In the Mammalia, the liver is more or less distinctly divisible into minute lobules, each of them composed of a parenchyma of hepatic cells, through which the blood-vessels are dis tributed in a close and solid plexus. The hepatic cells appear to occupy the entire space left in the meshes of this plexus, the bile ducts having been usually regarded as not traceable, under any form, into the interior of the lobule. Mr. Kiernan, however, has always regarded the bile-ducts as forming a plexus in the substance of the lobule, interlacing with the plexus of capillaries ; his belief being chiefly founded on the anastomotic distribu and biliary organs are so intimately connected in this genus, that it is not easy to point out tion of the bile-ducts in the left lateral liga ment, which Ire considers as in itself a rudi mental liver, exhibiting the structure of the entire organ in its most simple form. By Nit-. Bowman it was supposed, that the limitary membrane forming the wall of the minute biliary ducts is not continued into the sub stance of the lobule, but that the epithelial lining of the ducts is continuous with the mass of hepatic cells which forms its parenchyma. There is an a priori improbability in such an idea, which would leave the glandular cells in immediate "contact with the surface of the blood-vessels ; an arrangement which does not exist in any other gland. We have been accustomed, therefore, to accord with the opinion of Dr. Thomas Williams*, that the limitary membrane of the bile-ducts is pro bably expanded over the whole of the paren chymatous portion of each lobule, moulding itself upon, and identifying itself with, the capsule or sheath of the vessels, and thus forming a sort of irregularly reticulated cavity, which may be described as the whole space occupied by the lobule, minus the series of passages containing the capillary plexus. The manner in which the lining membrane of the uterine sinuses with the cellular decidua are prolonged into the placenta, and reflected over the capillary tufts of the fcetal vessels, so as to divide the whole cavity of the placenta into a series of irregularly shaped chambers, freely communicating with each other, into vvhich the maternal blood is conveyed, will convey an idea of this method of viewing the disposition of parts in the liver ; the uterine sinuses representing thebile-ducts ; the cellated cavities of the placenta, corresponding with the spaces occupied by the cells of the hepatic parenchyma ; and the fcetal vessels occupying the place of the capillary plexus from which the secretion is formed.
The observations recently published by Dr. Leidy harmonise precisely with the view pro mulgated by Mr. Kiernan, and seem to confirm the idea that here, as elsewhere, the hepatic cells are enclosed in a limitary membrane. " The lobules are composed of an intertextnre of biliary tubes (fig. 317.); and in the inter spaces of the network the blood-vessels ramify and form among themselves an intricate anastomosis, the whole being intimately- con nected together by a combination of the white fibrous and the yellow elastic tissue. In structure, the biliary tubes (figs.:318,319.) respond with those of Invertebrata, consisting of cylinders of basement membrane, containing numerous secreting cells, and the only differ ence exists in the arrangement, the free tubes of the lower animals becoming anasto mosed on forming an intertexture in the Ver tebrata. The tubuli vary in size in an un
important degree in different animals,and also in the same animal, being generally from two to two and a half times the diameter of the secreting cells. The tubes of one lobule are distinct from those of the neighbouring lobuli, or only communicate indirectly by means of the trunks or hepatic ducts, originating from the tubes, and lying in the interspaces of the lobuli. The secreting cells (fig. 320.) are irre gularly angular or polygonal in form, from mutual pressure, and line the interior surface of the tubes. They vary in size in a moderate degree in different animals, and also in the same animal, appearing to depend upon cer tain conditions of the animal and liver."* We have ourselves verified these important observations to a certain point, not having been able to obtain a view of the regular and complete plexus of' ducts figured and described by Dr. Leidy, but having satisfied ourselves that a system of canals, prolonged from the bile ducts, exists in each lobule. The recently published observations of Dr.Natalis Guillott are to the same effect. He has not been able, any more than ourselves, to distinguish mem branous parietes around these canals ; and he considers that they are simply channelled out in the parenchyma of the liver, the particles of which form its sole borders. It appears probable to us, however, that these canals correspond to the spaces left in the centre of the binary tubuli of insects, &c.; and that the membranous walls, if they exist at all, would be found to invest the cells which immediately bound these passages.
The binary cells of the Mammalia (fig.320.) usually contain a certain number- of adipose particles ; their size and number, however, vary considerably according to the food of the animal, the amount of exercise which it has been taking, and other circumstances. If an animal be very fat or well fed, especi ally with farinaceous or oleaginous substances, the proportion of adipose particles is much greater than in an animal moderately fed and taking much exercise. The size of the glo bules varies from that of mere points, scarcely distinguishable from the granular contents of the ceils but by their intense blackness, up to one-fourth of the diaimeter of the cell. The finely granular matter is the portion from which the colour of the cell is derived ; it seems to fill the space not occupied by the oil globules ; and it often obscures the nu cleus,so that the latter cannot be distinguished until acetic acid is added, which makes the granular matter more transparent without affecting the nucleus.
The following are the dimensions of the hepatic cells in various animals, according to the measurements of Dr. Leidy (loc. cit.).
When the foregoing facts are duly weighed, the conclusion seems irresistible, that the cells containing the biliary matter are the only in variable constituents of the hepatic appa ratus ; and that the manner in which these cells are arranged, and brought into relation with the blood-vessels, may vary indefinitely without producing any change in the charac ter of the product. Consequently we cannot but look upon the biliary cells as the essential portion of' the secreting structure ; and we must, in like manner, consider their agency as the essential part of the secretory function.