SECRETION. 



447 



viewed by the naked eye, as in the flesh-fly, 

 Musca carnaria (fg. 31 1. a, 6) ; and generally 



Fig. 311. 



Biliary Organs of Musca carnaria. 



a, portion of a trunk and two branches of one of 

 the biliary tubes of the flesh -fly, viewed by re- 

 flected light, and magnified eight diameters ; b, 

 portion of a biliary tube of the flesh-fly highly 

 magnified, exhibiting the arrangement of the se- 

 creting cells, and the mode of distribution of the 

 trachea? ; c, a secreting cell from the liver of the 

 flesh-fly, very highly magnified. (After Leidy.) 



towards the free extremities the sides of the 

 tubes are so irregular, that they appear as if 

 merely folded upon the secreting cells to keep 

 them together. The secreting cells are round, 

 oval, or nearly cylindrical from elongation. 

 Their average measurement is about '09 

 millim. The contents are white, yellowish, 

 or brownish, and consist of a finely granular 

 matter, numerous fine oil globules, a granular 

 nucleus, and a transparent nucleolus. The 

 cells in the extremity of the tubes are not 

 more than half the size of those a little further 

 on (or nearer the termination), and contain 

 less granular matter and no oil globules, so 

 that they are more distinct, and the nucleus 

 more apparent. Upon advancing a very little, 

 the cells are found to be of an increased size, 

 and full of granular matter, so as considerably 

 to obscure the nucleus from view. A little 

 further, we find the addition of fine oil 

 globules, readily distinguishable by their thick, 

 black outline when viewed in a certain focus. 

 Sometimes the cells become so filled with oil, 

 as to be distended with it, rendering the 

 granular matter and nucleus so transparent as 

 totally to destroy all appearance of the for- 

 mer, and the latter only is to be perceived in 

 faint outline. Such a state I have frequently 

 observed in Dcrmestcs, Ateuc/nis, &c. The 

 nucleus (fg. 31 I. c) is generally central, glo- 



bular, and pretty uniform in size in the same 

 species, averaging in measurement about -025 

 millim. The nucleolus is always transparent, 

 and measures about '006 millim. The cen- 

 tral passage of the tubes, or separation of the 

 cells in the middle line, is usually found filled 

 with fine granules, and a great amount of oil 

 globules. The biliary tubes of insects are 

 bathed in blood, or the nutritive fluid, and the 

 respiratory tracheae are distributed to them 

 with extreme minuteness, but are separated 

 from the secreting cells by the intervention of 

 the basement membrane."* According to 

 Dr. T. Williams (op. cit.), some at least of 

 the large cells which give the sacculated ap- 

 pearance to a biliary tubulus are really parent 

 cells, filled with a second generation of hepa- 

 tic cells ; they are, therefore, analogous to fol- 

 licles, save that they have no proper outlet, 

 for we shall hereafter see that the follicle in 

 its earliest condition is probably nothing else 

 than a parent cell. From the above descrip- 

 tion, it would appear that the hepatic cells 

 originate towards the upper or caecal end of 

 the tubulus, that they are gradually being 

 pushed onwards towards the outlet by the 

 growth of new generations behind them ; and 

 that, as they thus advance, they acquire an 

 increase in size by their own inherent powers 

 of development, at the same time drawing 

 into themselves the peculiar matters which 

 they are destined to eliminate from the circu- 

 lating fluids. The cells, having attained their 

 full growth, and completed their term of life, 

 give up their contents by the rupture or deli- 

 quescence of their walls, and these pass down 

 the central cavity of the tube, to be discharged 

 into the alimentary canal. 



In the higher Crustacea we find a condition 

 of biliary structure much more closely allied 

 to that of Mollusca than to that of Insects ; 

 the liver being a pair of massive tabulated 

 bodies, each of them made up by the aggre- 

 gation of numerous ceecal follicles, from every 

 one of which passes off a narrow duct, to join 

 a trunk that is common to all the vesicles on 

 one side. " In structure," says Dr. Leidy 

 (loc. cit.), "the caeca resemble the tubes of 

 insects, being composed of a sac of basement 

 membrane, within which, originating from 

 the inner surface, are numerous secreting 

 cells (fig. 312. a). The cells are more or less 

 polygonal in form, from mutual pressure. At 

 the bottom of the caeca the cells are small, 

 with an average diameter of 02 millim., and 

 contain a finely granular matter of yellowish 

 hue, with a granular nucleus, and a trans- 

 parent nucleolus. As we proceed from the 

 bottom upwards, the cells (/, e, rf, c, b) are 

 found to increase in size, and to obtain a gra- 

 dual addition of oil globule's, until beyond 

 the middle of the tube, where they are found 

 filled with oil, so as to have the appearance of 

 ordinary fat cells, and have a diameter avera- 

 ging -06 millim. From this arrangement of 

 the cells, when a caecum is viewed beneath 

 the microscope, its lower half appears filled 



* Leidy, in American Journal of the Medical 

 Sciences for Jan. 1848. 



