144 The Vertebrate Organ Systems 



and gradually results in the formation of the feces. Occasionally after 

 a meal, movements known as niass peristalsis occur. These move the 

 undigested remains of food from one region to another. 



Defecation is brought about by a combination of many movements. 

 The v^alls of the rectum are contracted, forcing the feces downward. 

 In addition, the abdominal walls and the diaphragm push inward and 

 downward, helping to force the feces out. 



Movements of the Villi. — In the small intestine, the movements 

 of the villi are important for the absorption of the digested food. Two 

 types of movement are observable. One is simply a slow waving back 

 and forth ; the other is a sort of pumping movement which aids the 

 passage of the food into the vessels of the circulatory system. 



FOODS AND NUTRITION 



Vertebrates vary considerably in their food habits. Both struc- 

 turally and physiologically they are adapted to these various habits. 

 Carnivorous animals such as the dog and cat rely upon meat as their 

 main food ; herbivorous animals such as cattle use vegetable materials. 

 An animal such as man is omniverous in his habits, that is, he eats 

 both animal and vegetable foods. No matter what its source, food is 

 utilized for either energy or repair. Energy is derived chiefly from 

 the oxidation of carbohydrates and fats while the proteins contribute 

 to the building and repair of tissues. 



All the foods used by the body produce a measurable amount 

 of heat. This heat of combustion can be determined, and is measured 

 by the unit known as the Calorie. A Calorie is defined as the amount 

 of heat needed to raise the temperature of one liter of water one de- 

 gree centigrade. In general, carbohydrates produce four Calories per 

 gram, fats produce nine, and proteins four. It is more difficult to be 

 accurate in the measurement of the caloric value of proteins as their 

 combustion is not always complete. 



The caloric needs of various individuals vary with age and activity. 

 Usually it is recommended that a man leading a sedentary life needs 

 about 2,500 per day; a woman, about 2,100. Children have a higher 

 demand, and a 16- to 20-year-old youth may need as many as 3,800 daily. 



In addition to fats, carbohydrates, and proteins, certain other 

 materials are needed in the diet. These do not contribute calories, but 

 perform other essential functions. Among these necessary items are 

 the various vitamins, inorganic salts, and water. 



