-serous sebaceous fat owes its solidity and firmness; whereas the elasticity and softness of the sub cutaneous adipose tissue, and the marrow, depend upon the predominance of elaine.