140 FUNCTIONS OF NUTRITION. 



powerful agencies. Lastly, the fatty substances are reduced to a state 

 of emulsion, and in this condition diffused through the digestive fluids. 

 This effect, which is mainly due to the contact of an albuminous liquid, 

 may be aided by a partial acidification and saponification ; but the prin- 

 cipal mass of the fat, in undergoing the digestive process, only assumes 

 the form of a chylous emulsion. All the alimentary substances are 

 accordingly made ready for absorption, without losing the essential 

 features of their chemical constitution. 



In the following pages the properties of the digestive fluids will be 

 considered in detail, together with the action exerted upon the food in 

 different parts of the alimentary canal. 



Mastication. 



The process of mastication, which takes place in the mouth, consists 

 of a mechanical trituration of the food by the teeth. At the same time 

 it is mingled with the saliva, which is so worked into the alimentary 

 mass as to reduce it to a pasty condition. By this means the solid 

 substances of the food, finely divided and thoroughly moistened, are 

 rendered susceptible to the action of the digestive fluids. Food swal- 

 lowed either in large masses or in a dry condition would be slowly 

 affected by the alimentary secretions, and would be consequently diffi- 

 cult of digestion ; but, when comminuted and softened by mastication, 

 it presents a large surface of contact and a ready permeability, favor- 

 able to the prompt action of the digestive solvents. 



The form of the teeth and their physical action vary in different 

 animals according to the nature of their food ; being adapted, in the 

 carnivora, mainly for wounding and lacerating in the rodentia for 

 gnawing and cutting, and for grinding in the herbivora. In man they 

 are adapted for a mixed diet of animal and vegetable food, and com- 

 bine a general resemblance to each other, with certain special characters 

 in different parts of the mouth. The incisors, four in number, in each 

 jaw, are more or less chisel-shaped, with a cutting-edge running from 

 side to side. They are useful in separating from a mass of food the 

 proper quantity to be taken into the mouth. The canine teeth, one on 

 each side, in each jaw, placed immediately behind the former, are some- 

 what pointed in form, and are immediately followed by the two anterior 

 molars, which are thicker and stronger. Finally, the three posterior 

 molars, on each side of each jaw, complete the dental arch posteriorly. 

 They are the largest and strongest of the set, firmly planted in the 

 jaw, and present upon their free extremity a number of conical and 

 ridge-like elevations, separated by shallow furrows. They are espe- 

 cially adapted, by their form, size, and situation, for the comminution of 

 resisting substances, and they perform, with the anterior molars, the 

 main part of the work of mastication. The enamel which covers the 

 crowns of all the teeth, and which is the hardest substance in the body, 

 protects their substance from injury, and enables them to exert the 

 necessary physical action upon the food. 



