CHAPTER IV 



Spinal Frog 



DESTROY the brain of a frog by inserting a pointed bit of wood 

 into the skull cavity from the gap between the back of the skull 

 and the spinal column, which can be felt by taking the frog in a 

 cloth and bending the head downwards. A large pin may be 

 found more convenient to destroy the brain, since it penetrates the 

 skin more easily ; but the bit of wood should afterwards be inserted 

 to stop bleeding. 



After a few minutes' time, reflex movements can be elicited by 

 pinching the toes, dropping dilute acid on the skin, applying a hot 

 wire or an electrical stimulus. All of these are effected without 

 conscious sensation, since the brain is absent. 



Central Nervous System 



The general arrangement of the nerve centres in the frog should 

 be observed at this stage. Take a frog which has lain in methy- 

 lated spirit for a day or two. Remove the bony covering of the 

 skull by inserting one point of a strong finely pointed pair of 

 scissors through the membrane between the skull and the spinal 

 column. Also remove the arches of the vertebrae posteriorly from 

 the same place. Note the nerves connecting the nerve centres 

 to all parts of the body ; especially the optic nerves conveying 

 impulses from the eyes and the fact that the sciatic nerve, which 

 we have seen to cause muscular movement, is given off by the 

 spinal cord. 



Nerve 



Structure. Tease a piece of fresh sciatic nerve of the frog in 

 0.7 per cent, saline, obtaining the fibres as long and straight as 

 possible. It is not very easy to separate individual fibres uninjured. 

 Note the double contour of each fibre, due to the highly refracting 

 medullary sheath, and the interruptions of this sheath at intervals 

 (Nodes of Ranvier). The material of the medullary sheath may 

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