124 THE VERTEBRATE ANIMAL: IRRITABILITY 



of the animal in a given situation. It should be clear to the 

 student that in any organism the vast majority of adjustments are 

 not conscious, and that, in general, the simplest of these adjust- 

 ments occur in the spinal cord, the more complex in the brain. 



In summarizing the details of functional locaHzation, it may be 

 said that the central nervous system has the general function of 

 adjustment of impulses, while the peripheral system furnishes 

 transmission paths between all parts of the body and the central 

 system. By the combined functions of the nervous system as a 

 whole, the activities of the organism are correlated so that it 

 behaves as a unit. This result is also known as coordination or 

 integration. The degree of integration of which an animal is 

 capable determines its ability to react successfully to the environ- 

 ment, which is the most important factor in its survival. The 

 increase in the complexity and specialization of the nervous system 

 has therefore been said to be the chief factor in evolution. As 

 Gaskell puts it, " The law of progress is this: The race is not to the 

 swift, nor to the strong, but to the wise." 



Reception, Transmission, and Discharge 



It has been pointed out that nervous coordination depends 

 essentially upon three factors. In the first place, the organism 

 must be able to be aware of changes in its environment, that is, 

 to receive stimuh. The stimulus sets up an impulse which must be 

 transmitted. And, finally, the animal responds to the stimulus 

 because the impulse is discharged at some effector. Theories of 

 reception, transmission, and discharge are rather compHcated, and 

 the processes are not well understood. However, certain state- 

 ments can be made. 



Animals like the vertebrates possess sense-organs which are 

 specialized for the reception of stimuli. These sensory areas, or 

 receptors, are sensitive to special kinds of stimuli. The retina of 

 the eye is a receptor for changes in the environment brought about 

 by light waves (Fig. 67). In the lower fishes, the ear is primarily 

 a receptor for stimuli produced by changes in the animal's equilib- 

 rium. The semicircular canals of the ear in higher vertebrates re- 

 tain this function, while the cochlea becomes specialized to receive 

 sound waves (Fig. 68). Chemical substances in solution stimulate 

 the olfactory epithelium of the nose and the taste-buds of the 



