356 



THE MAINTENANCE OF THE INDIVIDUAL 



The origin of the autonomic system has been the subject of con- 

 siderable speculation. Some investigators believe that it has been 

 secondarily derived from the central nervous system probably by the 

 migration of cells. Others support the idea that it is in reality a 

 primitive ancestral apparatus which is more or less homologous with 

 the nervous system of invertebrates. According to this theory the 

 autonomic system has become secondarily subservient to the volun- 

 tary nervous system of the vertebrates. 



The Sense Organs — Receptor Devices 



The mechanism and functioning of many of the different parts of 

 the vertebrate nervous system have been considered in some detail 

 for the purpose of showing how the voluntary system controls actions, 

 and also how the involuntary system has taken over the burden of 

 running the body. It now remains to trace the various devices that 

 have been developed to help an animal keep in touch with its environ- 

 ment, in other words the sensory receptors, which range from special- 

 ized to rather generalized structures and are usually classified as 

 organs of taste, smell, sight, hearing, and the tactile sense. 



Taste 



In the lower vertebrates the sense of taste is quite widely dis- 

 tributed. For example, in some of the fishes sensory cells of chemical 



reception are scattered 

 ^,fV^,'P^2-^-,— ^-^-^ somewhat widely over the 



body surface. In higher 

 vertebrates such organs are 

 mostly restricted to the sur- 

 face of the tongue and are 

 known as taste buds. Most 

 people labor under the de- 

 lusion that they can dis- 

 tinguish between a great 

 variety of flavors. Actually, 

 however, buds are sensitive 

 to only four kinds of stim- 

 uli, sweet, sour, bitter, and 

 salty. The confusion results from the inclusion of interpretations 

 of sensations received by the olfactory senses. 



epithelial 

 cell 



tosLe- 

 Cell... 



■nerve/ 



!trrh\".^. 



A taste bud. Explain how it functions. 



