34 CLASSIFICATION OF TISSUES. 
What is the medullary sheath ? 
It is a sheath enveloping the axis cylinder and is made up of a semi- 
fluid translucent substance called myelin. When treated chemically it 
sometimes appears as though it had a number of incisions through its 
substance. These are called the incisures of Schmidt. 
Describe the neurilemma. 
It is a thin, structureless tube enclosing the medullary sheath. It 
is broken up into segments in each of which is an oval nucleus. 
What is the node of Ranvier ? 
At intervals in the course of a nerve fibre there are constrictions 
which extend through the fibre to the axis cylinder. In this way the 
medullary sheath is divided completely into segments. The neurilemma 
is also constricted and these constrictions are joined together by a 
cement substance which extends to the axis cylinder. 
What Ee the portions of nerve fibre between the nodes of Ranvier 
called ? 
Internodes or interannular segments. 
What are bundles of nerve fibre called ? 
Fasciculi or funicult. 
How is the connective tissue of nerves divided and what is each 
division called ? 
The fasciculi are bound together by connective tissue called the 
epineurium or peri-fascicular connective tissue. ach fasciculus is 
surrounded by several concentric lamelle called the perineurium, or 
lamellar sheath. While running between the nerve fibres are pro- 
longations of the tissue of the perineurium called the endo-neurium or 
intra-fascicular connective tissue. 
What is Henle’s sheath ? 
It is a tubular sheath formed by endothelial cells cemented together 
edge to edge and surrounding individual nerve fibres. 
How do nerve fibres terminate in nerve centres ? 
In nerve centres it is supposed that the axis cylinder uninvested 
by medullary or primitive sheath joins a process of the nerve cell, or 
that the axis cylinders break up into their primitive fibrils before 
entering the nerve cells. 
How do nerve fibres terminate at their point of distribution ? 
They may terminate, as in voluntary muscle, in distinctly nucleated 
flattened structures, called end plates, or as in involuntary muscle 
