NERVOUS TISSUE 145 



sion in the myelin. Toward its distal end the fiber usually branches, 

 and the branches are given off at the nodes. The myelin then becomes 

 thin, so that the fiber is surrounded merely by neurolemma (Fig. 131, d), 

 and finally this ends. The naked axis cylinder then breaks up in its 

 terminal arborization, forming the motor organs attached to striated 

 muscle fibers. In comparison with the size of its cell body, the neuraxon 

 shown in the diagram is too short; in extreme cases, as in the neuraxons 

 extending from the spinal cord to muscles in the foot, it may be actually 

 more than a meter long, or several thousand times the diameter of the cell 

 body from which it comes. 



The medullated nerve fibers were the first parts of the nerve to be studied micro- 

 scopically, and were referred to as "cylinders;" the central fiber was called the axis 

 cylinder. Remak (Obs. anat. et micr. de syst. nerv. structura, Berlin, 1838) was the 

 first to describe non-medullated nerves, which are still known as "Remak's fibers," 

 but their nervous nature was not readily admitted. Moreover. Remak recognized 

 that nerve fibers proceed from cells. Deiters (Untersuchungen iiber Gehirn und 

 Riickenmark, Braunschweig, 1865) supplemented these observations by showing 

 that all "ganglion cells" (referring to nerve cells within the spinal cord and brain) 

 are centers for two systems of true nerve fibers, (i) the generally broader and always 

 single and undivided axis cylinder process; and (2) the protoplasmic processes with 

 their extensive system of minute branches. He discussed whether the nerve cells 

 anastomose with one another, and concluded that all such anastomoses which had 

 been reported were due to deceptive appearances. Thus the nerve cells were 

 believed to communicate by contact and not by continuity. 



The confused mass of interwoven fibers which sections of nervous tissue ordinarily 

 present, is, therefore, not a general syncytium from which sensory and motor fibers run 

 out, but an orderly arrangement of branching cells. Striking proof of this was afforded 

 in Golgi's description of the olfactory bulb (1875). In the plate which accompanied 

 his publication, the cells in the different layers, and their various processes, were 

 drawn in black with absolute assurance; similar figures of "Golgi preparations" 

 are now seen in all treatises on the anatomy of the nervous system (Fig. 132). Golgi 

 found that if fresh tissue is placed in a solution of potassium bichromate and osmic 

 acid, and is later transferred to a solution of silver nitrate, a heavy black deposit 

 occurs in certain nerve cells, extending throughout their minutest ramifications, 

 whereas adjacent cells are wholly unaffected. The process must be carried out 

 with great care, and even then it is capricious; but this method has afforded funda- 

 mental information in regard to the forms of individual nerve cells. 



In order to emphasize that the nervous system is built up of separate 

 cells, the term neurone has been widely used to designate a complete nerve 

 cell, with all its branches. Fig. 131, therefore, represents a neurone, to- 

 gether with certain sheath cells. 



Recently, however, there has been a tendency to regard such a neurone as a 

 syncytium, and in the latest editions of his "Lehrbuch," Stohr adopts this interpreta- 

 tion. He states that in so far as the neurone includes peripheral nerve fibers, it is 

 a biological or syncytial unit, but not a single cell. It is considered to be a "biological 

 unit" since it is well known that the cell body of the nerve cell is the nutritive or 



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