732 



NEUROLOGY 



regarded as the cranial portions of the sympathetic system. The sympathetu 

 nervous system includes those portions of the nervous mechanism in which a medul- 

 lated nerve fiber from the central system passes to a ganglion, sympathetic or 

 peripheral, from which fibers, usually non-medullated, are distributed to such 

 structures, e. g., bloodvessels, as are not under voluntary control. The spinal and 

 sympathetic ganglia differ somewhat in the size and disposition of the cells and in 

 the number of nerve fibers entering and leaving them. In the spinal ganglia (Fig. 

 638) the nerve cells are much larger and for the most part collected in groups near 

 the periphery, while the fibers, which are mostly medullated, traverse the central 



portion of the ganglion; whereas in 



Nerve-cells of ganglion the sympathetic ganglia (Fig. 639) 

 the cells are smaller and distributed 

 in irregular groups throughout the 

 whole ganglion; the fibers also are 

 irregularly scattered; some of the 

 entering ones are medullated, while 

 many of those leaving the ganglion 

 are non-medullated. 



Neuron Theory. The nerve cell 

 and its processes collectively con- 

 stitute what is termed a neuron, and 

 Waldeyer formulated the theory 

 that the nervous system is built up 

 of numerous neurons, "anatomically 

 and genetically independent of one 

 another." According to this theory 

 (neuron theory} the processes of 

 one neuron only come into con- 

 tact, and are never in direct con- 

 tinuity, with those of other neu- 

 rons; while impulses are transmitted from one nerve cell to another through 

 these points of contact, the synapses. The synapse or synaptic membrane seems 

 to allow nervous impulses to pass in one direction only, namely, from the terminals 

 of the axis-cylinder to the dendrons. This theory is based on the following facts, 

 viz. : (1) embryonic nerve cells or neuroblasts are entirely distinct from one another; 

 (2) when nervous tissues are stained by the Golgi method no continuity is seen even 

 between neighboring neurons; and (3) when degenerative changes occur in nervous 

 tissue, either as the result of disease or experiment, they never spread from one 

 neuron to another, but are limited to the individual neurons, or groups of neurons, 

 primarily affected. It must, however, be added that within the past few years the 

 validity of the neuron theory has been called in question by certain eminent histol- 

 ogists, who maintain that by the employment of more delicate histological methods, 

 minute fibrils can be followed from one nerve cell into another. Their existence, 

 however, in the living is open to question. Mott and Marinesco made careful 

 examinations of living cells, using even the ultramicroscope and agree that neither 

 Nissl bodies nor neurofibrils are present in the living state. 



For the present we may look upon the neurons as the units or structural elements 

 of the nervous system. All the neurons are present at birth which are present in 

 the adult, their division ceases before birth; they are not all functionally active 

 at birth, but gradually assume functional activity. There is no indication of any 

 regeneration after the destruction of the cell-body of any individual neuron. 



Fasciculi, tracts or fiber systems are groups of axons having homologous origin 

 and homologous distribution (as regards their collaterals, subdivisions and ter- 

 minals) and are often named in accordance with their origin and termination, the 





FIG. 639. Transverse section of sympathetic ganglion of cat. 

 A. Ganglion. X 50. o. A nerve cell. X 250. 



