FIBRES  may be medullated or non-medullated, but, whichever they are, they consist of the

long process or axon of a nerve cell; in a non-medullated nerve this process is either naked or enclosed in a delicate membrane called the primitive sheath or neurilemma, but in a medullated nerve the process or axis cylinder is encased by a white fatty substance called myelin, and so the term " myelinated " is often used instead of " medullated " for these nerves (see fig . 1) . Axis Outside this white sheath the neurilemma is cylinder present in most nerves, but is lost when they are massed to form the white matter of the central nervous system and in the optic nerve . Myelin At regular intervals the myelin is interrupted by some substance which stains deeply with silver nitrate, and these breaks are known as nodes of Ranvier . They do not, however, affect the axis cylinder . In a large nerve, such as the median, the nerve fibres are collected into small Primitive bundles called funiculi, enclosed in a connective sheath tissue sheath, the perineurium, and separated from it by a lymph space . From this sheath delicate processes penetrate among the fibres, and these are known as the endoneurium . The funiculi are collected into bundles called fascicuti, and the whole nerve consists of a variable number of fasciculi surrounded by a dense fibrous sheath, the epineurium . The various bundles do not remain distinct, but break up and re-arrange themselves, so that following them up with the scalpel is a difficult and tedious work . The nerve fibres, however, never join one another and are often several feet in length .