Adipose tissue, according to Toldt, must not be classed with the connective tissue either by its histological structure or by its func tions. Its embryological development, too, does not proceed from the connective tissue but from definite germ layers, and in its further growth is connected with the preformed adipose tissue.
According to a widely differing assumption by Virchow and Flem ming, there is no special difference between adipose and loose connec tive tissue, and the fat cells are said to be identical with the fixed connective-tissue cells. The connective-tissue cells and other cells may change into fat cells by the absorption of fat if they are favorably situated in relation to the blood current so as to take up the lipoge nous material transuding more or less freely from the blood after the ingestion of food. The transudation from the vessel wall occurs in greater amount wherever there is a considerable retardation of the circulation by reason of a circumscribed dilatation of smaller vessels.
According to both theories, intimate relations exist between adi pose tissue and the vascular apparatus, and a deposit of fat occurs only where there is a rich vascularization and retardation of the blood current, while tissues containing few vessels do not become filled with fat. The amount of the deposition of fat, however, rests withal upon conditions which vary with the individual and the time. Under the assumption of a specific adipose tissue (Toldt), copious nutrition, slight tropho-plastic activity of the tissues, and deficiency of oxygen of the blood do not suffice for the accumulation of fat in the body ; but the secretory energy of the adipose tissue, its quantity, and its power of proliferation enter largely into the consideration and there fore will differ in amount in various individuals and at different times. The amount of the accumulation of fat therefore may show extensive variations under otherwise equal conditions in a concrete case.
If we do not consider the adipose tissue as a special tissue forma tion, then the amount of the deposition of fat is based upon the vaso motor innervation and the irritability of the vessel wall dependent upon the more frequent or rarer occurrence of a local vascular dila tation in the loose connective tissue, and is influenced moreover by the number of cells in the adventitia and the power of proliferation of its cellular elements, which may show more or less individual variations. On the other hand, Toldt and his followers do not doubt that the con nective-tissue cells produce fat under certain circumstances and accu mulate it in their interior in larger or smaller drops. In advancing obesity, according to Toldt the preformed adipose tissue from defi nite germ layers will increase in extent and thickness, and at the same time more and more connective-tissue cells are changed into fat cells ; while according to Flemming the deposition of fat takes place in the connective-tissue cells.
Recently Unna has advanced another theory of the formation of fat and the adipose tissue.
Unna assumes three sites for the formation of fat, the first of which is said to be in the skin, the second in the muscles, and the third in the intestine. In the skin the fat is produced mainly by the activity of the convoluted glands. The kidney fat he interprets as muscle fat, the fatty lymph current being filtered through the veins of the capsule of the kidney. The source of the intestinal fat is said to be situated in the mesentery where the fat is formed by the co operation of certain blood corpuscles. Basing his argument on these results of his investigations, Unna has attempted to explain why one and the same method of forced nutrition and of fat-reduction cannot have the same effect on all the deposits of fat.
How far the facts will bear out Unna's views, which would be likely to advance materially our knowledge of the pathogenesis and the course of obesity, must be settled by future investigations.
Hence the pathogenesis of obesity is by no means as yet fully elucidated by the histological and physiological examinations bearing upon it. The frequent early development, the course, the rapid or tedious involution of corpulence, especially by therapeutic measures, present absolutely unexplained phenomena observed not only in dif ferent cases but also at different times in a concrete case. Most in complete, however, is our knowledge of the various factors which in duce obesity, if we endeavor to trace its etiology and particularly to answer the question about the disposition to an extensive accumula tion of fat or to obesity.
The decision whether the fat accumulated in the body has exceeded the normal limit, whether it has led to corpulence, obesity (lipomatosis universalis, adipositas, obesitas, polysarcia, corpulentia morbosa), and occurs as a pathological phenomenon or a disease sui genenis, will depend upon the following two criteria : 1. Whether the external proportional somatic relations and the symmetry of the limbs are disturbed by an unequal increase of vol ume in consequence of excessive deposition of fat; and 2. Whether a limitation of the functional activity of the various organs, especially of the circulatory and respiratory organs, is caused by an excessive accumulation of fat.
According to this limitation of the conception of increased fat for mation and deposition, therefore, the abnormal development of fat such as occurs in fatty degeneration of the tissue elements and in processes leading to atrophy, is completely excluded and relegated to another chapter in pathology.