General Physiology of the Nerves and 

 Electro-Physiology. 



321. STRUCTURE OF THE NERVE ELEMENTS. The nervous elements 

 present two distinct forms : 



T -Nerve V\hvP / Non-medullated. TT n f Of various forms 



I. Nerve-Fibres, j Medullate d. tt Nerve-Cells. j and functions 



An aggregation of nerve-cells constitutes a nerve-ganglion. The fibres represent 

 a conducting apparatus, and serve to place the central nervous organs in connection 

 with peripheral end-organs. The nerve-cells, however, besides transmitting impulses, 

 act as physiological centres for automatic or reflex movements, and also for the 

 sensory, perceptive, trophic, and secretory functions. 



I. (1) The non-medullated nerve-fibres occur in several forms : 



1. Primitive Fibrils. The simplest form of nerve-fibre, winch is visible with a magnifying 

 power of 500 to 800 diameters linear, consists of primitive nerve-fibrils. They are very delicate 

 fibres (fig. 368, 1), often with small varicose swellings here and there in their course, which, 

 however, are due to changes post-mortem. They are stained of a brown or purplish colour by the 

 gold-chloride method, and they occur when a nerve-fibre is near its termination, being formed 

 by the splitting up of the axis-cylinder of the nerve-fibre, e.g., in the terminations of the corneal 

 nerves, the optic nerve-layer in the retina, the terminations of the olfactory fibres, and in a plexi- 

 form arrangement in non-striped muscle (p. 461). Similar fine fibrils occur in the grey matter 

 of the brain and spinal cord, and in the finely divided processes of nerve-cells. 



2. Naked or simple axial cylinders (fig. 368, 2), which represent bundles of primitive fibrils 

 held together by a slightly granular cement, so that they exhibit very delicate longitudinal 

 striation with fine granules scattered in their course. The best example is the axial cylinder 

 process of nerve-cells (fig. 368, I, z). [The thickness of the axis-cylinder depends upon the 

 number of fibrils entering into its composition.] 



3. Axis-cylinders surrounded with Schwann's sheath, or Remak's fibres (3 '8 to 6 "8 jx broad), 

 the latter name being given to them from their discoverer (fig. 368, 3). [These fibres are also 

 called pale or non-medullated, and from their abundance in the sympathetic nervous system, 

 sympathetic.'] They consist of a sheath, corresponding to Schwann's sheath [neurilemma, or 

 primitive sheath, which encloses an axial cylinder ; while lying here and there under the sheath, 

 and between it and the axial cylinder are nerve-corpuscles. These fibres are always fibrillated 

 longitudinally]. The sheath is delicate, structureless, and elastic. Dilute acids clear the 

 fibres without causing them to swell up, while gold chloride makes them brownish-red. They 

 are widely distributed in the sympathetic nerves, [e.g., splenic], and in the branches of the 

 olfactory nerves. All nerves in the embryo, as well as the nerves of many invertebrata, are of 

 this kind. [According to Ranvier, these fibres do not possess a sheath, but the nuclei are 

 merely applied to the surface, or slightly embedded in the superficial parts of the fibre, so that 

 they belong to the fibre itself. These fibres also branch and form an anastomosing net-work 

 (fig. 370). This the medullated fibres never do. These fibres, when acted on by silver nitrate, 

 never show any crosses. The branched forms occur in the ordinary nerves of distribution, and 

 they are numerous in the vagus, but the olfactory nerves have a distinct sheath which is 

 nucleated.] 



(2) Medullated fibres occur also in several forms : 



4. Axis-cylinders, or nerve-fibrils, covered only by a medullary sheath, or white substance 

 of Schwann, are met with in the white and grey matter of the central nervous system, in the 



