772 General and Applied Biology 



"fat-soluble A" factor and is now called vitamin A (Fig. 369). A char- 

 acteristic function of vitamin A is the maintenance of a normal con- 

 dition of the eyes. A dietary deficiency of vitamin A produces a con- 

 dition known as night blindness (inability to see well in dim light). 

 Further deficiency of vitamin A results in a severe disease called xeroph- 

 thalmia (Fig. 370) which will result in real blindness if the diet is not 

 corrected. 



A dietary deficiency of vitamin Bi causes a loss of appetite, diminished 

 digestive secretions, muscular atrophy, lesions in the central nervous sys- 

 tem, and finally paralysis. This disease in man is called beriberi, and in 

 other animals polyneuritis (Fig. 371) . 



A dietary deficiency of nicotinic acid (niacin) (of the vitamin B com- 

 plex) causes pellagra ("skin seizure") in man, with characteristic 

 skin lesions (Fig. 372), diarrhea, anemia, and lesions of the central nerv- 

 ous system which result in confusion, dementia, and mania. Pellagra 

 is quite common in southern United States. A similar dietary disease 

 called "black tongue" occurs in dogs. For a summary of the more com- 

 mon vitamins, consult the chapter on the Biology of Man. 



Toxins, Split Proteins, Antibodies, and Hypersensitiveness (Allergies) 



Real toxins are proteinlike substances of unknown chemical composi- 

 tion produced by the metabolic activities of the living protoplasm of 

 certain bacteria. Split proteins are produced by the decomposition 

 (probably enzymatic) of nonliving proteins or by the death and subse- 

 quent decomposition of any kind of bacterial cell. Real toxins stimu- 

 late the tissues of animal bodies to form specific chemical substances 

 known as antibodies (antitoxins) which act specifically on the toxins in 

 question. Split proteins do not excite the formation of antibodies, al- 

 though they may institute a type of tolerance with no definite immunity. 

 Toxins are specific in that they have a chemical affinity for certain cells 

 or tissues on which they produce specific eflfects. Toxins are usually 

 quite injurious. One-millionth of a cubic centimeter of botulism toxin 

 kills a guinea pig weighing 250 grams in a rather short time. Such 

 bacteria as diphtheria, tetanus, botulism, and gas gangrene organisms 

 produce true toxins. Bacterial toxins prevent body cells from using 

 foods in a normal manner, or they may destroy the cells because of the 

 irritating eff'ects of chemical constituents of the toxin. Toxins do not 

 attack tissues as readily when the latter are well nourished, not over- 

 worked, and subjected to normal temperatures. A protein may be a 

 perfectly good material, but, when split into smaller particles of pro- 

 teins, the latter may become quite poisonous. 



