THE TISSUES. 33 



structure of these nerve-fibres is therefore histologically of a similar 

 grade to that of muscular-fibre, and the only difference between 

 the two is in the quality of the differentiated protoplasm, which in 

 one case gives rise to muscle, and in the other to nerve-substance. 

 Such fibres are to be seen in the Invertebrata as well as in 

 Amphioxus and the Cyclostomi. In the higher Vertebrata they are 

 present only in the sympathetic nervous system. 



b) Further differentiation gives rise to a second stage of the 

 nerve-fibre. The nerve-substance, which lies beneath a more or less 

 delicate envelope, is differentiated into a chord which traverses the 

 axis of the fibre — the axis-cylinder — and into a fatty substance 

 which surrounds it. The latter, known as the medullary cylinder 

 (medullary sheath), gives a highly refractive contour to the nerve- 

 fibre, and can be separated from the axis-cylinder only by artificial 

 means. The homogeneous sheath which surrounds the medullary 

 cylinder — the neurilemma — contains the nuclei which are the remains 

 of the cells from which the fibre was formed. So far as is yet 

 known this form obtains in the Gnathostomous Vertebrata only. 



2) The other form-element of nervous tissue is represented by 

 cells, which are called ganglion-cells, as they are principally 

 present in the swellings (ganglia) of the nervous system. They 

 form the central apparatus. Their substance is generally finely 

 granular in character, with many other peculiarities which cannot 

 be entered into more closely here. The nucleus, which as a rule 

 is provided with distinct nucleoli, lies in the middle of the granular 

 substance ; this latter is often limited by an external membranous 

 and firmer layer. A very complicated structure is ascribed to these 

 cells, and is explained by every observer in essentially different 

 ways, so that the questions involved appear to be still far from 

 being settled. 



The ganglion-cells possess processes by which they are con- 

 nected partly to one another, and partly to nerve-fibres. They 

 form therefore the points of origin of the nerve-fibres. It is not 

 yet settled how ganglion-cells, which are devoid of processes, and 

 therefore completely isolated, can be of any service. The fact is, 

 that the belief in their existence grows less and less every day. 

 The processes of the nerve-cells vary greatly in number, as well 

 as in their relation to the fibres ; the only point to be noted about 

 them is that in the differentiated fibres it is the axis-cylinder which 

 is continued into the substance of the cell, while the medullary 

 cylinder ceases at some distance from it, or, rather, is no longer 

 differentiated. The relations of the axis-cylinder to the substance 

 of the cell appear to vary greatly, and are in many points a problem 

 still* 



* 



* Solbrig, A., Ueb. d. fein. Strucfcnr der Norvenelemente der Gasteropoden. 

 Leipzig, 1S72. 



