172 THE INTERNAL ENVIRONMENT OF THE BODY Part Til 



voyages that took a year or more, the great dread of sailors was scurvy, a dis- 

 ease caused by the long steady diet of dried and salted foods lacking in vita- 

 mins. When it was discovered that eating limes would prevent scurvy, no ship 

 went to sea without them. Sailors ate limes and unknowingly treated them- 

 selves to vitamin C (Table 11.1). 



Vitamin research really began when it was discovered that animals needed 

 vitamins and could be used as subjects in experimentation with deficiency dis- 

 eases. The first clearcut results (Eijkman, 1893) were obtained upon chickens. 

 When they were fed on polished rice, the chickens developed a disease similar 

 to beriberi, common among human rice-eating populations (Fig. 11.2). As 

 soon as they were fed the previously cast off rice polishings they recovered 

 from the disease. As often happens, the wide significance of these results was 

 not recognized until some time later. By 1915, however, it was fully realized 

 that in addition to the regular foods, more than one vitamin was essential for 

 health. The discovery of vitamin A came about through attempts (1913- 

 1915) to use pure fats in the diets of experimental animals. It was observed 

 that for no apparent reason butterfat was far superior to other fats, such 

 as lard. When young rats were fed diets containing only lard, they were 

 stunted and had a scaly, infected condition of the eyes known as xeroph- 

 thalmia (Fig. 11.3). In contrast to this, when butter was substituted for lard 

 in the diet, the rats grew and remained healthy. Oleomargarine made from 

 vegetable oils has a food value identical with that of butter now that sufficient 

 vitamin A is added. 



Minerals. Minerals required by the body are usually obtained with the food 

 or drinking water. Several such substances are essential to plants and animals, 

 but in minute quantities. These are called micronutrients and trace elements, 

 the latter not to be confused with radioactive tracer substances. Experimental 

 diets given to animals have revealed most that is known about the use of 

 micronutrients. 



Types of Nutrition 



There are three principal types of diet: herbivorous, carnivorous, and om- 

 nivorous. Herbivorous animals feed on vegetation. They include grazing cattle, 

 leaf-eating insects such as Japanese beetles, seed-eating birds, and rodents. 



Carnivorous animals are flesh-eaters. Among the most voracious are the fresh- 

 water protozoans Didinium nasutum. When they are placed among a population 

 of paramecia, each one immediately attaches its trunklike proboscis onto a Para- 

 mecium which is speedily "swallowed" (Fig. 11.4). An individual Didinium 

 may devour paramecia until its own body splits open. Among other carnivores 

 no tiger can be more bloodthirsty than female mosquitoes and blood-sucking 

 leeches. Most fishes are typical carnivores; so are snakes, owls, and hawks. 

 The order Carnivora is a group of mammals that includes cats, tigers, dogs, 



