196 



The Vertebrate Organ Systems 



is questionable if such a simple reflex arc actually ever exists, but all 

 activity except that of the highest type is composed of various combina- 

 tions of these simple, inherited automatic responses, reflexes. 



The simplest activity, such as the burning of the finger and with- 

 drawing it, would actually involve a number of these simple reflex arcs. 

 In this action, the conscious portions of the brain become aware of 

 what has happened only after the response of the muscular movement 

 has occurred. Another example of this simple type of reflex response 

 to a stimulus is the familiar knee jerk which occurs when the area below 

 the knee is hit sharply. 



.DORSAL FISSURE 



Fig. 58. — Diagram of a reflex arc. 



The Central Nervous System: the Brain. — It is among the 

 vertebrates that the great development of a centralized controlling 

 brain is found. Essentially the brain is but a series of enlargements of 

 the anterior portion of the dorsal nerve cord. Its gradual development 

 and increased complexity can be traced through the various vertebrate 

 groups. 



Nearly all groups have the same essential regions. Beginning at 

 the anterior end, the regions are as follows : the telencephalon, the 

 diencephalon, the mesencephalon, the metencephalon, and the myelen- 

 cephalon. 



Two main parts are recognizable in the telencephalon. The an- 

 teriormost of these are the olfactory lobes w'hich are concerned with the 

 sense of smell. Leading forward from these lobes are the olfactory 

 tracts which terminate anteriorly in the olfactory bulbs. From the 

 bulbs, nerves pass to the sensory receptors in the nasal region. In fish 

 and amphibians, these lobes are a very conspicuous part of the brain, 



