CHAPTER 12 



THE NERVOUS SYSTEM AND SENSE ORGANS 



To survive, an animal must be constantly adjusting to an ever- 

 changing environment. These adjustments are mediated through the 

 nervous system with its complex organization. The many stimuli af- 

 fecting the organism from the environment are received by the indi- 

 vidual sense organ, the interpretation of these sensations is made by 

 the central nervous system, and the response to the stimulus is made 

 by muscular movements or glandular secretions. The nervous system 

 is able to perform this important protective function because it inte- 

 grates all the varied activities of the body. Perhaps it could even be 

 said that the tissue composing the nervous system actually represents 

 an extreme development of that fundamental property of all proto- 

 plasm, irritability. 



Every animal has some type of system which coordinates its bodily 

 activities and allows it to respond to environmental changes. Even 

 among the single-celled protozoans (ciliates), some forms have special 

 fibrils which coordinate the beat of the cilia. The first nervous system 

 that is composed of definite units is that found among the coelenterates. 

 Here there is no central brain, but there is a network of nerve cells 

 which ramify throughout the body cells. In general, the whole animal 

 responds to any one stimulus. Among the worm groups are found the 

 first true beginnings of centralized nervous systems. The flatworms 

 have the "ladder type system" in which two longitudinal nerves extend 

 posteriorly from a head concentration of nerve cells. The longitudinal 

 fibers are connected by cross fibers. The concentration of nerve cells 

 or head ganglion is the beginning of centralized control. In the an- 

 nelids (earthworm) this centralization of control is further developed. 

 From a dorsal head ganglion or "brain", two nerve connectives pass 

 around the esophagus and join ventrally to form the ventral nerve cord 

 which then traverses the length of the body. Along its length, nerve 

 fibers are given off from ganglia in each segment. The nervous system 



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