Sect. II.] Physiology, 299 



before, it is obviously expedient to direct the chief 

 attention to the former branch of the subject. 



Galen supposed heat to be the principal cause 

 of digestion, and this opinion so generally prevail- 

 ed for a long time that the term coction was used 

 by the greater part of physiologists instead of 

 digestion. But, though the effect of heat in assist- 

 ing and expediting digestion is universally admit- 

 ted, no person will now contend that it is the sole 

 cause. 



During the eighteenth century, the theorists of 

 digestion have ascribed it either, sijigly, to fcr- 

 mentation^ mechanical action, or the operation of a 

 solvent in the stomach j or to the combined efl'ects 

 of two or all of these agents. 



Dr. Boerhaave, dissatisfied with the opinions of 

 all who had gone before him on this subject, and 

 leaning "strongly to mechanical theory, admitted 

 fermentation as one cause of digestion, but princi- 

 pally ascribed to it trituration, pressure, and power- 

 ful quassation. The analogy of digestion, as per- 

 formed in certain birds, seems to have led him into 

 this doctrine. He had observed the ostrich to 

 swallow pieces of iron and glass, evidently for the 

 purpose of trituration, because the sound of grind- 

 ing was perceptible to those who listened. In tiie 

 ^ranivorous birds he had noticed, in addition to 

 the crop furnished with salivary glands to macerate 

 and soften their food, a gizzard, or second sto- 

 mach, provided with strong muscles to triturate 

 the grain, and the eagerness with which they swal- 

 low gravel to assist the operation. Considering the 

 predominance of mathematical doctrines at thai 

 period, it is not wonderful that this great mechanic 



