THE HISTORY OF FAT. ADIPOSE TISSUE. 473 



The fat in the interior of bones forming the yellow marrow appears to 

 have the same general structure and to be formed in the same way as the 

 rest of the adipose tissue. 



418. The fat thus deposited in a fat-cell sooner or later disappears. It 

 is not injected bodily into the surrounding lymph-spaces of the connective 

 tissue, but passes away either into the blood stream or into the lymphatics 

 by some processes not as yet fully understood. The shell of cell substance 

 which forms the envelope of the fat-cell is probably of a differentiated nature, 

 and may have properties which assist the escape of the fat ; but on this 

 point we have no exact knowledge. The disappearance of the fat appears 

 to take place in two different ways. On the one hand, and this perhaps is 

 the more ordinary method, the fat gradually disappears, little by little, and 

 the rounded distended vesicle gradually assumes the characters of a connec- 

 tive-tissue corpuscle, even of a branched one. On the other hand, especially 

 when the disappearance is rapid and total, the space previously occupied by 

 fat becomes filled with a clear fluid resembling lymph, the fat vesicle being 

 transformed into a lymph vesicle. This condition, however, is temporary 

 only, the lymph is subsequently absorbed and the vesicle shrinks. At times 

 the emptying of the cell, whether by the one method or the other, is followed 

 by a rejuvenescence of the cell, the nucleus by division gives rise to several 

 nuclei, and the cell divides into new cells, each of which may, under appro- 

 priate conditions, develop again into a fat-cell. 



419. The fat thus lodged in adipose tissue varies somewhat in composi- 

 tion in various animals, but is chiefly composed of olein, palmitin, and stearin 

 in varying proportions, with small quantities of the glycerin compounds of 

 such fatty acids as butyric, capronic, caprylic, etc., together with a little 

 lecithin and cholesterin. The " fat " of one animal, that is, the fat thus con- 

 tained in adipose tissue, differs from the fat of another animal partly by the 

 presence of more or less of one or more of these less abundant fats, but 

 chiefly by the proportion in which the three main fats, olein, palmitin, and 

 stearin, are respectively present in the mixed fat. The melting-points of 

 these three fats being different, the melting-point of the fat of the body will 

 differ according to the relative proportions in which the three are present. 

 Thus the subcutaneous fat of man melts at from 15 to 22 or higher, the 

 fat round the kidney being firmer and not melting until 25 ; the fat of the 

 dog melts at about 22, that of the goose at about 25, of the ox at about 

 40, and of the sheep at 50, the less resistant fat of the man and dog con- 

 taining relatively more olein than that of the ox or of the sheep. 



420. When we come to consider the question, By what processes does 

 the fat make its appearance in the fat-cell? we are brought face to face with 

 much the same kind of problem as that which occupied us in dealing with 

 glycogen. On the one hand we may suppose that the fat is brought to the 

 fat-cell as fat and is in some way taken up by the cell and deposited in the 

 cell substance with little or no change. On the other hand, we may suppose 

 that the fat is manufactured by the fat-cell, in some such way as mucin or 

 pepsin is manufactured by a mucous or a gastric cell, out of and by means 

 of its cell substance, and that the process of fattening, or of producing fat in 

 fat-cells, consists essentially in feeding and so building up the cell substance 

 which subsequently breaks down into fat, and does not consist merely in 

 bringing fat within reach of the cell. Which of these views is the true one, 

 or how far are both these operations carried on in the animal body? 



In support of the latter view it may be urged that, not only the more 

 complex living substance, but, as we have more than once urged, the simpler 

 proteid constituent of living substance, obviously contains what we may call 

 a fatty radicle, so that we might expect fat to be formed out of its metah- 



