. Text-book of normal histology: including an account of the development of the tissues and of the organs. inal ganglia, and send out fibre-pro-cesses in two directions, one set growing intothe nerve-centre {centripetally), while a secondgroup of fibres extends towards the periphery{centri/ugally). In all cases the nerve-fibresare formed as outgrowths from the prhnarynerve-cells; in later stages the cells concernedin extending the nervous path may disappearafter the establishment of the tract. The spongioblasts, on theother hand, are especially concerned in the production of the neuro-glia-cel
. Text-book of normal histology: including an account of the development of the tissues and of the organs. inal ganglia, and send out fibre-pro-cesses in two directions, one set growing intothe nerve-centre {centripetally), while a secondgroup of fibres extends towards the periphery{centri/ugally). In all cases the nerve-fibresare formed as outgrowths from the prhnarynerve-cells; in later stages the cells concernedin extending the nervous path may disappearafter the establishment of the tract. The spongioblasts, on theother hand, are especially concerned in the production of the neuro-glia-cells, these ultimately becoming transformed into the close reticu-lar formation supporting the nervous elements. The nerve-fibres are at first pale and possess neither medullarysubstance nor neurilemma. The acquisition of the white substanceof Schwann occurs much later, the exact mode of its production,however, being by no means certain; whether the medullary sub-stance owes its formation to the influence of the axis-cylinder, or itsorigin must be referred to the more or less direct agency of the ele- 6. Ganglion nerve-cell withspiral fibre from the sym-pathetic of frog : Sp F, thespiral fibre surrounding thestraight process (,Cf) anddividing at a node of Ran-vier {Th); s, neurilemma.(After Schiefferdecker.) g2 NORMAL HISTOLOGY. ments represented by the nerve-nuclei, future investigation mustdetermine. If traced to the axis-cyhnder, the sheath must be classedas ectodermic tissue ; as mesodermic, on the other hand, if referredto the nerve-corpuscles. The period at which the medullary coatappears in the various groups of nerves is variable, but constant forgiven tracts ; account has been taken of this fact with great advantagein the laborious investigations of tracing the i)ath of many nerve-tracts composed of medullated fibres. The neurilemma may beregarded as certainly derived from the differentiation of surroundingmesodermic cells, as likewise the more general connective-tissuee
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