24 



DIGESTION. 



are immediately connected with the external 

 senses. Its functions are excited or depressed 

 by various causes, which can only act through 

 the medium of the mind or imagination; while 

 it is argued that in all cases its various condi- 

 tions and the changes which its functions expe- 

 rience can be referred to no cause, except to 

 corresponding changes in the nervous system.* 

 This hypothesis, like that of the vital prin- 

 ciple, has been supported by the consideration 

 of the inadequacy of all the other modes of 

 explaining the phenomena, and the impossi- 

 bility of referring them either to mechanical 

 or to chemical principles. But it has this clear 

 and decided advantage, that it rests upon the 

 co-operation of an actual agent of great and 

 acknowledged power, one the existence of 

 which is universally recognized, the only ques- 

 tion being whether it is applicable to this indi- 

 vidual case. But although we admit the facts 

 in their full force, we must still demur to the 

 conclusions that must be deduced from them. 

 If we inquire upon what principle, or by what 

 medium the nervous system can operate on the 

 digestive functions, two modes present them- 

 selves to the mind. We may ascribe the 

 effect either to the general operation of the 

 nervous energy, whatever this may be, which 

 pervades eveiy part of the system, and the 

 stomach among the rest, and which gives it 

 those powers which distinguish living from 

 dead matter; or we may conceive that the ner- 

 vous system is, in some way, more especially 

 concerned in the production of the gastric 

 juice, and that consequently whatever tends to 

 decrease or diminish the nervous energy, may 

 operate in the increased or diminished produc- 

 tion of this secretion, and thus indirectly, al- 

 though necessarily, affect the digestive func- 

 tion. But although we may admit the truth 

 of both these suppositions, we gain no specific 

 answer to our inquiry. It is not enough to be 

 informed that the stomach acts upon its con- 

 tents because it is alive, or that whatever pre- 

 vents the secretion of the gastric juice puts a 

 stop to the digestion. Oar inquiry embraces 

 a farther object, and leads us to investigate 

 the nature of the connexion between these facts 

 and the ultimate effect produced, or to discover 

 the reason why certain acknowledged effects 

 are connected with certain acknowledged causes; 

 but to this question the nervous hypothesis 

 gives us no satisfactory answer. It indeed 

 rather involves the theory of secretion than of 

 digestion, for even were it to be clearly proved 

 that the nervous power (whether, according to 

 the hypothesis of Dr. Philip, we identify it 

 with the galvanic influence, or we act the more 

 cautious part of not attempting to explain its 

 nature,) is the immediate agent in the forma- 

 tion of the secretions, still we are left equally 



* It was on facts of this description that Vanhel- 

 xnont founded his hypothesis of the stomach being 

 the immediate seat of the soul ; Onus Med. p. 

 248, 49, 50. See also on the same subject the 

 remarks of Hartley, on Man, v. i. p. 189, and 

 Scemmering, 179.. .4, who may be respectively 

 considered as among the most accurate metaphysi- 

 cians and anatomists of modern times. 



uninformed concerning the mode in which this 

 fluid, when secreted, performs its appropriate 

 function.* 



From this brief review of the different the- 

 ories of digestion we may conclude, that the 

 hypothesis of trituration is decidedly incorrect, 

 and that those of the vital principle and the 

 nervous energy do not resolve the question. 

 We are therefore reduced to the two chemical 

 hypotheses, which, although not without con- 

 siderable difficulties, are not so palpably defec- 

 tive or erroneous. In deciding between these 

 two hypotheses it must be our first object to 

 ascertain the exact sense in which the term 

 fermentation was used by the older physio- 

 logists, and how far, according to the modern 

 use of the term, it is applicable to the phe- 

 nomena in question. The word was originally 

 employed in a very extensive, and, as may be 

 supposed, in a somewhat vague manner, to 

 designate every spontaneous change which took 

 place between bodies that were placed in con- 

 tact, and which generally manifested itself by 

 the extrication of some gaseous or volatile 

 matter. Thus all the spontaneous changes in 

 the body, whether natural or morbid, were 

 considered to be different kinds of fermen- 

 tations, and many of the changes that take 

 place among inorganic substances, as well as 

 various processes in the laboratory, were dis- 

 tinguished by the same appellation. 



As our knowledge of the nature of these 

 processes was extended, and we were thus 

 enabled to ascertain more correctly what was 

 the change which was produced, our language 

 became more correct and better denned, and 

 the term fermentation was restricted to a spe- 

 cific operation, in which certain proximate 

 principles, derived from organized bodies,f act 

 upon each other, and enter into new elementary 

 combinations. The process is generally pro- 

 moted by the addition of a substance called 

 the ferment, which is employed to enable the 

 bodies to act upon each in the first instance, 

 although, when the action has commenced, its 

 presence may be no longer necessary. The 

 most familiar kind of fermentation is that by 

 which a mixture of sugar and mucilage is con- 

 verted into alcohol, and that by which the 

 same substances, when exposed to the atmos- 

 phere, and to a certain temperature, are con- 

 verted into acetous acid. How far we are to 

 extend the number of fermentations is a point 

 respecting which chemists are not agreed, and 

 indeed there appears to be no reason but that 

 of convenience which can decide the point. 

 We accordingly find that while Mr. Brande is 

 disposed to restrict the term to the vinous and 

 acetous fermentation,! others extend it to three, 

 four, or with Dumas, even to six processes. 



* We may refer our readers to the judicious re- 

 marks of Dr. Prichard, in his Essay on the Vital 

 Prin. sect. 8. 



t Some of the most eminent chemists confine the 

 process of fermentation to the proximate principles 

 derived from vegetables ; but this restriction is not 

 universally adopted, nor does it appear to be neces- 

 sary. 



J Ut supra. 



Ut supra. 



