Nervous System and Sense Organs 193 



of the arthropods is very similar to this except that there is fusion, 

 and hence reduction of the number of gangha in the ventral nerve cord. 



The vertebrate nervous system shows many unique features whose 

 ancestral counterparts among the invertebrates are difficult if not im- 

 possible to find. The vertebrates have the greatest development of the 

 central nervous system. This central nervous system is arbitrarily 

 divided into the brain, with its many complex and specialized regions, 

 and the spinal cord. In contrast to the invertebrates, the central nerv- 

 ous system of the vertebrates is wholly dorsal and is hollow. The 

 brain is differentially developed among the various groups of vertebrates. 



THE FUNCTIONS OF THE NERVOUS SYSTEM 



The nervous system has two primary functions : one relates the 

 organism to its external environment, the other coordinates the ac- 

 tivities within the organism. Both of these functions are really inte- 

 grative activities by which the organism is able to live in its environ- 

 ment as a unified whole. 



1. Interpretation of the Environment. By means of its 

 many sense organs, such as the eyes, the ears, and the nose, the animal 

 is made aware of changes in its environment. These specialized organs 

 are known as receptors. The information received by them is relayed 

 to the central nervous system for interpretation. A response to the 

 change of the environment is brought about by effectors. An effector 

 may be a muscle which brings about a movement or a gland which is 

 stimulated to secrete. 



2. Integrative Activities Within the Animal. The activi- 

 ties of the many different organ systems must be coordinated for the 

 proper functioning of the animal as a whole. It may be recalled that 

 during the discussion of the different organ systems, mention was often 

 made of the role played by the nervous system in regulating the move- 

 ment or response to different stimuli. The complex interworkings of 

 various systems was particularly well illustrated by the circulatory and 

 muscular systems. During strenuous exercise, the amount of blood re- 

 turning to the heart is greatly increased. This results in stretching of 

 the wall of the auricle and in stimulation of special sense endings. 

 This stimulation results in an increased heart rate. Thus the in- 

 creased demands of the muscular system are met by a response from 

 the circulatory system which is mediated by the nervous system. 



