NERVES AND THEIR CONTROL 



313 



2. Just back of the cerebrum find the mid-brain. The 

 dorsal part is enlarged into two large lobes, known as optic 

 lobes. Are there branches leading from 



them ? Where do they lead ? Note. 

 The term 'optic' refers to sight. 



3. How do the optic lobes compare in 

 size with the cerebrum ? 



4. The hindmost part of the brain is 

 known as the medulla. To what does it 

 join posteriorly ? 



Conclusion. Write a paragraph tell- 

 ing what you can of the nervous system 

 of the frog. 



Observation. Study Figure 79. The 

 cell much branched is known as a neuron, 

 or a unit of the nervous system. The 

 brain and spinal cord contain many 

 millions of them. 



Conclusions. 1. If a branch from one 

 should end in a muscle, in which direc- 

 tion would messages travel through it ? 



2. If a branch from a neuron should 

 end in the sensitive part of the tongue 

 or tip of the finger, in what direction would messages travel 

 through it? 



--^Dendritea 



Cell Body 



Neuraxon 



-Medulla 



. Node ofHanvier 



---Neurilemma 



Nerve-ends 



7> 



FIG. 79. Diagram of a 

 neuron or nerve unit. 



b. Reactions 

 1. REFLEX 



Observations. Note. Reflex action is involuntary action tak- 

 ing place before the brain has had time to aid or interfere in 

 any way. 



1. Feign a blow at a person's face. Result ? 



2. Tickle the inside of the nose with a feather. Result ? 

 Conclusion. What do you judge is the especial reason for 



each of the reflex actions just noted ? 



