The processes of the nerve cells are invested by sheaths to form nerve fibres. From their distribution these may be described as:
1. Peripheral, extending between the nerve cell and the body structures generally; the peripheral fibres are gathered into bundles held together by connective tissue, and constitute the peripheral or distributed nerves.
2. Central, extending between the nerve cell and the deeper parts of the central nervous system; these make up the tracts of the brain and spinal cord.
The essential component of each nerve fibre is the nerve cell process which forms the central core or axis cylinder (Attention may be directed to the fact that, althoubh the axis cylinders of sensory nerves are dendrites and of motor nerves are axons, no difference between them in structure can be made out, even with the microscope.), It consists of a series of fine axisfibrillae extending along the whole length of the fibre and continuous with the neurofibrillae of the cell body (Fig. 14). The majority of the axis cylinders, are surrounded by a thick whitish fatty material known as the white substance of Schwann or medullary sheath. This substance is not uniformly continuous along the course of the fibre but is interrupted at regular intervals, which are known as the nodes of Ranvier (Fig. 15).
Nerve fibres possessing the medullary sheath are spoken of as medullated fibres; they make up the greater part of the peripheral nerves and the tracts of fibres within the brain and spinal cord. Fibres belonging to the sympathetic nervous system are not sheathed by this substance of Schwann; they are, therefore, described as non-medullated fibres (Fig. 16).
On the outside of the medullary sheath - or, in the case of a non-medullated fibre, forming the only covering for the axis cylinder - is a thin structureless membrane known as the neurilemma or primitive sheath of Schwann.
It is a continuous sheath and is not interrupted at the nodes of Ranvier; each internodal portion, however, possesses a nucleus. There is no neurilemma round the nerve fibres in the cerebro-spinal axis.
At their extreme terminations all the nerve fibres lose their sheaths and end as naked axis cylinders.
Human nervous system
The Animal Cell
Nerve Cells and Nerve Fibres
Nature of the Nerve Impulse
Specific Energy of Nerves
General Construction and Development of the Nervous System
The Spinal Cord
The Chief Fibre Systems of the Cerebro-Spinal Axis
The Areas of Localisation on the Cerebral Cortex
The Sense Organs
Human Brain Anatomy
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