NERIUM. 



[ 536 ] 



NERVES. 



Flagellate Infusoria. Free, solitary; fla- 

 gella two, arising- from the middle of the 

 ventral side. N. oHvitcea,Temiorm; length 

 1-1000" ; freshwatei-. (Kent, Inf. 405.) 



NERIUM. See Stomata, and Liber 

 (p. 461). 



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 ; cord, 

 with which must also be placed the ganglia. 

 These parts are composed essentially either 

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

 elements. 



The nei-ve-tibres may be arranged in two 

 groups, the meduUated and the non-medul- 

 lated. 



The meduUated nerve-tubes are most nu- 

 merous in the white portion of the nervous 

 centres and in the nerves. They are slender, 

 soft, cyhndrical filaments, varving in diam- 

 eter from 1-20,000 to 1-1 iOO". When 

 quite recent, they are transparent and appa- 

 rently homogeneous (fig. 509, 1), but they 

 really consist of three distinct parts— an 

 enveloping membrane or sheath ; a tenacious 

 liquid, the white substance of Schwann ; 

 and a soft, elastic, and probably fibrous 

 axis, the axis-cylinder, or band of Remak. 



The outer sheath or tubular membrane 

 of the nerve-tubes is a thin structureless 

 transparent membrane (fig. 510, 2, 3 a, 4 «), 

 containing nuclei. It is very visible and 

 thick in the nerves of the mesentery of the 

 frog, and in the electrical organs of the tor- 

 pedo. It is furnished with annular con- 

 strictions at regular intervals. 



Within the outer sheath is a hollow 

 cylinder or tube (figs. 509, 5i, 510, 3b, 4 b)— 

 the white substance of Schwann. It is 

 homogeneous 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. 510, Ji, 511), or becoming granular 

 externally, and remaining liquid internally. 

 It is also easily altered by pressm-e, some- 

 times escaping in globides or masses of 

 various forms (myeline), 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 vari- 

 cose appearance (fig. 511). It is frequently 

 called the medullary sheath, and those nem-es 

 possessing it are meduUated. 



The third structure exists within the last, 



in the form of a rounded or flattened, pale, 

 fibrous band, occupying the axis of the tube, 

 and called the axis-cylinder (figs. 509, 2 b, 

 3 c, 4 a, 5; 510, 1 b)\ this is the essential 

 nervous element. 



Fig. 509. 



/. 



5- 



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

 in tlie 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 con- 

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

 with serum ; a, sheath ; b, w hite substance with a double 

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

 from the human fourth ventricle : n, axial fibre. 5. Two 

 isolated axial filires, with a portion of the white sub- 

 stance adherent to the right-hand one. 



Magnified 350 diameters. 



These three structures of nerve are some- 

 what difiicult of demonstration. The outer 

 sheath may sometimes be shown by pressing 

 the nerve-tube, which forces out the white 

 substance. Boiling the nerves in absolute 

 alcohol, with the subsequent addition of 

 caustic alkali, or in acetic acid, when crys- 

 tals of fat separate from the white substance 

 (fig. 510, 1), will answer the same purpose. 

 Strong nitric acid, and afterwards potash, 

 causes the white substance to exude; and 

 the axial fibre being dissolved, the yellow 

 sheath is left empty and very distinct. So- 

 lution of corrosive .^sublimate has also been 



