88 THE NERVOUS SYSTEM OF THE FROG. 
centres whence impulses originate, while the nerve fibres serve 
to convey those 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: these form the greater part 
of the cranial and spinal nerves. 
Take a small piece of the sciatic or some other nerve from 
a freshly killed frog: spread out and tease in a drop of normal 
salt solution: examine with low and high powers: 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. 
Tease in glycerine a small piece of nerve that has been treated 
with osmic acid: examine with high power a single nerve fibre : 
note 
i. The medullary sheath: stained black in conse- 
quence of its fatty nature: interrupted at inter- 
vals by 
ii, The nodes of Ranvier: spots where the medullary 
sheath is absent, and the primitive sheath forms 
constrictions touching 
iii, The axis cylinder: a central cylindrical rod, the 
essential part of the nerve fibre: visible at the 
nodes: much less deeply stained than the medul- 
lary sheath. 
iv. Nuclei: project into the medullary sheath about 
midway between the nodes. 
