A GENERAL VIEW OF THE NERVOUS SYSTEM 



33 



The last is continuous for the whole length of the fiber, 

 but the medullary sheath is broken at short intervals by 

 little spaces called nodes. Some fibers 

 have only the neurilemma, and are there- 

 fore gray in color ; for it is the medullary 

 sheath which gives the characteristic shin- 

 ing white appearance to nerves and nerve 

 fibers. Those fibers possessing the sheath 

 are called medullated nerve fibers; those 

 without it are called nonmedullated. 



>_ Node of 

 Kanvier 



-Neurilemma 



\--Nucleua 



DEMONSTRATIONS 



Neuraxon 

 ~~ or Axis 

 Cylinder 



Medullary 

 ' 'Sheath 



Fig. 26. -Por- 

 tion of a me- 

 dullated nerve 

 fiber. 



6. The Brain. The brain of the sheep will be 

 found to be very satisfactory in demonstrating to a 

 class the general structure of this portion of the cen- 

 tral nervous system. The brain of the cat, dog, or 

 ox may be used instead. The brain can be removed 

 from the skull by sawing away the roof of the latter 

 and with a scalpel cutting the attaching membranes 

 and nerves. The brain should be prepared some 

 days, or even weeks, before it is needed by the class, 

 and hardened and preserved in some suitable me- 

 dium. Strong alcohol, a 2 to 5 per cent solution of 

 formalin (formol) in water, and a 2 to 5 per cent 

 solution of bichromate of potash are very good hard- 

 ening and preserving reagents. But more satisfactory is the following 

 mixture : 95 per cent alcohol, six parts ; 2 per cent solution of formalin, 

 four parts. When specimens are preserved in a fluid containing for- 

 malin, they should be soaked in water a short time before using, to 

 avoid the irritating effects of formalin vapor on the eyes, etc. Where 

 possible, each student should be provided with one of the preserved 

 specimens. A brain recently removed should be at hand, but it will 

 be found to be too soft for much careful study. With care one pre- 

 served brain may be made to suffice for an entire class. After exami- 

 nation the specimens may be preserved for more detailed study later 

 on. In studying the brain follow the descriptions of the general text. 

 MACY'S PHYS. 3 



