222 THE INTELLIGENCE SERVICE 



This sheath is present on the nerve fibres which are concerned 

 with rapid adjustments to alterations in environment. Peripheral 

 non-medullated fibres generally belong to the second system of 

 nerves and have a slower reaction rate as befits the slower adjust- 

 ments of the structures they innervate. 



1. Structure. A nerve is really a compound structure composed 

 of parallel bundles of nerve fibres (funiculi), each of which is 

 enclosed in lamellar connective tissue (perineurium). The nerve 

 fibres in a funiculus are supported by a delicate fibrous investment 

 — the endoneurium. The whole collection of funiculi — the nerve — 

 is united together and to neighbouring tissues by epineurium. 



Each fibre is an anatomical and physiological unit, and runs an 

 uninterrupted course between central cell (brain, spinal medulla, 

 sympathetic ganglion, etc.) and peripheral end organ (in muscle, 

 gland, sense organ, etc.). Each consists of a long thread of 

 protoplasm (axon) drawn out from and continuous with the 

 cytoplasm of the nerve cell. 



The neuron (cell and fibres), like any other cell, is a colloidal 

 fluid mass, (i.) This may be demonstrated by examination of the 

 living nerve by means of the ultra-microscope, when particles in 

 Brownian movement will readily be seen. Some of these particles 

 at times clump together to form large aggregates which again 

 dissociate, (ii.) Carlson has shown that nerves may be stretched 

 without altering their efficiency, judged by rate of conduction of 

 an impulse, (iii.) Macallum states that alterations in surface tension 

 can be detected especially in the growing nerve, (iv.) It has been 

 urged by Gothlin that, as a nerve is doubly refracting to a slight 

 extent just like muscle it must have a similar composition. These 

 facts all go to prove that nerve is of a liquid nature. 



2. Its function is to conduct. One cannot lay too much stress 

 on the fact that it does not conduct an impulse originating outside, 

 as a telephone wire conducts current from a battery. The battery 

 is an integral part of the neuron (cf. Irritability of Cell, Chap. XII.). 



3. The nature of the stimulus seems immaterial. Mechanical, 

 electrical or chemical stinudi all cause the nerve to propagate the 

 same kind of impulse. Furthermore, the excitatory result of the 

 propagated impidse depends not on the nature of the " trigger " 

 stimulus but on the nature of the mechanism to which the nerve 

 goes. That is, stinudation of the sciatic nerve by electrical, 

 mechanical, thermal or chemical means causes contraction of the 

 gastrocnemius muscle ; stimulation of the vagus ner\'c by any 

 means slows the heart, stimulation of the chorda tympani causes 

 the salivary glands to secrete, no matter how the stimulation is 

 effected. These are all outgoing nerves. The predominating 

 fibres in them carry impulses to the periphery to produce action of 



