SAEGENT: THE OPTIC KEFLEX APPARATUS OF VERTEBKATES. 145 



from the Avail of the canal. This, too, perhaps accounts for his state- 

 ment that he has observed two or more such fibres running parallel iu 

 the canal. 



In transverse sections of the spinal cord Reissner's fibre may usually 

 be found lying near the centre of the lumen of the canal. In such 

 sections it is inconspicuous, as it is small, and is often obscured by the 

 presence around it of corpuscles and loose cells (Sargent, :00, Fig. 13). 

 In thin sections the fibre is likely to drop out, since it is so slightly 

 supported in the relatively large lumen of the canal, which is usually 

 from ten to twenty times the diameter of the fibre ; or it may become 

 displaced and be seen in a more or less oblique position. 



The fibre in transverse section is best studied in tliose forms where it 

 is large, as in Cynoscion, Pomatomus, and Lophiatilus, in which it attains 

 a diameter of 10 micra, and in some selachians, where it is even larger. 

 In such sections the fibre shows a circular cross-section, and its cylin- 

 drical form is clearly made out by focussing. In series of transvei'se 

 sections the fibre can be followed from section to section throughout its 

 course, though it is studied to much greater advantage in longitudinal 

 sections. 



c. Internal Stmdure. The fibre of Reissner is highly organized, 

 having a definite internal structure. It consists essentially of numerous 

 axis-cylinders closely applied to each other, and surrounded by a single 

 thin medullary sheath of myelin. Cross-sections of the fibre in Cynoscion, 

 when double-stained with Ehrlich's hematoxylin and Congo red, show the 

 central portion stained red. Surrounding this is a hyaline layer, which 

 takes the stain very lightly, if at all. Outside this, on the periphery, is a 

 thin medullary sheath taking a deep hematoxylin stain (Plate 7, Figs. 53, 

 54). This sheath is brought out still more clearly in cross-sections of the 

 cord of the lizard prepared by vom Rath's osmic-acid method or by the 

 Weigert methods. Here the sheath is deeply stained blue or black, 

 the central portion of the fibre remaining unstained. The existence of 

 this sheath is often apparent in cross-sections stained by other methods, 

 and also in longitudinal sections. In sagittal sections of the posterior 

 portion of the cord of the chick stained with Heidenhain's iron hema- 

 toxylin, the central portion of the filn-e is seen darkly stainedwithin the 

 semi-transparent and lightly stained sheath. Finally, in the permanent 

 preparations of the fibre that was withdrawn from the canal of the 

 shark and double-stained with anilines, the sheath is found in many 

 places wrinkled upon the fibre, and the fibrillar nature of the internal 

 portion is clearly seen. In all its staining reactions, the sheath is 



