STOMACH AND INTESTINE. 



401 



charged with the water, fat, albumen, salts, 

 and extractive, which it has taken up from 

 the food, and from the secretions of the diges- 

 tive organs, reaches a large gland. There it 

 breaks up, as it were, into two streams of fluid: 

 bile, and hepatic-venous blood. And hence, 

 the composition of these two fluid products, 

 compared with its own, might be expected 

 to give us a clue to the process by which they 

 originate, if not to the action of the secreting 

 structure itself. 



Such an examination would show that the 

 hepatic blood has lost almost all the fibrin, 

 half the albumen, much of the water, and 

 half the fat (even more of the elain) present 

 in the portal vein. It has gained in extrac- 

 tive, and especially (ten to sixteen times as 

 much) in sugar. And its pale corpuscles are 

 increased in number. 



On the other hand, the organic constituents 

 of the bile are chiefly fatty substances, espe- 

 cially the fatty cholic acid and its congeners. 

 The quantity and quality of most of these sub- 

 stances show that they have probably been 

 formed in the liver : and hence that their pre- 

 sence in the bi e is not to be explained as a 

 mere transudation of certain dissolved con- 

 stituents of the blood, followed by their con- 

 centration in the gland, such as might be 

 alleged in the case of most of its salts. 



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. 



DEVELOPMENT. 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." 

 Supp. 



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 laminae being bent down- 

 wards and inwards, so as to give rise to two 

 shallow pits or fossae: which are named the 

 fovea cardiaca, seu aditus ad intestinum ante- 

 rior ; and the foveola caudalis, seu adilus ad 

 intestinum posterior. These two fossae, 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 layers gives the canal two sides, the 

 laminae 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, 



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