But here for the present we rest. Sugar on the one hand, and certain fatty acids on the other, appear to be formed in the liver ; at the expense of fat, albumen, and fibrin. Until accurate quantitative researches establish whether the disappearance of the protein compounds is sufficiently accounted for by the total increase of extractive and of pale corpuscles in the bile and hepatic vein, the exact source of these substances must re main a mystery. Schmidt, indeed, suggests, that the fat of the portal blood is decomposed in the liver into the sugar and cholic acid which its elements would exactly make up. But while we are justified in giving every consideration to a view which seems so con sonant with the facts hitherto known, we must be careful to remember that it is on these facts, and not on the neatness of any formula, that its value entirely depends. Un supported by them, it would be a mere ar rangement of certain letters and figures, de void of all real significance, and destined to the oblivion to which thousands of its predecessors in the literature — not the science — of chemistry are daily being con signed.
— The development of the alimentary canal, like that of other or gans, offers a series of complicated changes, the details of which often have but little visible or direct relation with the future function of the part. Hence any minute description of the process would be quite out of place in this essay. The author therefore limits himself to a brief sketch of its general outline ; and for all further details begs to refer the reader to the article " Ovum." Just as the completely developed intestinal tube might almost be described as the involu tion of an extremely vascular cell-growth, so its origin distinctly refers it to those two germinal layers of the embryo from which such mucous and vascular structures are re spectively derived. The centre of the early ovum consists of three layers ; the upper or serous, the middle or vascular, and the under or mucous, lamina. A portion of each of the two latter is folded inwards, to form the rudiment of the alimentary canal. And the whole history of the subsequent development of this tube is little more than a recital of the various steps and processes, by which these mucous and vascular structures are so arranged as to result in the characteristic form, the nu merous segments, and the complex structure, which have been briefly described in the fore going pages.
The formation of the tube begins by the separation of the united vascular and mucous layers from the serous lamina immediately above them. An increase of this separation prolongs their attachment to the serous layer into a simple and rudimentary mesentery. Each end of the canal is then mapped out, by the conjoined laminx being bent down wards and inwards, so as to give rise to two shallow pits or fossm: which are named the fovea cardiaca, seu aditus ad intestinum ante rior ; and the foveola caudalis, sets aditus ad intestinum posterior. These two fossm, how
ever, do not correspond to the future mouth and anus; but to the cardiac aperture of the stomach, and to the middle segment of the rectum respectively. And between them, a lateral inflection of the conjoined mucous and vascular lajers gives the canal two sides, the lamintE intestinales ; which, like the similar vertebral plates of the serous layer, bound a shallow groove. This groove, the fissura intestinalis, is rapidly converted into a tube, by the closing in of its inferior or open surface. The process of closure begins at each ex tremity of the groove, and runs rapidly to wards its centre ; but is arrested here, so as to leave an opening or umbilicus, by means of which the intestine is connected with the umbilical vesicle that replaces the vitelline membrane and yolk. But there does not seem to be any direct continuity of the vitel line and intestinal cavities with each other through the channel formed by this umbili cal ("omphalo-enteric") duct :—at least not such an aperture as to allow of the yolk itself being immediately received into the intestine. As the umbilical vesicle gradually removes from the intestine, this duct undergoes a cor responding elongation. Its canal becomes obliterated prior to the degeneration and dis appearance of the tube itself.
The simple straight cylindrical canal, the development of which has thus been traced out, resembles the permanent intestinal tube of many of the lower animals ; with the ex ception that, as above stated, it is deficient in both terminal segments. These it next ac quires. And at the same time that it does so, it assumes the length, form, and convolutions, proper to the perfect intestinal tube.
As it already occupies the whole length of the abdominal cavity, any elongation of the canal will of course give it a curved shape. And since, at this period of fcetal life, the abdomen opens by a wide vertical fissure in the situation of the future umbilicus, the first bend of the intestine renders it convex forwards, and then protrudes it through this aperture. Here it adjoins the base of the umbilical duct ; which opens into the point or angle of this convexity, so that the bowel appears like a bifurcation of the duct itself. The two forks of this bifurcation are soon produced into a spiral coil of intestine ; which still lies outside the abdominal cavity, and only recedes into it at about the middle of the third month of uterine life.