94 THE NERVOUS SYSTEM OF THE FROG. 



C. Histology of Nerves. 



Nervous matter consists histologically of elements of two 

 kinds, nerve cells and nerve fibres. The nerve cells are 

 branching nucleated cells, connected by their processes with one 

 another and with the nerve fibres. The nerve cells are the 

 centres whence impulses originate, while the nerve fibres serve 

 to convey these impulses from one part to another. A local 

 .accumulation of nerve cells is called a ganglion. 



1. Nerve Fibres are of two kinds, medullated and non- 

 medullated. 



a. Medullated nerve fibres form almost the whole of the 

 cranial and spinal nerves, and a large part of the brain 

 and spinal cord. 



Take a small piece of the sciatic or some other nerve from a 

 freshly killed frog : spread it out and tease it in a drop of normal 

 salt solution: examine with low and high p)owers: note 



i. The nerve fibres : unbranched. 



ii. The perineurium, or connective tissue binding the 

 nerve fibres into bundles, or " nerves." 



In each nerve fibre note 



iii. The primitive sheath, or sheath of Schwann: a 



very delicate external investment, seen with 

 difficulty, and only in certain places. 



iv. The medullary sheath : a thick fatty layer within 

 the primitive sheath ; it is highly refractive, and 

 gives the nerve fibre its double contour. 



Tease in glycerine a small jnece of nerve that has been treated 

 with osmic acid : examine with the high power a single nerve 

 fibre: note the following p)oints. 



i. The medullary sheath is stained darkly in conse- 

 quence of its fatty nature : it is interrupted at 

 intervals by the nodes of Ranvier. 



ii. The nodes of Ranvier are spots where the medul- 

 lary sheath is absent, and the primitive sheath 

 forms constrictions touching the axis cylinder. 



iii. The axis cylinder is the central cylindrical rod, 

 the essential part of the nerve fibre : it is clearly 



