NEKVES. 



[ 537 ] 



NERVES. 



recommended. The axial band is best seen 

 in nerves treated with strong acetic acid, or 

 cold absolute alcohol, ether, chromic acid, 



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

 acid and alcohol : a, sheath ; 6, axial band ; c, crystals 

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

 soda. 3. From the human fourth ventricle, after treat- 

 ment with soda : a, sheath ; b, white substance exuding 

 in drops : the axial band has been removed in the pre- 

 paration. 4. Human, treated with soda : a, sheath; 6, 

 white substance ; the axial band not visible. 

 Magnified 350 diameters. 



. 511. 



Human nerve-tubes, showing tubes of various sizes ; 

 some with a single, others with a double outline ; some 

 varicose, others with the white substance in a granular 

 state. 



Magnified 350 diameters. 



&c., and staining fluids. Osmic acid so- 

 lution hinders the coagulation of the medul- 



lary substance, which is oily and very re- 

 fractive, and turns it black. 



Chemically, the sheath and axial band 

 consist of a proteine compound, and the 

 white substance of a mixture of cerebrine 

 with lecythine. 



In the non-medullated nerves the fibres 

 are of three kinds. The primitive nerve- 

 fibrils present the simplest form, and are 

 very fine and thread-like. They are only 

 visible with powers greater than 500 linear, 

 and may be traced to their junction with 

 ganglion-cells and thicker nerve-fibres. No 

 internal structure can be distinguished ; and 

 they become varicose here and there with 

 reagents, such as osniic acid, finally becom- 

 ing diffluent. They abound in the neigh- 

 bourhood of the termination of other nerves, 

 for instance, in the retina (see EYE). 



The second kind of fibres have been called 

 naked axis-cylinders, and are thicker than 

 the primitive fibrils. They are transparent, 

 and more or less striated longitudinally, 

 being composed, like the axis-cylinders of 

 medullated nerves, of minute longitudinal 

 fibrils. When connected with nmltipolar 

 ganglion-cells, this fibrillation is often di- 

 stinct under the action of reagents. They 

 have only been traced for short distances j 

 and it is evident that they and the primitive 

 fibrils may become covered with a sheath 

 and merge into the medullated type. The 

 third kind consists of a thicker or thinner 

 bundle of primitive nerve-fibrils, according 

 to the kind of axis-cylinder present, united 

 together by a structureless, perfectly trans- 

 parent, extremely thin nucleated tissue 

 the tubular sheath of the medullated fibres. 



In the cerebro-spinal nerves, the nerve- 

 tubes are aggregated into bundles, and sur- 

 rounded by an envelope of connective tissue, 

 called the neurilemma, in which blood- 

 vessels ramify, thus corresponding with the 

 arrangement of the primitive fibrillse of 

 muscle. Sometimes, towards the termina- 

 tions of the nerves, the neurilemma appears 

 as a homogeneous membrane with elongated 

 nuclei. 



Branching or division of all the nerve- 

 fibres, except the non-medullated primitive 

 kind, occurs occasionally in and near the 

 nervous centres and in the nerve-trunks, and 

 frequently in the periphery. The best 

 example of peripheral divison is in the 

 electric eel, where one medullated fibre 

 divides millions of times to supply the sub- 

 cutaneous fat-like organ. Division into 

 fibres may be seen in sections of the spinal 



