24 bulletin: museum of comparative zoology. 



■central strand become separated (Plate 1, Figs. 40-43). If is difficult 

 to say whether this is due to the action of the reagents or whether it 

 is natural. In Figure 40 (Plate 4) some of the fibres are seen to lie in 

 ■clear spaces within the sheaths Avhile in some the sheath and central 

 portion are ever}'Avhere in contact. The outline of the sheaths alone 

 in the former case is about as extensive as the entire structure in the 

 latter case, and the central part is much smaller in the fibres where it 

 is not in contact with the sheath. This would lead one to think that 

 shrinkage of the central bundle had occurred; but in more posterior 

 sections of 'the same series (Plate 4, Fig. 41) there are more fibres 

 which are separated from the sheaths by a space. 



On the other hand, in Figures 42 and 43 (Plate 4), which are from 

 another series, and pass through the anterior part of the brain, the 

 axial strands are all about equal in size, but each is separated from 

 its sheath, the extent of the separation varying in different cases. 

 And in Figure 54 (Plate 5), which is from a section whose plane is 

 farther back in the brain than that represented in Figure 43 (Plate 4), 

 some of the fibres are distinctly separated from the sheaths, while 

 in the others the sheath could not be seen. But in this case all the 

 central strands are of ap]:)roximately the same size. If shrinkage 

 occurred at all, one would certainly expect it to affect the nerves at 

 points near the eye as well as farther back, but I have never observed 

 this in any series. 



It is evident from what has just preceded that the individual fibres 

 of the optic nerve preserve their identity from the retinal cells to the 

 brain. Figures 42, 43 (Plate 4), and Figure 54 (Plate 5) show that 

 twenty-eight fibres may be distinguished some distance posterior to the 

 point where the nerve enters the brain as readily as immediately 

 behind the eye. Figure 54 (Plate 5) is from a section at about the 

 level where the individual fibres in that series are no longer distinguish- 

 able. 



The fact that there are twenty-eight cells in the eye and the same 

 number of fibres m the optic nerve is strong a priori evidence that there 

 is one cell for each fibre, and if the distribution of the fibres to the 

 retinal cells is followed, one can hardly fail to be convinced that such 

 is the case. Sections stained by ]Mallory's connective-tissue method 

 (Plate 5) are especially favorable for tracing the fibres. It is hardly 

 necessary to state that not all preparations are equally valuable and 

 that even when the staining has been particularly successful in differ- 

 entiating the nerves from other tissue, it is not easy to make out the 

 course of the nerves. Furthermore, if in any case the plane of the 



