288 



THF. INTERNAL ENVIRONMENT OF THE BODY 



Part III 



Fig. 16.10. Photograph of a model of the body of a nerve cell from the dorsal 

 (or sensory) horn of the cat's spinal cord. It shows the enormous number of 

 fibers of nerve cells whose end bulbs are in synaptic relation with this cell. The 

 model was made by fitting together individual models made from serial sections 

 of the cell. (After Haggar and Barr. Courtesy, Ham: Histology, ed. 2. Phila- 

 delphia, J. B. Lippincott Co., 1953.) 



Tropisms, Responses, and Reflexes 



Tropisms and reflexes are responses to stimuli that have a definite standard- 

 ized pattern. Tropisms make up almost the whole behavior of plants and 

 lower animals. One sunflower plant turns toward the sun; all sunflower plants 

 respond to the sun in the same way. Turn a bright light on a cockroach and 

 it will scuttle to the nearest shadow; any other cockroach would do the same. 

 Almost anybody chokes when a crumb starts down his windpipe; and all 

 chokes have a more or less standard pattern. Tropisms are movements of 

 the whole body toward or away from the stimulus, as a housefly turns toward 

 the light. Reflexes are more often movements of a part of the body, the flick 

 of a cat's ear when its edge is touched, the snapback of one's hand at the 

 touch of a spark. 



Tropisms. Insects are drawn to light or dark, that is, they are positively 

 or negatively phototrophic to light. But they are so physiologically attuned 

 that their reactions to light are changed by temperature and humidity, and 

 vary with particular phases of their lives. In the mating flight the queen honey- 

 bee rises for the first time in her life, high into the sunshine, and for only the 

 second time she flies with the swarm on a brilliant day. Outside of these two 

 occasions, both concerned with the reproduction of her species, a queen bee 

 stays in complete darkness within the hive. 



