THE NERVOUS TISSUES. 



81 



FIG. 98. 



enclosing the individual nerve-cells. These latter possess, in 

 general, a spherical form, and are usually provided with one or two, 

 seldom more, processes ; in the bipolar cells the processes frequently 

 pass from opposite poles to become continuous with the afferent 

 and efferent fibres. In the ganglia of some of the lower vertebrates 

 bipolar cells occur in which one process becomes invested by the 

 turns of the other or spiral fibre. Unipolar cells exist in which the 

 single process divides into T-branches extending almost at right 

 angles ; such cells occur also in man. 



The development of all nerve-fibres and nerve-cells must be 

 referred to the elements derived from the invaginated ectoderm 

 forming the neural tube. Without entering 

 upon an exhaustive account of the process, 

 many details of which are still uncertain, it may 

 be accepted that the primary neural ectoderm 

 differentiates into two varieties of cells the 

 neuroblasts and spongioblasts. The nerve- 

 fibres are formed as outgrowths from the primi- 

 tive nerve-cells or neuroblasts. This may take 

 place either in one direction alone, from the 

 centre towards the periphery (centrifugally), as 

 in the formation of the efferent fibres of the 

 motor-nerve roots of the spinal cord ; or, as in 

 the production of the afferent (sensory) nerves, 

 the neuroblasts may be somewhat removed from 

 the central nervous mass, occupying the position 

 of the spinal ganglia, and send out fibre-pro- 

 cesses in two directions, one set growing into 

 the nerve-centre (centripetally}, while a second 

 group of fibres extends towards the periphery 

 (centrifugally}. In all cases the nerve-fibres 

 are formed as outgrowths front the primary 

 nerve-cells ; in later stages the cells concerned 

 in extending the nervous path may disappear 

 after the establishment of the tract. The spongioblasts, on the 

 other 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 medullary 

 substance nor neurilemma. The acquisition of the white substance 

 of 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 its 

 origin must be referred to the more or less direct agency of the ele- 



6 



Ganglion nerve-cell with 

 spiral fibre from the sym- 

 pathetic of frog : Sp F, the 

 spiral fibre surrounding the 

 siraight process (Cf) and 

 dividing at a node of Ran- 

 vier (7A); s, neurilemma. 

 (After Schiefferdecker.) 



