HISTORY OF PHYSIOLOGY. 



in its progress through the intestines, chyle is absorbed by the lac- 

 teals ; and this, poured into the blood by the thoracic duct, repairs 

 the waste and nourishes the growth of the animal. But by what 

 powers is the food made to undergo these transformations ? Can we 

 explain them on mechanical or on chemical principles ? Here we 

 come to a part of physiology less certain than the discovery of vessels, 

 or of the motion of fluids. We have a number of opinions on this 

 subject, but no universally acknowledged truth. We have a collec- 

 tion of Hypotheses of Digestion and Nutrition. 



I shall confine myself to the former class ; and withc^it dwelling 

 long upon these, I shall mention some of them. The philosophers of 

 the Academy del Cimento, and several others, having experimented 

 on the stomach of gallinaceous birds, and observed the astonishing- 

 force with which it breaks and grinds substances, were led to consider 

 the digestion which takes place in the stomach as a kind of tritura- 

 tion* Other writers thought it was more properly described as fer- 

 mentation ; others again spoke of it as a putrefaction. Varignon 

 gave a merely physical account of the first part of the process, main 

 taining that the division of the aliments was the effect of the disen- 

 gagement of the air introduced into the stomach, and dilated by the 

 heat of the body. The opinion that digestion is a solution of the food 

 by the gastric juice has been more extensively entertained. 



Spallanzani and others made many experiments on this subject. 

 Yet it is denied by the best physiologists, that the changes of diges- 

 tion can be adequately represented as chemical changes only. The 

 nerves of the stomach (the pneumo-ffastric) are said to be essential to 

 digestion. Dr. Wilson Philip has asserted that the influence of these 

 nerves, when they are destroyed, may be replaced by a galvanic cur- 

 rent. 8 This might give rise to a supposition that digestion depends 

 on galvanism. Yet we cannot doubt that all these hypotheses, me- 

 chanical, physical, chemical, galvanic are altogether insufficient. 

 "The stomach must have," as Dr. Prout says, 7 ' ; the power of organi/- 



5 Bourdon, Physiol. Comp. p. 514. 



6 Miiller (Manual of Physiology, B. iii. Sect. 1, Chap, iii.) speaks of Dr. Wil- 

 son Philip's assertion that the nerves of the stomach being cut, and a galvanic 

 current kept up in them, digestion is still accomplished. He states that he and 

 other physiologists have repeated such experiments on an extensive scale, and 

 have found no effect of this kind. 



' Bridgewater Tr. p. 4'J3 



