STRUCTURE AND ARRANGEMENT OF THE NERVE ELEMENTS. 625 



tudinal growth of the fibers takes place through elongation of the individual 

 interannular segments and at the same time by the formation of new segments. 



The ganglia have been considered partly as cells, partly as more complex 

 structures. There are to be distinguished: 



i. Central ganglia (Fig. 220, I), occurring partly as large cells (up to 150 // in 

 diameter, visible to the naked eye, in the anterior horns of the spinal cord), 

 partly as small cells (from 4 to 9 // in diameter, deficient in protoplasm, in the pos- 

 terior horns, in many parts of the cerebrum and cerebellum and in the retina), 

 spherical, ovoid, or pear-shaped, with numerous processes that often give to the cells 

 a star-shaped appearance. The brothers Landois found the ganglia of young insects 

 much smaller than those of adult insects. A similar statement is made also by 

 Schwalbe with respect to these cells and their nuclei. The cell-body is without 

 a capsule, of soft consistence and exhibiting a reticular structure, or a finely 

 fibrillar structure which extends into the processes. Between the fibrillae. is 

 everywhere distributed yellow or brown finely granular pigment either heaped 

 up at some particular part of the cell or disseminated throughout the entire cell. 

 The relatively large nucleus is clear, granular or reticular, and in early life without 

 a membrane. It contains little or no chromatin-substance. The nucleus contains a 

 nucleolus, which in the fresh state is angular and motile, and after death is spheri- 

 cal, highly refractile, staining feebly, and often in turn contains a smaller granule. 



On treatment with precipitating and coloring materials (alcoholic methylene- 

 blue) the cell-substance can be demonstrated to contain chromophilic granular 

 masses, so-called Nissl bodies, which are discernible with greater difficulty in the 

 fresh and living state, and which appear with varying degrees of distinctness in 

 different nerve-cells. In the motor ganglion-cells these bodies are arranged in 

 concentric parallel layers. Electrical irritation induces contraction of the cell, 

 causes it to appear darker and brings about a closer approximation of the Nissl 

 bodies. Section of the motor fiber emanating from the ganglion-cell leads to 

 granular degeneration and diminution in the number of the bodies. If the nerve- 

 fiber undergoes regeneration the cell again acquires its normal appearance. Ac- 

 cording to Lugaro the cells of the spinal ganglia become altered and finally 

 degenerate after division of their peripheral fibers, the nucleus becomes smaller, 

 while the Nissl bodies increase in number and size and become grouped about the 

 nucleus. Various poisons (strychnin, alcohol, tetanus, also uremia, autointoxica- 

 tions, inanition, anemia, prolonged high fever, heat-stroke, etc.) cause various 

 changes in the ganglion-cells, mainly affecting the Nissl bodies. The cells of the 

 cerebral cortex are affected in a peculiarly specific manner by each poison. 



Of the processes given off by the ganglion-cells there are a large number that 

 break up soon after their origin, like an intricately branching root, into numer- 

 ous, delicate fibers presenting a varicose appearance, and designated den- 

 drites or protoplasmic or secondary processes (Fig. 220, I, y). These dendrites 

 conduct cellulipetally and form an intricate terminal network. The dendrites 

 of adjacent cells do not anastomose with one another, but lie in close rela- 

 tion, entering merely into contact. Neither do the fibers of the dendrites 

 give rise to nerve-fibers passing in a peripheral direction. The group of terminal 

 filaments of a dendrite is designated a terminal network or a telodendron. In 

 addition to the dendrites, the ganglion-cell gives off a process of considerable 

 length, arising by a conical base, then pursuing a uniformly simple course, and 

 conducting in a cellulifugal direction. This process is designated an axone or 

 a neurite (an axis-cylinder or principal process) (I, z). The neurite is often con- 

 tinued peripherally into a nerve-fiber. Within the central nervous system it gives 

 off here and there delicate branches that are designated collaterals. These 

 break up into a fine terminal network, the radicles of which penetrate between 

 the elements of the central organs. The neurites from the ganglion-cells of the 

 cerebral and cerebellar cortex divide after a short course in a complex manner. 

 Also these fine divisions come in contact with other nerve-elements in the central 

 organ. Thus nerve-cells are connected only by the contact of their delicate 

 processes. Moreover, a nerve-fiber never passes directly over into the histological 

 elements of a nerve end-organ, the conducting fiber being only in telodendritic 

 contact with that organ. If certain nerve-tracts are especially used and exercised, 

 the altered function of the ganglion-cells in question may perhaps be explained 

 by a further penetration of the dendrites into additional areas of the interstitial 

 tissue, into which they had hitherto not penetrated. Demoor and Heger ob- 

 served changes in the dendrites and neurites of the brain after irritation, cocain- 

 anesthesia and the application of cold. 

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