STOMACH AND INTESTINE. 



307 



7. Finally, we may close these vague con- 

 jectures by attempting to include, in one for- 

 mula, most of the varieties seen in the whole 

 animal kingdom. The complexity of the di- 

 gestive apparatus varies with that of the 

 digestive function. And this is again the 

 product of two chief elements : the kind of 

 food used; the nature of the animal to be 

 nourished. 



In respect to the food, we might almost 

 form a scale of decreasing simplicity, begin- 

 ning with the rich chylous fluid that bathes 

 the intestinal parasite, and passing through 

 the various gradations of liquor sanguinis, 

 blood, flesh more or less decomposed, vege- 

 table juices, fruits, vegetables, and grains: 

 gradations which, however increased in 

 number and minuteness, would all find their 

 corresponding representatives in Natural His- 

 tory. And we have already seen that, through- 

 out the Vertebrate series, there is a constant 

 association of a long intestine or a compli- 

 cated stomach, with a vegetable diet. 



As regards the nature of the animal, the 

 Acalephan, Crustacean, Cephalopod, Fish, 

 Bird, Cetacean, all prey upon fish. Yet not 

 only are their organs of digestion most di- 

 verse, but they even exhibit a certain corre- 

 spondence with the general development of 

 each animal. Nor is it difficult to imagine why 

 this is the case. Looking only at the unity 

 of the organism, we might a priori expect, 

 that a high development of the whole would 

 imply an equal advance in the complexity of 

 its chief parts. To this we may add, that 

 one organ seems in a certain sense comple- 

 mentary to another, the necessary, and not 

 merely the formal, result of an increased 

 evolution of its fellow. And, in conclusion, 

 it is not unlikely that the complexity of the 

 digestive organ in the higher animals may be 

 referred to causes even more immediate than 

 either of the preceding : viz., to the more 

 composite chemistry of their structure, and 

 the more rapid and energetic change of their 

 substance. The structure of every animal is so 

 far self-regulative, as to determine the perma- 

 nence of its own composition, by a process of 

 which the blood is one main agent, and the tis- 

 sues generally another. But there is no rea- 

 son why we should exclude a third why we 

 may not suppose that the chemical assimila- 

 tion or likening of the foreign substances 

 taken as food is commenced in the course of 

 the digestive act why, in short, the absorp- 

 tion of more numerous, abundant, and com- 

 plex alimentary principles may not necessitate 

 the co-operation of a more highly developed 

 digestive organ. 



HUMAN ANATOMY. The alimentary canal 

 of Man is a long membranous tube, which, 

 commencing at the mouth, successively 

 occupies the regions of the neck, chest, belly, 

 and pelvis, to terminate at the lower orifice of 

 the latter cavity in the aperture of the anus. 



In this course, the canal first forms at 

 the back of the mouth a dilatation, called 

 the PHARYNX. It next contracts into a 

 straight cylindrical tube, the (ESOPHAGUS, 



which is continued through the neck and chest. 

 Immediately after perforating the diaphragm, 

 or septum which divides the thorax from 

 the abdomen, it expands into the STOMACH 

 (c,fig. 241.). An external constriction and 

 an internal valve (P, Jig. 241.) mark the 

 boundary between this organ and the INTES- 

 TINE, which forms the remainder of the tube. 

 And, finally, at about five-sixths of its length, 

 the intestine is subdivided into two portions, 

 by an alteration in size and character, which 

 commences at a point corresponding to the 

 presence of a caecum or blind pouch exter- 

 nally, and of a valve internally. Of these two 

 segments, the upper, longer, and narrower, 

 is called the SMALL INTESTINE (j, i,fig. 239.) ; 

 and the lower or wider, the LARGE INTESTINE 

 (cc, AC, TC, DC, sc, R,^. 239.). 



Fig. 239. 



Stomach and intestinal canal of the adult human 

 subject. 



c P, stomach ; c, cardiac ; p, pyloric orifice ; j i, 

 small intestine; j, jejunum; i, ileum; c c to A, 

 large intestine, viz. : -C c, caecum ; A c, ascending 

 colon ; T c, transverse colon ; D c, descending colon ; 

 s F, sigmoid flexure or sigmoid colon ; n, rectum ; A, 

 anus. 



It is the three latter portions of the ali- 

 mentary canal, viz., the stomach, small in- 

 testine and large intestine, which form the 

 especial subject of the following article. They 

 all possess the same general structure ; being 

 composed of three coats or tunics an exter- 

 nal and serous, a middle and muscular, and 

 an internal and mucous coat. The first of 

 these constitutes their means of attachment to 



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