694 LIFE : OUTLINES OF GENERAL BIOLOGY 



Nerve fibres arise as outgrowing prolongations of ganglion cells, 

 which extend themselves in the embryo in a manner reminiscent of 

 an Amoeba sending out pseudopodia. In a suitable medium the 

 fibres may continue growing for weeks from a fragment of isolated 

 embryonic nervous tissue, and the occurrence of something like 

 amoeboid movement at the tip is a good instance of what may be 

 called the conservatism of Organic Evolution. 



Nervous tissues always arise from the outer or ectodermic layer 

 of the embryo, as might indeed be expected. For this is the layer in 

 which, in the course of history, it has been most important that 

 protoplasmic irritability should find expression. For the outer 

 surface is, to begin with, the layer most directly amenable to 

 external stimulus. But why some ectodermic cells should form 

 epidermis and scales, while others form sensory patches or sink in 





Fig. 103. 

 Extraordinarily branched nerve-ending of a sensory neuron. .^fterCajal. 



to become ganglia, is one of the unanswered questions of embryology, 

 raising as it does the whole problem of differentiation. 



A cross-section of a nerve usually shows several bundles of nerve 

 fibres, each bundle enclosed in a connective tissue sheath (peri- 

 neurium) with ingrowing partitions. Like electric wires in some non- 

 conducting substance, the bundles are embedded in a matrix of 

 connective tissue; and in this there may be seen an artery, a vein, 

 and a lymphatic vessel, l^nsheathing the whole nerve there is an 

 outer sheath of connective tissue, the epincurium. 



A nerve fibre itself is soft, cylindrical, glassy, with constrictions 

 or nodes at intervals, and with peripheral nuclei in the intcrnodal 

 regions. In the centre of a main fibre there is the neuraxon or axis 

 cylinder, and between it and the external neurilemma there is, as 

 mentioned, in most Vertebrate nerve fibres a whitish "medulla" of 

 fatty myehn. When the axis cylinder reaches a muscle fibre it spreads 

 out into a branched motor-ending with many nuclei. The neuri- 

 lemma sheath of the nerve fibre becomes continuous with the 



