512 DIGESTION. 



BOOK II. 



NUTRITIVE FUNCTIONS. 



THE human body, from the moment of its formation to the cessation 

 of existence, is undergoing constant decay and renovation decomposi- 

 tion and composition : so that at no two periods can it be said to have 

 exactly the same constituents. The class of functions about to engage 

 attention, embraces those that are concerned in effecting such changes. 

 They are seven in number ; digestion, by which the food, received 

 into the stomach, undergoes such conversion as fits it for the separa- 

 tion of its nutritious and excrementitious portions ; absorption, by 

 which this nutritious portion, as well as other matters, is conveyed into 

 the mass of blood ; respiration, by which the products of absorption 

 and venous blood are converted into arterial blood ; circulation, by 

 which the vital fluid is distributed to every part of the system ; nutri- 

 tion, by which the intimate changes of composition and decomposition 

 are accomplished ; calorification, by which the system is enabled to 

 resist the effects of greatly elevated or depressed atmospheric tempera- 

 ture, and to exist in the burning regions within the tropics, or amidst 

 the arctic snows; and secretion, by which various fluids and solids are 

 separated from the blood ; some to serve useful purposes in the ani- 

 mal economy ; others to be rejected from the body. 



CHAPTER I. 



OF DIGESTION. 



THE food, necessary for animal nutrition, is rarely found in such a 

 condition as to be adapted for absorption. It has, therefore, to be 

 subjected to various actions in the digestive organs ; the object of 

 which is to enable the nutritive matter to be separated from it. These 

 actions constitute the function of digestion ; in the investigation of 

 which we shall commence with a brief description of the organs con- 

 cerned in it. These are numerous, and of a somewhat complicated 

 nature. 



1. ANATOMY OF THE DIGESTIVE ORGANS. 



The human digestive organs consist of a long canal, varying con- 

 siderably in its dimensions in different parts, and communicating ex- 

 ternally by two outlets, the mouth and anus. It is usually divided 

 into four chief portions the mouth, pharynx, oesophagus, stomach, and 

 intestines. These we shall describe in succession. 



