126 TEXT-BOOK OF PHYSIOLOGY 



xanthin. They are present in the coffee seeds, the tea leaves, and the cocoa 

 bean to the extent of 1.7 per cent., 1.4 per cent., and 1.6 per cent, respectively. 

 When prepared as a beverage, however, there is three times as much caff ein 

 in coffee as thein in tea. 



Alcohol when taken in small quantities stimulates the digestive glands 

 to increased activity and thus promotes digestive power. Its absorption into 

 the blood is followed by increased action of the heart, dilatation of the cutane- 

 ous blood-vessels, a sensation of warmth, and an excitation of the brain. 

 In large quantities it acts as a paralyzant, depressing more especially the 

 vaso-constrictor nerve-centers and certain areas of the brain, as shown by 

 an impairment in the power of sustained attention, clearness of judgment, 

 and muscle coordination. 



Alcohol is undoubtedly oxidized in the body, as only about 2 per cent, 

 can be obtained from the urine and expired air. It thus contributes to the 

 store of the body-energy. Whether for this reason it can be regarded 

 as a food that is, whether it can be substituted in part at least for fat or 

 carbohydrate material without impairing the protein metabolism is at 

 present a subject of experimentation and discussion. According to some 

 investigators, alcohol does not retard protein metabolism, for when it is 

 introduced into the body in amounts equivalent to the carbohydrates with- 

 drawn from the food there is at once a rise in the amount of nitrogen excreted. 

 Hence it cannot be regarded as a food. According to other investigators, 

 alcohol retards or protects protein metabolism just as effectually as an 

 equivalent amount of starch or sugar. Many more experiments are required 

 to decide this question. When taken habitually in large quantities, alcohol 

 deranges the activities of the digestive organs, lowers the body temperature, 

 impairs muscle power, lessens the resistance to depressing external con- 

 ditions, diminishes the capacity for sustained mental work, and leads to the 

 development of structural changes in the connective tissues of the brain, 

 spinal cord, and other organs. In infectious diseases and in cases of depres- 

 sion of the vital powers it is most useful as a restorative agent. 



THE ENERGY OR HEAT VALUE OF THE FOOD PRINCIPLES 



The food consumed not only restores the materials metabolized and 

 discharged from the body, but also the energy which has been expended as 

 heat and mechanic motion. The food principles are products of the con- 

 structive processes taking place in the vegetable world during the period of 

 growth and activity. At the time of their formation there is an absorption 

 and storing of the sun's energy which then exists in a potential condition. 

 During the metabolism of the animal body these compounds are reduced 

 through oxidation to relatively simple bodies, such as carbon dioxid, water, 

 urea, etc., with the liberation of their contained energy. All of the energy 

 of the body, whatever its manifestations may be, can be traced to chemic 

 changes going on in the tissues, and more particularly to those changes 

 involved in the oxidation of the food principles. 



It becomes, therefore, a matter of interest to determine the heat loss 

 from the body in twenty-four hours for the purpose of subsequently deter- 

 mining if the energy contained in the foods, expressed in terms of heat, is 

 present in amounts sufficient to compensate for the loss. The total quan- 

 tity of heat liberated in the body and dissipated from it in twenty-four hours 



