328 



ORIGINS OR ROOTS OF THE NERVES. 



The fibres of a nerve may be traced to some depth in the substance of the brain 

 or spinal cord, and hence the term " apparent or superficial origin " has been 

 employed to denote the place where the root of a nerve is attached to the surface, in 

 order to distinguish it from the " real or deep origin " which is beneath the surface 

 and concealed from view. If the deep origin be traced out, it will usually be found 

 that the nerve-fibres arise from portions of the grey substance of the nerve-centre : 

 such a portion of grey substance is termed the " nucleus of origin " of the nerve. 



In the case of the efferent nerves it would appear that the individual nerve-fibres 

 originate as prolongations of the nerve-cells in the grey substance. In fact, as the 

 researches of His have shown, these nerve-fibres have grown out from the nerve-cells 

 (neuroblasts) within the embryonic nerve-centre (see Embryology, Development of 



Fig. 383. TRANSVERSE SECTION OF THE SPINAL CORD OF A CHICK ON THE QTH DAY OF INCUBATION, 

 PREPARED BY GOLGI'S METHOD (Ramon y Cajal). 



A. Axis-cylinders of anterior root-fibres issuing from large cells of the anterior cornu, C. 



B. Posterior root-fibres passing from the bi-polar cells of the spinal ganglion, E, into the posterior 

 column of the spinal cord (D), where they bifurcate (d) and become longitudinal. 



e, f, g, collateral branches from these fibres, passing into the grey matter. 



Nerves). In the case of afferent nerves it seems to be clear that these in most if not 

 all cases have grown into the nerve-centre from the cells of the ganglia, or from nerve- 

 cells in the peripheral organs of special sense. Having entered the nerve-centre (fig, 

 383, D) the afferent fibres appear usually to bifurcate at least this is the case with 

 the fibres of the posterior spinal roots and the two resulting branches become 

 longitudinal, sending off lateral ramuscles (e,f, g) into the grey matter, in which they 

 appear to break up into fine ramifications, without being directly continuous with 

 nerve-cells of the grey substance. 



In the nerve-roots the fibres are bound up together, as they pass towards the 

 foramina of exit from the cranio-vertebral canal, by a stout external sheath continuous 

 with the pia mater and receiving an accession from the arachnoid and dura mater as 

 the roots pass through those membranes. This sheath sends in strong septa which 

 branch and unite with one another in the nerve-root, and thus divide it up irregu- 

 larly into bundles of fibres, which have not the same cylindrical character, with 

 special lamellated sheaths of perineurium, as the funiculi of the peripheral nerves, 



