306 



STOMACH AND INTESTINE. 



sion of intestine ; a liver ; a pancreas ; and, 

 finally, a compound character of mucous 

 membrane, by virtue of which the whole tube 

 might be compared to one vast expanse or 

 aggregation of glands. Some of these par- 

 ticulars will again force themselves upon our 

 attention. Hence we may here limit ourselves 

 to the remark, that the main elements of this 

 advance consist in the evolution or separation 

 of accessories, and the increase and subdi- 

 vision of surface: and that both of these 

 conditions imply a division of labour which, 

 here as elsewhere, enhances both the quantity 

 and quality of its product. 



2. Respecting the homologies of the intes- 

 tinal canal, scarcely anything can be said. 

 As might be expected, form seems always 

 subordinate to purpose: in other words, 

 neither general nor individual development 

 offers us any permanent or temporary organs 

 of digestion, from which we can deduce a 

 shape that can be considered as a common 

 pattern or archetype.* In rare instances, as 

 in the Earth-worm and Arachnidan, the form 

 of the internal canal approaches that of the 

 body and limbs respectively. But even this 

 peculiarity of form is probably teleological. 



3. Sufficient allusion has already-)- been made 

 to vegetative or irrelative repetition, as a pos- 

 sible explanation of the complex canal seen in 

 many of the lower Invertebrata. Some of 

 these ramified canals such as those of the 

 Acalephae, and, with less probability, of the 

 Distoma? may be conjectured to represent a 

 vascular, rather than intestinal, system. But 

 there are others such as those of the Leech 

 and Spider which seem to be true processes 

 of the digestive canal, used as reservoirs of 

 food. 



4. Some complications seem mainly de- 

 pendent upon circumstances which may be 

 termed collateral or subordinate to digestion 

 itself. Thus, the crops of many animals, 

 like the cells of the Camel's stomach, are 

 connected with the more or less necessary 

 habit of gorging large quantities of food at 

 distant intervals. While the gizzard, which 

 is possessed by such very different orders as 

 Polyps, Molluscs, Fishes, and Birds, appears 

 to be closely related, not only to the food, 

 but to the mechanical conditions of the 

 animal. This is especially the case with the 

 Bird, whose long neck, and habits of flight, 

 could scarcely be rendered compatible with a 

 heavy masticatory organ occupying the or- 

 dinary position. 



5. The import of some of those numerous 

 blind tubes or pouches which we have so 

 often noticed as opening into the intestinal 

 canal, has already been suggested in the 

 preceding remarks. They are generally, and 



* In this respect the intestinal canal may probably 

 be contrasted with both the vascular and nervous 

 systems. At least the author feels sure that the 

 latter of these will ultimately be found reducible 

 to that serial homology of the skeleton which the 

 researches of Professor Owen have done so much to 

 elucidate. 



t See p. 295. 



no doubt correctly, regarded as earlier 

 developmental forms of the various conglo- 

 merate glands which are appended to the 

 canal in higher animals. But as regards the 

 principles of their diagnosis, and the limits of 

 its application, it seems important to remind 

 the reader, that, in the present state of organic 

 chemistry, the situation of their apertures, 

 and the order of their appearance, often 

 constitute our only guides. Thus, for in- 

 stance, tubes which open into the commence- 

 ment of the canal, especially in connection 

 with a higher development of the masticatory 

 organs, are probably salivary. In like man- 

 ner, those which empty themselves in the 

 neighbourhood of the pylorus are supposed 

 to be biliary. And any which, by communi- 

 cating with the anus and exterior of the 

 body, appear to aim at an immediate and direct 

 extrusion of their contents, naturally remind 

 the physiologist of the highly poisonous 

 characters of the urinary secretion, and so far 

 entitle him to suspect that they serve to expel 

 this important product of animal life. Here, 

 however, chemistry would often assist his de- 

 cision. The colour of the bile sometimes 

 affords a less certain aid to the diagnosis of 

 this secretion. The order of appearance only 

 helps our conjectures by showing, that, of the 

 two glands which open into the median por- 

 tion of the digestive tube, the liver is the more 

 constant and important: and hence, that it 

 is probable a solitary set of tubes are chiefly 

 hepatic. But it is obvious that, in many in- 

 stances, all these aids to conjecture may leave 

 us in doubt as to the true nature of a set of 

 secerning tubes. 



6. In many of the Vertebrata such as 

 Birds and Edentata there are caeca to which, 

 as to the smaller vermiform appendix of man. 

 the above explanation cannot apply ; since 

 the ordinary accessory glands are also present. 

 And some of the tubes seen in Insects are 

 probably quite as supplementary. The struc- 

 ture of all these tubes seems to indicate that 

 they are true organs of secretion. But whe- 

 ther this is their main function or if so, what 

 is the nature of their product is utterly 

 unknown. The supposition of their possess- 

 ing a special absorptive function only increases 

 this obscurity, by leaving it doubtful whether 

 the lower parts of such tubes reclaim a por- 

 tion of the secretion poured out by the upper 

 just as the intestine absorbs the bile after its 

 entry into the duodenum or whether they 

 absorb materials derived from the general cavity 

 of the intestine. But that increase of surface 

 which facilitates mere absorption is effected 

 by folds and projections so much better than 

 by tubes, that, even supposing this latter re- 

 absorption to obtain, we ought at least to 

 concede some modifying power to the secret- 

 ing surface. The ordinary situation of their 

 apertures near the junction of the small 

 and large intestine scarcely assists our spe- 

 culations. It may, however, indicate an ex- 

 posure of their secretions to the long and 

 energetic absorption effected by the large in- 

 testine. 



