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Fat-colls insinuate themselves between the primitive muscular fasciculi, and lie of course in stripes in the direction of the muscular fibres, which may remain unchanged. The development in this case has its origin in the interstitial tissue of the muscle. At the commencement of the

development, and when it proceeds with very great regularity, it may happen, that single rows of fat-cells lying one behind the other alternate with the rows of muscular elements. In this case, where the primitive fasciculi are forced asunder, and the circulation in the muscle is generally disturbed in consequence of the abundant development of fat, so that the flesh becomes pale—it looks to the naked eye as if there no longer existed any muscular tissue whatever. If, for example, in an inferior extremity, which in consequence of an anchylosis of the knee has remained unexercised, the gastrocnemii are examined, we find nothing but a yellowish mass exhibiting scarcely any striae and without any appearance of flesh, but upon a more minute examination it is discovered, that the primitive muscular fasciculi still pass, essentially unaltered, through the fat. In this case the fat forms an impediment to the use of the muscle, but the primitive fasciculi still exist and are to a certain extent capable of action. This process therefore is essentially different from necrobiosis, where the muscular fibres as such completely perish. Here we have a purely interstitial formation of adipose tissue, ordinary connective tissue becoming converted into adipose tissue,

Fig. 108. Interstitial growth of fat in muscle (fattening). //. Rows of interstitial fat-colls; M, m, m, primitive muscular fasciculi. 300 diameters.

TRANSITORY FATTY INFILTRATION. 3!i5

and the term, fatty degeneration, which is so very liable to be misunderstood, should be avoided.

This form occurs pretty frequently, especially in the heart, and may, when it attains a great extent, produce considerable derangement in the motor power of the muscular substance of this organ, but in pathological importance it stands far below real fatty metamorphosis, although this again in its outwardly visible results much resembles it. The hearts described by the old anatomists as fatty were in a great measure only hearts infiltrated with fat; on the other hand, what is meant at the present day when genuine fatty degeneration (metamorphosis) of the heart is spoken of, is not this obesity of the heart, this interlarding of its fibres with fat-cells, but rather a real transformation of its substance, going on in the interior of the fibres (Fig. 23). In the latter case the fat lies in, in the former between the primitive fasciculi.

The second series of processes consists in the transitory accumulation of fat in certain organs, as we meet with it in a typical form in digestion. When a fatty substance has been eaten, and has passed into the state of emulsion, we find that, when it has reached the upper end of the jejunum, and to some extent even in the duodenum, the villi of the mucous membrane become whitish, clouded and thick, and more minute examination shews, that they are filled with extremely minute granules, much more minute than can be produced by any artificial emulsion. These granules, which are found even in the chyme, come in the first instance into contact with the cylindrical epithelium with which every single intestinal villus is invested. On the surface of every epithelial cell we find, as was first discovered by Kolliker, a peculiar border which, when the cell is seen in profile, exhibits minute and fine striae ; when viewed from above, and seen upon the surface, the cell appears hexagonal and, as it were, dotted over with a number• of minute points (Comp. the epithelium of the gall-bladder, Fig. 14. and also Fig. 109, A). Kolliker has put forward the conjecture, that these fine striae and dots correspond to minute pore-canals, and that the absorption of the fat is effected by its minute particles being taken up through these minute pores upon the surface of the epithelial cells. But the object is one which is accessible only to the highest powers of our optical instruments, and it has therefore hitherto been impossible to obtain perfectly clear notions as to whether the striae really correspond to fine canals, or whether, as Briicke supposes, the truth is rather that the whole of this upper border is composed of little rods or

Fio. 109.

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pillars resembling cilia. I must confess that my own investigations also have rather disposed me to adopt this latter opinion, especially as comparative histology shows us real

Fig. 109. Intestinal villi, shewing the absorption of fat. A. Normal human intestinal villus from the jejunum; at a the cylindrical epithelium in part still investing it with the delicate border and nuclei; c, the central lacteal vessel; v, v, blood-vessels; in the rest of the parenchyma the nuclei of the connective and muscular tissue. B. Villas in a state of contraction, from a dog. C. Human intestinal villus during the absorption of chyle, D, in a case of retention of chyle; at the apex a large fat-drop, emerging from a crystalline envelope. 280 diameters

ABSORPTION OF FAT IN THE INTESTINES. 327

ciliated epithelium to be the equivalent structure in the same parts. At all events this much is certain, that, a short time after digestion has taken place, the fat no longer lies only outside, but is found also inside, the cells, and first at their outer end; then it gradually advances farther and farther inwards in the cells, and indeed so distinctly in rows, that it might easily give rise to the impression, that fine canals ran throughout the whole length of the cells themselves (Fig. 109, C, a). But this too is a question which will not, I think, with our present optical instruments, be so very speedily settled. At any rate, the plain fact remains, that the fat passes through the cells, and this indeed in such a way, that at first only their outer end is filled with it, then a time comes when they are quite full of fat, then a little later the outer part again becomes entirely free from it, whilst the inner still contains a little, until at last all the fat entirely vanishes from the cells. In this manner its gradual progress may be followed from hour to hour. After the fat has advanced as far as the inner extremity of the cells, it begins to pass into the so-called parenchyma of the villus (Fig. 109, C). Whether the epithelial cells have an orifice below, and whether, as has been quite recently maintained by Heidenhain junior, they are connected with extremely minute canals formed by the connective-tissue-corpuscles, is not quite decided, though it is very probable. It is extremely difficult to come to any definite conclusions with regard to these extremely minute arrangements of the substance of tissues. In the interior of the villi we generally find the network of blood-vessels a little below the surface (Fig. 109, A, v, v), whilst in its axis there is a tolerably wide canalicular cavity with a blunt extremity, the commencement of the lacteal vessel, as far as it can at present be determined with certainty (Fig. 109, A, c). At the periphery of the villi Brucke has discovered a layer of muscular fibres, which is of great importance in digestion, inasmuch as by its help an approximation of the apex of the villus to its base, a shortening, is effected, as may very readily be seen. Upon cutting off villi from the intestine of an animal just killed, they may be seen under the microscope to contract, become wrinkled, thicker and shorter (Fig. 109, B); thereby a pressure from without inwards is manifestly produced, which promotes the onward movement of the juices. 'So far the matter is tolerably clear, only what sort of a structure the rest of the parenchyma has, it is extremely difficult to see. Upon the outer side of the muscular layer, smallish nuclei are seen, which, as I pointed out many years ago, are now and then pretty distinctly enclosed in fine, cellular elements. But whether these parenchymatous cells anastomose with one another so as to form a special network, I am unable to say. During the process of absorption it looks as if the fat which keeps penetrating farther and farther into the interior of the villi, filled up the whole parenchyma.1 At last it reaches the central lacteal, and there the regular current of chyle begins.

The whole process therefore presupposes an emulsive condition of the fat, which penetrates through the parts everywhere in a state of extremely minute division; in the regular course of events the particles are so extremely minute that, if the chyle is examined when fresh and still warm, scarcely a trace of solid particles can be detected in it. But every disturbance which occurs in the process of absorption, and impedes the onward movement of the fatty particles, causes them to run together; larger granules separate in the tissues, drops appear which continually increase in volume, until at length they attain quite a large size. These are found even in the epithelial cells or within

1 I have quite recently convinced myself by the examination of transverse sections of villi, filled with chyle, in man, that the fat does not lie scattered in the parenchyma, but forms deposits in the interior of special minute cavities (cells?).—Note to the second edition.

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