GENERAL PHYSIOLOGY OF THE NER- 

 VOUS SYSTEM AND ELECTRO- 

 PHYSIOLOGY. 



GENERAL CONCEPTION OF THE NERVOUS SYSTEM. 



STRUCTURE AND ARRANGEMENT OF THE ELEMENTS OF THE 

 NERVOUS SYSTEM. 



With relation to the general comprehension of the structure and function of 

 the nerve-elements, two opposed views are held at the present time. According 

 to the one the neuron, that is a ganglion-cell with all of its processes, is to be 

 considered as the independent physiological unit of nervous tissue. The various 

 neurons are not in immediate and direct connection with one another. The axis- 

 cylinders of all nerve-fibers arise from ganglion-cells, and not from a network of 

 fibers. All nerve-fibers terminate finally by means of terminal arborescences or 

 telodendrites. It is only through these terminal filaments that the nerve-elements 

 are connected by contact, the minute radicles being applied to one another. The 

 nerve-cells and the nerve-fibers have each a distinct physiological importance, the 

 cells acting as the physiological centers (for automatic or reflex movement, for 

 sensation, perception, for trophic and secretory functions), the fibers, which 

 always originate from the nerve-cells as processes, representing a conducting 

 apparatus. 



The more recent view rejects the neuron as the physiological unit and con- 

 siders the fibrillary substance or the neuropile as the medium of nervous activity. 

 The fibrillary substance is present in the great mass of gray matter, which repre- 

 sents a fine lacework or network of nerve-fibrils. It can be seen further in the 

 nerve-cells and in the fibers passing off from them. The higher the plane of devel- 

 opment in the animal scale the less numerous are the nerve-cells in proportion to 

 the fibrillary structure, the ganglion-cells serving only as nutritional centers for 

 the metabolism of the nerve-tissue. As Bethe has shown that reflex activity 

 persists in crabs even after the ganglion-cells have been extirpated, conduction 

 must obviously take place in the mass of fibers exclusively. The neuron thus 

 loses its significance in the physiological sense and also from the histological 

 standpoint. 



The nerve-fibers are of several varieties: 



1. The simplest form of nerve-fibers are the primitive fibrils or axis- fibrils 

 (Fig. 220, i), distinguishable only with high powers of the microscope. They 

 occur as delicate fibers, presenting at varying intervals slightly varicose or spindle- 

 shaped thickenings (postmortem change), and they can be recognized by the 

 brown color that develops after the application of gold.chlorid. They appear in 

 part in the vicinity of tne terminal distribution of the nerves, resulting from the 

 fibrillation of the axis-cylinders, as, for example, in the layer of fibers of the 

 optic nerve in the retina, in the terminal distribution of the olfactory fibers, in 

 the net-like connection at the terminal distribution in unstriped muscle, and in 

 part in the gray substance of the brain and the spinal cord as the most delicate 

 processes of divided dendrites. 



2. Naked axis-cylinders (Fig. 220, 2) represent bundles of primitive fibrils, 

 which are characterized by most delicate longitudinal striation, separated by a 

 number of fine granules. They are encountered in most exquisite form as neurites 

 of central ganglion-cells (Fig. 220, I, z). 



3. Axis-cylinders surrounded by neurilemma or the sheath of Schwann, from 

 3.8 to 6.8 [i in thickness, and designated non-medullated or gray nerve-fibers. The 

 sheath of these fibers is a delicate, elastic cylinder composed of flattened cells 



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