l6o THE TISSUES. 



The segmental structure of nerve-fibers would seem to give 

 the impression that they are formed by a number of cells fused end 

 to end. After what has been said with regard to ganglion cells and 

 their processes, this can be the case only so far as the nerve-sheaths 

 are concerned. According to this theory, the formative cells of the 

 latter gather in chains along the neu raxes or dendrites, forming a 

 mantle around them, and in the adult nerve-fibers taking the shape 

 of the segments or internodes just described (His, 87 ; Boveri, 85). 

 The points at which the sheath-cells are joined would then corre- 

 spond to the nodes of Ranvier. Other investigators have concluded 

 that the whole nerve-fiber is developed from a terminal apposition 

 of ectodermal cells. In this case not only the sheaths of the fibers 

 but also the corresponding portions of the nerve processes are 

 formed by them (KupfTer, 90). In both theories the neurilemma 

 corresponds to the cell-membrane ; in the former the neurilemma 

 nucleus corresponds to that of the sheath-forming cell, in the latter 

 to that of the formative cell of the whole nerve segment. It should 

 be noticed that, according to the second theory, a fiber segment is 

 the product of a single cell, while according to the first it is evolved 

 from at least two cells (ganglion cell (process) and sheath-forming 

 cell). The former theory is now very generally accepted. 



The nonmedullated nerve-fibers, Remak? s fiber's, possess no 

 medullary sheath ; the axial cord shows nuclei which can be re- 

 garded as belonging to a thin neurilemma. The majority of the 

 neuraxes of the neurones of the sympathetic nervous system are of 

 this structure, although small medullated nerve-fibers (the neuraxes 

 of sympathetic neurones) are found in certain regions. 



All nerve-fibers, medullated as well as nonmedullated, in the 

 central and peripheral nervous systems lose the sheaths here de- 

 scribed before terminating ; the axis-cylinders (axial cords) ending 

 without special covering (naked axis-cylinders). These terminal 

 branches are, in fixed and stained preparations, beset with small 

 thickenings varicosities which vary greatly in size and shape. 

 Nerve-fibers presenting such appearances are spoken of as varicosed 

 fibers. The varicose enlargements may be regarded as small 

 masses of neuroplasm ; the fine uniting threads, as representing the 

 axial fibrils. 



In the peripheral nervous system the nerve-fibers are grouped 

 to form nerve -trunks. The nerve-fibers, as has been stated and as 

 will be seen from the diagram (Fig. 122) on the next page, are the 

 neuraxes of neurones, the cell-bodies of which are situated in the 

 spinal cord or brain and in the sympathetic ganglia, and the den- 

 drites of peripheral sensory neurones, the cell-bodies of which are 

 found in the spinal and homologous cranial ganglia. 



In the nerve-trunks the nerve-fibers are gathered into bundles 

 termed funiculi. The nerve-fibers constituting such a bundle are 

 separated by a small amount of fibro-elastic tissue, containing here 

 and there connective-tissue cells, the cndoneuriinn. This is continu- 



