363 GENERAL STRUCTURE AND FUNCTIONS 



The manner in which this is accomplished is as follows : 



The nervous system, however simple or however complicated it 

 may be, consists always of two different kinds of tissue, which are 

 distinguished from each other by their color, their structure, and 

 their mode of action. One of these is known as the white substance, 

 or the fibrous tissue. It constitutes the whole of the substance of 

 the nervous trunks and branches, and is found in large quantity on 

 the exterior of the spinal cord, and in the central parts of the brain 

 and cerebellum. In the latter situations, it is of a soft consistency, 

 like curdled cream, and of a uniform, opaque white color. In 

 the trunks and branches of the nerves it has the same opaque 

 white color, but is at the same time of a firmer consistency, owing 

 to its being mingled with condensed areolar tissue. Examined by 

 the microscope, the white substance is seen to be composed every- 

 where of minute fibres or filaments, the "ultimate nervous fila- 

 ments," running in a direction very nearly parallel with each other. 

 These filaments are cylindrical in shape, and vary considerably in 

 size. Those which are met with in the spinal cord and the brain 

 are the smallest, and have an average diameter of T<j^<j(7 f an 

 inch. In the trunks and branches of the nerves they average ^W 

 of an inch. 



The structure of the ultimate nervous filament is as follows: 

 The exterior of each filament consists of a colorless, transparent 

 tubular membrane, which is seen with some difficulty in the natural 

 condition of the fibre, owing to the extreme delicacy of its texture, 

 and to its cavity being completely filled^with a substance very 

 similar to it in refractive power. In the interior of this tubular 

 membrane there is contained a thick, semi-fluid nervous matter, 

 which is white and glistening by reflected light, and is called the 

 "white substance of Schwann." Finally, running longitudinally 

 through the centrab part of each filament, is a narrow ribbon- 

 shaped cord, of rather firm consistency, and of a semi-transparent 

 grayish color. This central portion is called the "axis cylinder," 

 or the " flattened band." It is enveloped everywhere by the semi- 

 fluid white substance, and the whole invested by the external tubu- 

 lar membrane. 



When nervous matter is prepared for the microscope and exa- 

 mined by transmitted light, two remarkable appearances are ob- 

 served in its filaments, produced by the contact of foreign sub- 

 stances. In the first place the unequal pressure, to which the fila- 

 ments are accidentally subjected in the process of dissection and 



