SENSE ORGANS AND NERVOUS COORD/NAT/ON 503 



association areas lie between the sensory and motor regions (Fig. 29.15). 

 Presumably such complex mental processes as learning, memory, thought 

 and imagination occur here. It these areas are destroyed, one loses the 

 ability to comprehend symbols and formulate expressions, a condition 

 known as aphasia. In one type of aphasia, words are heard, but they 

 might as well be in an unknown language for they cannot be recog- 

 nized. The ability to learn and understand is not localized in any 

 particular association area; instead, the cerebral cortex appears to 

 function as a whole in the higher mental processes. In many injuries 

 the nature of the lost ability is correlated more with the amount of 

 cortex destroyed than with the specific part destroyed. Biologists and 

 psychologists are just beginning to understand the functioning of the 

 human brain, and many of its aspects are beyond our comprehension 

 at present. 



Questions 



1. Describe what happens to a ray of dim light that enters the eye from a point near the 

 observer. Through what structures does it pass; what, if any, adjustments are neces- 

 sary to make it fall upon the retina; and how does it activate a receptor cell? 



2. What effect did the transition from water to land during the course of vertebrate 

 evolution have upon the eyeball and surrounding structures? 



3. Describe how we become aware of a loud sound of low frequency. 



4. How has the ear changed during evolution from fish to mammal? What part of the 

 ear has changed very little? 



5. Describe the electrical changes that occur in a neuron during the transmission of a 

 nerve impulse. 



6. List the major categories of neurons that make up the nervous system. Which ones 

 are involved in a spinal reflex? 



7. Distinguish between the roots and rami of a spinal nerve. 



8. \Vhat are the major differences between the cranial nerves of mammals and fishes? 



9. Define the autonomic nervous system. How does autonomic innervation differ from 

 the innervation of other organs? 



10. How do the dorsal and ventral columns of the spinal cord differ? 



11. List the five divisions of the brain and the major brain structures that develop in 

 each. 



12. In what ways has the structure and function of the cerebral hemispheres changed in 

 the evolution from fish to mammals. 



13. Briefly state the function of each of the following: medulla, cerebellum, thalamus, 

 hypothalamus. 



14. What is believed to be the function of the association areas of the cerebral hemi- 

 spheres? What happens if they are destroyed? 



Supplementary Reading 



The structure and physiology of all of the sense organs of man are considered care- 

 fully by Geldard in The Human Senses. The fascinating story of the evolution of the 

 vertebrate eye and its adaptation to all environments in which vertebrates live are con- 

 sidered in Walls' monograph, The Vertebrate Eye. Stevens and Davis. Hearing, Its Psy- 

 chology and Pliysiology, is a valuable reference work on the ear. Recent investigations on 

 the nerve impulse are summarized in an article by Katz, The Nerve Impulse, in Flan- 

 agan's The Physics and Chemistry of Life. Gardner's Fundamentals of Neurology is a 

 good, concise account of the morphology and physiology of the human nervous system. 



