DIGESTION. 



25 



Among these, one which is the subject of daily 

 observation is the panary, or that by which 

 dough is converted into bread, a change which 

 appears to come strictly under the definition, 

 as a spontaneous action among the elementary 

 constituents of the body, by which a substance 

 is produced, essentially different from the one 

 from which it was composed. Now we are 

 disposed to think that the same principle will 

 apply to the conversion of aliment ito chyme, 

 and that it is little more than a difference in 

 the mode of expression, whether we say that 

 digestion depends upon chemical action gene- 

 rally, or upon that peculiar kind of chemical 

 action which has been termed fermentation. 



The foregoing remarks apply immediately to 

 the production of chyme, and it still remains 

 for us to consider whether the same mode of 

 reasoning can be applied to the further conver- 

 sion of chyme into chyle. And it must be 

 confessed that this part of our subject presents 

 us with new difficulties, and that the analogy, 

 which in the former case was imperfect, is 

 apparently still more so, when we apply it to 

 the action of chylification. Here we have a 

 chemical change in the constituents, without 

 the intervention of any assignable agent, at- 

 tended with the production of a new substance, 

 in consequence, as far as we can judge, of the 

 spontaneous action of the elements upon each 

 other, and with the separation of the substance 

 thus formed from the remainder of the mass. 

 But although the operation may be somewhat 

 more complicated, and although we may find 

 it less easy to assign an efficient cause for each 

 step of the process, there will be found nothing 

 contrary to the recognized effects of chemical 

 affinity. And with respect to the question, 

 how far these effects should be referred to the 

 specific action of fermentation, we may remark 

 that the result of the proper fermentative pro- 

 cesses is to form a new product, and to sepa- 

 rate the product thus formed from the residuary 

 mass. Upon the whole therefore we may con- 

 clude, that although there are many points in 

 the chemical theory of digestion that are still 

 unexplained and require to be further investi- 

 gated, yet that we have no facts which directly 

 oppose it, while the difficulties which we feel 

 on certain points would appear to be princi- 

 pally owing to the imperfect state of our know- 

 ledge on the subject. 



V. Peculiar affections of the digestive or- 

 gans. We now proceed, in the last place, to 

 offer some remarks on certain affections of the 

 stomach and its appendages, which are only 

 indirectly connected with the function of diges- 

 tion. Of these the most important are hunger, 

 thirst, and nausea; we shall consider in suc- 

 cession the causes of each of them, and the 

 relation which they bear to the animal economy 

 in general. 



Hunger is a peculiar perception experienced 

 in the stomach, depending on the want of 

 food. Its final cause is obvious, but respecting 

 its efficient cause there has been considerable 

 difference of opinion among physiologists, 

 some referring it to a mechanical, others to a 

 chemical action, while by a third set of writers 



it is referred exclusively to a peculiar condition 

 of the nervous system. Before we enter into 

 the respective merits of these opinions it will 

 be necessary to remark concerning the feeling 

 excited by hunger, that it is one of a specific 

 nature, as essentially different from the mere 

 perception of touch, as the sense of sight is 

 from that of mechanical pressure made on the 

 ball of the eye. In physiological language 

 the stomach may be regarded as one of the 

 organs of sense, in the same way with the eye 

 and the ear; i. e. a part furnished with a spe- 

 cific apparatus for producing specific impres- 

 sions on a set of nerves appropriated to it, 

 which convey to the mind certain perceptions, 

 and which, by habit or by instinct, we connect 

 with certain conditions of the organ. In most 

 cases we are able to point out distinctly the 

 nature of the agent which produces these per- 

 ceptions, as light when applied to the eye, and 

 the undulations of the air to the ear ; in the 

 particular case of the stomach we are not able 

 to point out any corresponding agent of this 

 description, and in so far the analogy between 

 the stomach and the organs of sense must be 

 considered as defective. 



The mechanical physiologists ascribed hun- 

 ger to the friction of the different parts of the 

 internal membrane of the stomach on each 

 other, an opinion which, although sanctioned 

 to a certain extent by Haller,* must be aban- 

 doned, whether we regard the anatomical 

 structure of the part, which shows that such 

 friction is incompatible with its rounded form, 

 and the disposition of its muscular fibres, or 

 the nature of the sensation itself, which is 

 specifically different from that produced by 

 pressure, or any species of mechanical impulse 

 on the surface of the body. Nor can the hy- 

 pothesis be maintained, which supposes that 

 the action of the gastric juice, by its tendency 

 to decompose organized substances, exercises 

 a degree of this eroding quality on the internal 

 coat of the stomach, and thus produces the 

 uneasy sensation. But in this hypothesis the 

 great distinction, which has been so frequently 

 referred to, between living and dead matter as 

 to the action of the gastric juice is disregarded; 

 besides that from every analogy which we pos- 

 sess, it might be presumed that a substance so 

 mild and apparently so little active as the gas- 

 tric juice, could not produce effects, which 

 must be attributed to a body possessed of highly 

 acid or noxious qualities. And it may be fur- 

 ther remarked, that in cases of the most pro- 

 tracted privation of food, and where death has 

 occurred after the most severe pangs of hunger, 

 nothing like erosion of the stomach has been 

 observed, and that conversely, in those in- 

 stances where this effect has been produced 

 after death, we have no reason to suppose that 

 it was in any degree caused by the deficiency 

 of food, or had been preceded by hunger. 



From what has been stated above it may be 

 inferred that the view which we feel disposed 

 to take of the efficient cause of hunger is to 

 regard it as a specific perception, occasioned 



* Prim. Lin. 368 ; El. Phys. xix. 2, 12. 



