304 GENERAL ZOOLOGY 



natural adaptations for protection. Even its cells and 

 fluids have properties which effectively shield them from 

 dangers likely to be encountered. The skin usually keeps 

 out undesirable materials. If it is broken, however, the 

 clotting of the blood forms an effective plug which checks 

 contamination from the outside. If bacteria or poisons 

 do gain an entrance, they are eaten up by the leucocytes 

 (which swarm through the blood channels to repel invaders), 

 destroyed by antitoxins, or oxidized. Exposed parts of 

 the body are especially protected by heavy growths of hair 

 or callouses. Civilized man has devised clothing, houses, 

 and other protective devices to assist his natural defences. 

 Man is also one of the favored animals which have a con- 

 stant body temperature, maintained by delicate adjust- 

 ments and coordinations of metabolic activities, vaso- 

 motor nerves, and perspiratory glands. He is thus able 

 to survive critical periods of climatic stress, when a cold- 

 blooded animal would die. Man is also endowed with 

 discriminating courage, reasonable fear, and a great degree 

 of resourcefulness, so that he may fight, flee, or escape 

 danger by using his wits. 



Yet, despite all man's versatility in combating dangers, 

 he is continually subject to minor disorders and is often 

 killed outright by accident or disease. There are many 

 chances for the occurrence of defects in the bodily machine. 

 (1) A man may be born with some defect which cannot be 

 corrected hunchback is an example of such an affliction. 

 Among the most pathetic of such cases are those in which 

 the nervous system is defective. There is a malady known 

 as Little's disease in which the body and mind may be 

 perfect, but the nervous connections (pyramidal tracts) 

 never grow down completely from the brain, as they should 

 do about the time of birth. The bodily movements, 

 therefore, are not coordinated because messages cannot be 

 properly carried from the brain. A bright mind may thus 

 wear itself out in a helpless but perfect body over which 

 it has little or no control, and which ultimately dies from 



