) 



768 THE NERVOUS SYSTEM 



sent in from without, either as ingrowths of the developing pia mater, the most proximal of the 

 membranes, or is carried in with the blood-vessels, of the walls of which it is an abundant 

 component. Practically, the neuroglia as a connective tissue proper differs from white fibrous 

 tissue only in origin and in its chemical or staining properties. Based upon the latter, there are 

 methods of technique by which the two may be distinguished. White fibrous tissue is derived 

 from the middle germ layer or the mesoderm, whUe neuroglia comes from the ectoderm. The 

 epithelium lining the central canal of the spinal cord and the ventricles of the encephalon, with 

 which the canal is continuous, is the remains of the mother tissue of the neuroglia, and in the 

 adult is the only vestige representing its origin. The cells of this epithelium are known as 

 ependymal cells, and they are usually classed as a variety of neuroglia. 



Axones, with their medullated or non-medullated sheaths (nerve fibres) 

 comprise all nerves in the periphery and all nerve tracts in the central system. 



White substance [substantia alba] (''white matter") consists of a portion of 

 nervous tissue in which medullated fibres predominate. The myelin sheaths, 

 being in the form of a fat emulsion, reflect the entire spectrum and thus appear 

 white. 



Grey substance [substantia grisea] ("grey matter") is a portion of nervous 

 tissue in which medullated axones do not predominate. Thus sympathetic 

 ganglia and sympathetic nerves may be grey, though the term is usually applied 

 to grey portions of the central system, such as the cerebral cortex, the central 

 grey column of the spinal cord, etc. Such grey regions contain more cell-bodies 

 of neurones than other regions, though at least half of their volume may consist 

 of neuroglia, white fibrous connective tissue, blood-vessels, and axones of both 

 varieties. 



Neurone chains. — As noted above, the numerous neurones comprising the nervous system 

 are functionally and anatomically related to all the other tissues of the body and to each other. 

 A functionally complete nerve pathway extends from the tissue in which the nerve impulse is 

 aroused to the tissue in which a resultant reaction occurs. It is known that the simplest 

 possible of such paths necessarily comprises at least two neurones. The great majority involve 

 a greater number. The axone of one neurone bearing impulses from the peripheral tissue 

 transfers the impulses to the dendrites or cell-body of another by synapsis, and the axone of this, 

 in the same way, transfers them to another and so on tiU the final neurone receives the impulses 

 and the telodendria of its axone transfer the impulse to the tissue element which reacts in re- 

 sponse to the stimulus brought. Neurones are thus linked together in chains. A neurone 

 chain may be defined, therefore, as a number of neurones associated with each other in series 

 to form a functionally complete nerve pathway. Examples of the simplest forms of neurone 

 chains as contained in the spinal cord are illustrated in fig. 610. An impulse aroused in the skin 

 is borne by the spinal ganglion neurone to the spinal cord where, in the left half of the figure, 

 telodendria of one of the terminal branches of its axone form synapses with a neurone in the 

 ventral horn, and the axone of this bears the impulse out of the spinal cord to transmit it proba- 

 bly direct to skeletal muscle. This arrangement involves but two neurones and is supposed 

 to be relatively rare. In the right half of the figure, a third neurone is seen interposed. This is 

 a neurone, numerous in grey substance everywhere, whose axone is relatively short and branches 

 frequently, making possible several synapses in the near neighbourhood of its parent cell-body. 

 Its type is referred to as the Golgi neurone of type II. This interposed, gives a chain of three 

 neurones between the origin of the impulse in the periphery and the contraction of muscle in 

 response. Simple chains hke these can result only in reflex activities and such chains are often 

 called reflex arcs. Another chain is indicated in the figure in which the reflex action involves 

 involuntary or smooth muscle. This must involve at least one sympathetic neurone, and, should 

 the Golgi neurone of type II form synapses with the ventral horn neurone involved, a chain 

 composed of four neurones results. In the more extensive and complex neurone chains, such 

 as those in which the impulse from the skin, as above, ascends to the cerebral cortex and the 

 resultant muscular contraction is thrown under cerebral control, each of the several neurones or 

 links in the series is not only referred to by name according to the position of its cell-body, but 

 each is often called according to its order in the series, as "neurone of first order," "second 

 order," "third order," etc. 



A given axone may break into a considerable number of branches each of which forms 

 synapses with a diflFerent second neurone, or, if peripheral, the telodendria of each branch may 

 terminate upon a separate peripheral tissue element. Thus, a given impulse aroused in a 

 peripheral tissue element may be transmitted to an ever increasing number of neurones, 

 and the initial neurone may comprise the first link in a number of neurone chains. Such is 

 quite general in the structural plan of the nervous system throughout. It is thought possible 

 to consider each nourono interposed in a chain as a separate source of energy, a sort of relay 

 in the nerve path; that the impulse passing through the axone is gradually weakened in over- 

 coming resistance, but, when transferred to another neurone, it incites a splitting into usable 

 form of the substance represented by the tigroid masses and thus a liberation of energy or a 

 reinforcement of the impulse. Further, thus is made possible the economy of one neurone 

 serving as a link in a number of neurone chains. 



The axones (nerve fibres) taking part in the various neurone chains course in bundles of 

 varying size, the larger of which have names. And there is a general tendency with axones of 

 the .same function and the same origin to course in company with each other. A fibre bearing 

 impulses from the peripheral tissues to the central system is an afferent fibre or sensory fibre. A 

 fibre bearing impulses out of the central system to peripheral tissues is an efferent fibre or motor 



