604 FUNCTIONS OF ANIMALS. 



by chemists'and physiologists respecting both chyle and lymph 

 have been detailed in a preceding chapter of this volume. 



Such is the very imperfect view that can be at present given 

 of the process of digestion. The food in the mouth is converted 

 into a moist and comminuted mass, which in the stomach is act- 

 ed on by the gastric juice, and converted into chyme. The 

 chyme passes into the small intestines, where it is acted on by 

 liquids, there secreted and converted into chyle and excremen- 

 titious matter. The part played by the pancreatic juice is un- 

 known. But the soda of the bile neutralizes the acid in the 

 chyme, while the choleic acid facilitates the evacuation of the 

 excrementitious matter from the intestines. The chyle when 

 completed is taken up by the lacteals, carried to the thoracic 

 duct, where it is mixed with the lymph, and both together are 

 conveyed to the left subclavian vein, where they mingle with the 

 blood, and during the circulation through the blood-vessels, the 

 conversion of the chyle into blood is completed. 



CHAPTER II. 



OF RESPIRATION. 



THE function of respiration, so essential to the existence of 

 hot-blooded animals, and indeed of all animals, could not be un- 

 derstood till after the discovery of the circulation of the blood. 

 Accordingly, there is nothing respecting this function in the 

 writings of the Greek philosophers that is deserving of being 

 noticed. Plato, in his Timeus, says, that, " as the heart might 

 be easily raised to too high a temperature by hurtful irritation, 

 the genii placed the lungs in its neighbourhood, which adhere 

 to it and fill the cavity of the thorax, in order that their air ves- 

 sels (arteries) might moderate the too great heat of that organ, 

 and reduce the vessels to an exact obedience." And this opi- 

 nion respecting the use of the lungs, strange as it may appear, 

 was generally adopted by philosophers and medical men, till the 

 chemical discoveries respecting heat made by Dr Black about 

 the middle of the last century laid the foundation of another ex- 

 planation. 



The structure of the lungs seems to have been first explained 



3 



