RESPONSIVENESS IN VERTEBRATES 



consciousness. Sleep and anesthetics in some way lessen or completely block 

 the functioning of the cerebral cortex and produce unconsciousness. 



By combined functions of the nervous system as a whole, the activities of the 

 organism are correlated so that it behaves as a unit; the function of the nerv- 

 ous svstem is, therefore, coordination or integration. It has been pointed 

 out that nervous coordination depends essentially on three factors. In the 

 first place, the organism must be able to be aware of changes in its environ- 

 ment, that is, to receive different kinds of stimuli. Secondly, nerve impulses 

 which are set up in response to the stimulus are conducted over the neurons 

 of the reflex arcs. And, finally, the animal must respond to the stimulus when 

 the impulse is discharged at some effector. We shall now describe some of the 

 mechanisms concerned with reception, conduction, discharge, and response, 

 especially with respect to adjustment to changes in the external environment. 



Reception and Conduction. The eye is an organ of special sense con- 

 taining the retina which is made up of cells receptive to changes in the 

 environment brought about by light rays (Fig. 4.3). The cornea, lens, and 

 fluids of the eye function as refracting surfaces which, in the normal eye, bend 

 the light rays entering the eyeball from a distance of more than 20 feet so that 

 they are brought to a focus on the retina (Fig. 4.13). If, during its develop- 

 ment, the eyeball grows out of proportion to the refracting surfaces, defective 

 vision will result. The near-sighted individual has an eyeball too long for its 

 refracting surfaces, and light rays focus in front of the retina; objects at a dis- 

 tance seem blurred. Contrary to popular opinion, such an individual does 

 not see close objects any better than a normal person does. The far-sighted 

 person has an eyeball that is too short for the refracting surfaces, and light 

 rays focus behind the retina; this, too, results in blurred images. If appro- 

 priate corrective lenses are placed in front of the eyeball, the light rays can 

 be brought to focus on the retina, and objects are seen distinctly. 



When an object is closer than 20 feet, a change in the shape of the crystal- 

 line lens of the eye must occur in order for the light rays to be focused on 



Fig. 4.13. Eyeballs of different shapes; note 

 effect on the focal point for light rays travel- 

 ing more than 20 feet. 



Far-sighted 

 (Hyperopic) 



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