NERVE FIBRES OR TUBES. 261 



la appearing immediately within the membranous tube, and affording a 

 contrast to the portions which are toward the centre or axis. In this 

 state the nerve-tube presents the appearance of parallel lines toward its 

 periphery, the outer one corresponding to the membranous Membrane, 

 sheath, and the inner to the internal limit of the coagulated J^ l * tance 

 material. In this condition the tube is very prone to as- axis cylinder, 

 sume a beaded appearance, either by the influence of pressure, or even 

 spontaneously. Names have been given to distinguish these parts from 

 each other ; the central grayish portion is called the axis cylinder or axis 

 band, since it may be of a flattened shape ; and the material which sur- 

 rounds it, intervening between it and the membranous investment, is des- 

 ignated the medulla or white substance of Schwann. There can be no 

 doubt that the membranous tube, the white substance, and the axis cyl- 

 inder discharge different physiological functions. In chemical composi- 

 tion they also differ : the tube is a nitrogenized structure, the white sub- 

 stance oleaginous, and the axis cylinder is supposed to be nitrogenized 

 also. In the first development, the axis cylinder is first formed, and the 

 white substance then cast round if. 



If a portion of a nerve, a, Fig. 114, be placed 

 in concentrated acetic acid, the axis cylinders of 

 its included tubes will, in the course of a day 

 Axis cylinder of nerveiT or ^ ^ geen protruding in a brush-like form, 

 as at , the effect being very well shown when the nerve is sufficiently 

 slender to be subsequently examined by the microscope. 



The nerve fibres run in a direct course to their point of distribution. 

 Of their manner of termination we shall speak subsequently ; Terminal 

 here, however, it may be remarked, that occasionally they ex- branchings of 

 hibit preparatory terminal branchings, as shown in Fig. 115, 

 p. 262, observed by Kolliker in the case of the frog : tf, a being bifurca- 

 tions, b a trifurcation of a small twig from the cutaneous thoracic muscle. 

 Similar subdivisions of the ultimate ramifications have been noticed in the 

 amphioxus, fishes, insects, and it is certain that they also occur in man. 



The sheath of the nerve fibres is an elastic membrane, which is nei- 

 ther acted on by dilute alkalies nor by boiling, but is solu- Chemical reac- 

 ble in concentrated acetic acid and strong solutions of pot- 

 ash and soda. By nitric acid it is stained yellow, and, though fibres, 

 not identical with elastic tissue, has a certain resemblance thereto, ap- 

 proaching, however, more nearly to a protein substance. The axis cyl- 

 inder is, as is shown by its behavior with reagents, a protein substance, 

 differing, however, from syntonin and also from blood fibrin. From the 

 latter substance it is distinguished both by the difficulty with which it 

 dissolves in acetic acid and in a solution of nitre, from the former by its 

 insolubility in hydrochloric acid. 



