NERIUM. 



J 



NERVES. 



NERIUM. See STOMATA and LIBER, 

 p. 387. 



NERVES and NERVOUS CENTRES. 

 The nervous system is usually regarded 

 as consisting of two parts : the nerves, which 

 are divided into the cerebro-spinal and the 

 sympathetic; and the nervous centres, re- 

 presented by the brain and spinal chord, 

 with which must also be placed the ganglia. 

 These parts are composed essentially of 

 either nerve-tubes, nerve-cells, or of both 

 these elements. 



The nerve-tubes or primitive nerve-fibres 

 are most numerous in the white portion of 

 the nervous centres and in the nerves. They 

 are slender, soft, cylindrical filaments, vary- 

 ing in diameter from 1-20,000 to 1-1100". 

 When quite recent, they are transparent and 

 apparently homogeneous (fig. 514, 1), but 



Fig. 514. 



Nerve- fibres. 1 . From nerves of the dog and rabbit, 

 in the natural state : a, fine, b, moderate, c, large fibre. 

 2. From a frog, after the addition of serum: a, drop 

 forced out by pressure ; b, part of the axial fibre contained 

 in it. 3. From the human spinal marrow, treated with 

 serum : a, sheath ; b, white substance with a double out- 

 line; c, axial fibre. 4. Fibre with double outline, from 

 the human fourth ventricle : a, axial fibre. 5. Two iso- 

 lated axial fibres, with a portion of the white substance 

 adherent to the right-hand one. 



Magnified 350 diameters. 



they really consist of three distinct parts, 



an enveloping membrane or sheath, a tena- 

 cious liquid, and a soft but elastic internal 

 fibre. 



The sheath of the nerve-tubes is a very 

 delicate, structureless and transparent mem- 

 brane (fig. 515, la, 2, 3 a, 4 a) ; it is not 

 demonstrable in the smallest fibres, although 

 probably always present. 



Fig. 515. 



Nerve-tubes. 1. From a frog, after boiling with acetic 

 acid and alcohol : a, sheath ; b, axial band ; c, crystals of 

 fat. 2. Isolated sheath of a frog's nerve boiled with soda. 



3. From the human fourth ventricle, after treatment with 

 soda : a, sheath ; b, white substance exuding in drops : 

 the axial band has been removed in the preparation. 



4. Human, treated with soda : a, sheath ; b, white sub- 

 stance ; the axial band not visible. 



Magnified 350 diameters. 



Within the sheath is a hollow cylinder or 

 tube (%s. 514, 3 b, 515, 3, 4 b), called the 

 white substance of Schwann. It is homo- 

 geneous and tenacious in perfectly fresh 

 nerves, but soon after death becomes coagu- 

 lated, sometimes externally only, giving a 

 double outline to the walls of the nerve- 

 tubes (fig. 5 14, 2, 3, 4), or becoming granular 

 externally, and remaining liquid internally. 

 It is also easily altered by pressure, some- 

 times escaping in globules or masses of va- 

 rious form, from the ends or the broken 

 sides of the tubes, at others accumulating at 

 intervals in various parts of the tubes, giving 

 them an elegant varicose appearance (fig. 

 516). 



The third structure exists within the last, 

 in the form of a rounded or flattened, pale, 

 elastic band or fibre, occupying the axis of 

 the tube, and called the axial band (figs. 514, 

 2 b, 3 c, 4 a, 5 ; 515, 1 a). 



These three structures of nerve are some- 

 what difficult of demonstration. The outer 

 sheath may sometimes be shown by pressing 



