212 



PRACTICAL HISTOLOGY. 



[XVII. 



the paraffin is extracted by turpentine, and the sections mounted in balsam. 

 They may be stained with a watery solution of Bismarck brown. Instead of 

 this, after fixing the nerve in osmic acid, and hardening in 90 per cent, alcohol, 

 place it for 24-48 hours in a strong saturated watery solution of acid fuchsin 

 (S.N. 30), wash in alcohol, embed and cut in paraffin. The ends of* the fibrils 

 on the axis-cylinder are stained red (fig. 195). 



(a.} Note the axis-cylinder in the centre surrounded by a dark ring (figs. 

 195, 196, b), the myelin blackened by the Os0 4 . If the section of a nerve- 

 fibre is through incisures, the double contour of the myelin may be seen 

 sometimes with a narrow black edge, at others with a broad black edge 

 externally. 



(ft.) If a piece of the nerve be placed for forty-eight hours in a solution of 

 Bismarck brown, and then teased, the appearance shown in fig. 195, a, is 

 obtained, when the axis-cylinder presents a longitudinally striated appearance. 



13. Size of Nerve-Fibres. The osmic acid method has yielded the best 

 results. Some of the fibres are broad, and others are narrow or fine. Thus the 

 anterior roots of the upper cervical nerves, and the third cranial nerve, contain 

 only broad nerve-fibres, while the second and succeeding thoracic nerves contain 

 broad and fine fibres (fig. 196). In nerves going to muscles there are many 

 large and few small medullated fibres, while in nerves going to viscera the 

 fine medullated fibres are far more abundant than the broad fibres. 



FlG. 196. A. T.S. of the anterior root of a spinal nerve below the first dorsal nerve. 

 B. T.S. of a part of a cervical nerve. Osmic acid. 



14. Living Nerve-Fibres are readily studied in the inflated lungs of a newt 

 or frog. The frog's lung is best kept inflated by Holmgren's apparatus. 1 

 Nerve-fibres are also readily seen in the tongue of a frog arranged as for 

 studying the circulation of the blood (Lesson XIX.). 



15. Marchi's Method for Degenerated Fibres. Harden a nerve in Miiller's 

 fluid for eight days and then in the following fluid : 



Marchi's Fluid. 



Miiller's fluid ... 2 parts. 



Osmic acid (i per cent) . . I part. 



Embed in celloidin and mount in warmed balsam. It is well not to employ 

 balsam dissolved in chloroform, as then the darkened parts lose their dark 

 colour. The degenerated parts of nerve-fibres are black. This method is 

 particularly useful for degenerations in the nerve-centres before sclerosis has 



1 Beitrage z. Awit. u. Fhys., C. Ludwig, gewidmet, Leipzig, 1874, p. cxvi. 



