426 THE TISSUES. 



elastic fibre (Fig. 274), in the centre of the tube, and usually. occu- 

 pying about one-third of its diameter. It is generally homogeneous, 

 though rarely faintly striated or finely granular ; is usually through- 

 out of uniform size, solid, and resembles coagulated albumen. It 

 generally pursues a straight course, but may be curved or slightly 

 undulating (never varicose), with an irregular border. Chemical 

 reagents also show that it contains not a trace of fat, but is an albu- 

 minous compound ; though it is not identical with the fibrine of 

 the blood, nor the peculiar element of muscular tissue (musculine). 

 Analogically with the latter immediate principle, it may be termed 

 nervine. 



The axis-fibre is found in all, even the very finest, nerve-tubes ; 

 and in the latter, it only can always be satisfactorily demonstrated. 

 Its size varies with that of the nerve-fibre itself. During life, how- 

 ever, it cannot be distinguished from the medulla which surrounds it. 



In the acoustic nerve of the sturgeon, Czerm&k has demonstrated 

 the existence of bifurcating axis-fibres in dividing nerve-fibres. 



2. The neive-medulla, or pulp, is a thick, viscid fluid, like thick 

 oil of turpentine, mostly composed of fatty matter, and filling all 

 the space between the axis-fibre and the inner surface of the neuri- 

 lemma. Consequently it occupies the two remaining, or external, 

 thirds of the diameter of the tube. In other words 5 it is itself a 

 viscid, fluid, hollow cylinder, completely inclosing and isolating the 

 solid axis-fibre, which is placed within it. Hence its designation, 

 also, as the medullary sheath. It has also been called the "white 

 substance of Schwann." 



The entirely different chemical reactions of the medulla and the 

 axis-fibre, would seem to demonstrate their distinct existence and 

 function; though the microscope does not distinguish them during 

 life. It is the medulla that gives the dark border to nerve-tubes 

 under the microscope ; and such are therefore termed medullated or 

 dark-bordered tubes. 



Although the axis-fibre exists in every nerve-fibre, many are met 

 with which have no medulla non-medullated fibres. These consist 

 of the neurilemma, the axis-fibre, and an intervening fluid, some- 

 times identical in appearance with the latter, and sometimes similar 

 but more clear. These rcow-medullated fibres are also found to occur 

 in continuation of the medullated, both when they communicate 

 with the nerve-cells in the encephalon and spinal cord, and also at 

 the peripheral extremities of the fibres. 



