130 



NERVOUS TISSUE. 



certain amount of granular protoplasm, may be seen near the middle of 

 each segment. The primitive sheath turns in at the nodes and surrounds 

 the axis-cylinder as this passes from one segment to the other, so that 

 tlie sheath is in a manner perforated by the axis-cylinder. The sheaths 

 of successive segments are not directly continuous over the nodes, but 

 are there separated by a certain amount of clear intersegmental substance 

 (shown in A. fig. 82), which forms the " constricting band " of 

 Eanvier, and is probably similar in nature to ordinary intercellular or 

 cementing substance (see p. 15), being like that stained by nitrate 

 of silver. The last-named reagent stains also the axis-cylinder in the 

 neighbourhood of the node, so that the fibres after this treatment 

 appear marked with little crosses (fig. 83) ; the transverse limb of the 

 cross being due to the ring of intersegmental substance, the longitu- 

 dinal to the axis-cylinder. Other staining fluids, such as the picrocar- 

 minate of ammonia, also act on the axis-cylinder at the nodes ; they 

 are probably prevented from reaching it elscAvhere owing to the presence 

 of the fatty matter in the surrounding medullary sheath. 



Fig. S3. 



Fig. S4. 



Fig. 83. — A Portion of a Small Nerve-Trunk from the Thorax of the IVIouse, 



TREATED WITH NiTRATE OF SiLVER (RilUvier). MaGNIFIEP. 



Cross markings are seen at the nodes, and tlie layer of flattened epithelioid cells which 

 covers the surface is also brought into view. 



Fig. 84. — Fibres from the Root of a Spinal Nerve. 



At a, where they join the spinal cord, they are varitose ; lower do\\ii at h, they are 

 uniform and larger (from Valentiu). 



No structure corresponding to Eanvier's nodes has, it is believed, 

 hitherto been observed in the non-medullated fibres ; they appear to be 

 absent also in the white fibres of the brain and spinal cord. 



Varicose fibres. — ^lany of the medullated nerve-fibres, when sub- 

 jected to the microscope, appear dilated or swollen out at short distances 



