THE NERVOUS SYSTEM 



delicate neurofibrils embedded in a semifluid neuroplasm. It is surrounded by 

 a relatively thick myelin sheath and a nucleated membranous neurilemma sheath. 



Fig. 25. Primary motor neuron (diagram- 

 matic): ah, Implantation cone of axon; ax, 

 axon; c, cytoplasm; d, dendrites; m, myelin 

 sheath; m', striated muscle; n, nucleus; ri, 

 nucleolus; nR, node of Ranvier; sf, collateral; 

 si, neurilemma ; tel, motor end-plate. (Barker, 

 Bailey.) 



Fig. 26. Portions of two nerve-fibers 

 stained with osmic acid (from a young rabbit). 

 Diagrammatic. 425 diameters: RR, Nodes of 

 Ranvier, with axis-cylinder passing through; a, 

 neurilemma; c, opposite the middle of the seg- 

 ment, indicates the nucleus and protoplasm ly- 

 ing between the neurilemma and the medullary 

 sheath. In A the nodes are wider, and the in- 

 tersegmental substance more apparent than in 

 B. (Schafer, in Quain's Anatomy.) 



The myelin sheath consists of a fatty substance, myelin, supported by a retic- 

 ulum of neurokeratin. The latter, not seen in the living fiber, may be a coag- 

 ulation product produced during fixation. The highly refractive myelin gives 



