The fibres in a nerve are bound together by loose connective tissue, the endoneurium, into the primitive bundles, which are again united by the perineurium, a membrane of laminated connective tissue, into more definite funiculi seen by the naked eye, the secondary bundles.
The secondary bundles are tied together by connective tissue, in which are found fat-cells and in which run the fine blood-vessels supplying the nerves. This connective tissue has been named the epineurium, and its condensed outer layers constitute the sheath of the nerve. It is important to observe that the connective tissue of the nerves is permeated by lymphatics which penetrate to the nerve-fibres, so that these are brought in contact with, and as it were, bathed in, the lymph.
Each nerve-fibre runs an isolated course from end to end, without anastomosing with other fibres, and near its peripheral termination it usually divides into two or more branches.
The fibres of the peripheral nerves depend for their integrity and nutrition upon their connection with central organs. The large multipolar cells of the anterior horns of gray matter of the spinal cord preside over the nutrition of the motor fibres; the ganglia on the posterior roots of the spinal nerves over the nutrition of the sensitive fibres.
If a nerve be severed from its connection with these centres of nutrition, it in a short while undergoes degenerative changes which result in complete destruction of its fibres.
The nerve-fibres when in a state of functional activity conduct impressions along their length to the end-organs or to the nerve-centres with which they are in connection. This property of the fibres we call their conductivity. Each fibre conducts impressions in an isolated manner, not communicating them to other fibres with which it may be in contact. The rapidity of this conduction in human nerve-fibres is estimated at 33.9 meters (about 38 yds.) per second. This rate may be diminished by cold or by the anelectrotonic condition which is induced in the nerve by the passage through it of an electric current.
The nerve-fibres are irritable; that is, the application to them of stimuli excites their functional activity, and the impression made by the stimulus is transmitted to their extremities.1
1 The nerve-fibres in man do not appear to attain their full irritability until the fifth or tenth month after birth (Soltman).
The natural or physiological stimuli of the nerves act upon their extremities. Either they act through the peripheral mechanisms, giving rise to impressions which are conducted centripetally to the cells of the nerve-centres and there cause sensations or reflex actions, or they act upon the nerve-centres, giving rise to impulses which are conducted centrifugally and cause the various phenomena of contraction of muscles, inhibition of contraction, secretion, etc. Besides the physiological, there are other stimuli which excite the functional activity of nerve-fibres when applied at any point along their course.
Mechanical stimuli, blows, concussions, pressure, traction, etc., excite the nerves, causing sensations when applied to sensitive nerves, or contraction of muscles when applied to motor nerves. When mechanical stimuli are pushed farther, the irritability of the nerves may be destroyed. The gradual application of mechanical stimuli may destroy the irritability of nerve-fibres without any exhibition of excitation, as in paralysis from pressure. In nerve-stretching it is probable that many of the results depend upon the mechanical stimulation of the nerve-fibres by the traction. With a certain amount of force used the irritability of the nerve may be increased; carried farther, both the irritability and the conductivity may be diminished, and finally destroyed. As the centripetal fibres are soonest affected in the stretching, we can see how this proceeding is most beneficial in neuralgias, where a potent factor, if not the cause of the disease, is an abnormal excitability of the nerve-fibres. It is to be observed, nevertheless, that in cases of continued pressure upon mixed nerves the motor fibres are the first to suffer loss of their conductivity.