83 



NERVOUS SYSTEM. 



parated and raised up ; on the white substance which in- 

 vests the tuber annulare and its four elongations received 

 from the brain and cerebellum, it is not so red, it becomes 

 firmer, more adherent, not so easily torn; on the spinal 

 marrow and nerves it increases in density, becoming whit- 

 ish and resisting. 



Q. What purpose does this density of the spinal pia 

 mater subserve? 



#. It serves as a protection and support to the spinal 

 marrow, especially where the marrow, not filling the ver- 

 tebral canal, would, without this firm coat, be liable to 

 concussion. 



Q. What effects have acids on this nervous coat? 



Jl. They harden it immediately, but it soon regains 

 its softness. Water hardens the nerves exposed to its ac- 

 tion. 



Q. Whence does the medullary matter of the nerves 

 originate? 



. It is continuous with the medulla of the brain and 

 spinal marrow. 



Q. In what nerves is it most abundant? 



ft.. In the auditory and optic nerves. In the progress of 

 a nerve to its termination it diminishes. 



Q. What general influence has loss of natural moisture 

 on the whiteness of parts? 



/?. It destroys their whiteness. 



Q. In what respects do the cerebral and nervous me- 

 dullary substances differ and agree ? 



#.. The cerebral medulla putrefies soon, the nervous 

 slowly. Acids harden both; alkalies soften them. The ce- 

 rebral substance is digestible; the nerve is not. 



Q. Is the medulla the same in all the nerves? 



