THE FROG 55 
ments proper, called nerve cells, ganglion cells or neurons, and 
(b)- neuroglia cells and connective tissue, whose function is to 
support and bind the nerve cells together. 
A nerve cell or neuron consists of a compact or angular cell 
body, containing the nucleus, and numerous slender processes of 
which one or more may be very long and are distinguished as 
axons, or nerve fibers. The essential part of an axon or nerve 
fiber is the axis cylinder, and in some cases this is the only part 
present; but usually the axis cylinder is enclosed in two additional 
layers, described below, which are not properly parts of the nerve 
cell but are added to the axis cylinder from without. 
1. Nerve fibers. Cut out a piece of nerve from a freshly-killed 
frog and with needles tease it apart in normal salt solution. The 
nerve will be seen to consist of a large number of straight, un- 
branched nerve fibers, bound together by connective tissue. 
Examine with the high power. In a fiber make out: 
(a) The axis cylinder, forming an apparently homogeneous 
central axis. 
{b) The medullary sheath, or white substance of Schwann, 
forming a thick, highly refracting coat around the axis cylinder. 
The medullary sheath is composed largely of a fatty substance 
called myelin, which often becomes aggregated into irregular 
masses. 
(c) The neurilemma, or sheath of Schwann, a very thin mem- 
brane outside the white substance. It is best seen at the nodes 
(described below). 
(d) Nodes of Ranvier, constrictions where the medullary 
sheath is interrupted. The segments of the nerve between the 
nodes are called internodes. The nodes occur only at consider- 
able intervals. 
(e) The incisures of Schmidt, oblique markings at rather close 
intervals, across the medullary sheath. These are sometimes 
difficult to make out. 
(f) In each internode is a nucleus, lying just beneath the 
neurilemma and surrounded by a small amount of cytoplasm. 
In the development of a nerve fiber the axis cylinder appears 
first; as it grows out from the cell body, making its way through 
the other tissues, it becomes surrounded by nucleated cells which 
