SECT. 109.] NERVOUS SYSTEM. 2I % 3 



or out of varicose and ruptured parts of the sheath, and forms 

 larger and smaller drops of all possible forms, regularly spherical, 

 clavatc, fusiform, cylindrical, filiform, or of the most odd shapes, 

 which drops, like the nerve-fibres, may be coagulated only upon the 

 surface or throughout ; and, accordingly, like them, also, appear 

 double contoured, or partially or completely grumous. But while 

 within the tube, also, the medullary sheath alters, being accumu- 

 lated at certain places in larger proportion, instead of forming a 

 cylinder of uniform thickness. It is in this manner that the well 

 known varicose nerve-tubes arise (fig. 91), in which the medulla 

 presents sometimes beautiful moniliform swellings, sometimes 

 irregularly distributed nodosities of various size, or even complete 

 interruptions at some places. All these forms, in which the sheath 

 may or may not take part, but in which the central fibre is not 

 concerned, arise artificially, and are especially liable to occur in 

 the finer fibres, and those Avith more delicate sheaths, such as are 

 found in the central organs. 



The central fibre of the nerve-tubes (primitive baud, Remak ; 

 cylinder axis, Purkinje) (figs. 90, 2, 3, 4, 5), is a cylindrical or 

 slightly flattened fibre, which, in unaltered nerves, is as little 

 recognisable as the sheath, as it is completely surrounded by the 

 medulla, and refracts the light in the same degree as this; but 

 when the nerve-tube is torn through, or is treated with various 

 re-agents, the axial fibre readily comes into view, partly in the 

 interior of the tube, partly denuded, and is a constant structure. 

 In the natural condition, it is pale, mostly homogeneous, more 

 rarely finely granular or striated, bounded by straight, or occa- 

 sionally irregular pale contours, and usually of equal thickness 

 throughout ; it is especially distinguished from the nerve-medulla 

 by the circumstance, that although soft and flexible, it is not fluid 

 and viscid, but firm and elastic, somewhat like coagulated albumen, 

 with which it appears, for the most part, also, to agree in its 

 chemical characters. 



This so-called axis-cylinder is found in all nerve-fibres having 

 nerve-medulla, also in the finest, everywhere presents the same 

 characters, and ecmals in thickness nearly one-half or one-third of 

 the diameter of the nerve-fibre. 



The nerve-tubes, in which the three structures now described 

 can be distinguished, and which Ave shall designate as the medul- 

 lated or d:r!;-bordered, form the preponderating majority of those 

 found in the body; but there are some other forms deserving a 

 more particular notice. These are the nerve-tubes, in which there 

 is no trace of a nerve-medulla, and which possess a nerve-sheath, 



