74 CLARKE, ON NERVE-FIBRE. 



With regard to the structure of the external or mem- 

 branous sheath of the primitive nerve-fibre, there is some 

 difference of opinion amongst anatomists. According to 

 Schwann, and Todd and Bowman, it is a structureless, homo- 

 geneous, and finely granular tissue; while Pontana, Valentin, 

 liemak, Henle, and others, have described it as composed of 

 fibres which cross each other in different directions. Ac- 

 cording to my own observations, it consists of fibres of 

 different shapes and sizes, but sometimes of such extreme 

 delicacy, that when in close apposition they appear to be 

 fused, as it were, into a homogeneous, finely granular, but 

 nucleated membrane. Some of these fibres are broad, flat or 

 riband-shaped, of a faint or shadow-like aspect, and spotted 

 at intervals with exceedingly pale and delicate granules ; they 

 are joined together at their edges and proceed from the whole 

 breadth of their nuclei, which are occasionally oval, but 

 generally very elongated and spotted with the same kind of 

 granules as the fibres ; they are frequently found covering, 

 as with a continuous nucleated membrane, the entire surface 

 of many of the smaller primitive-tubules. Other fibres 

 composing the external sheath are of smaller diameter, but 

 sometimes less delicate and branched ; they proceed from 

 the ends of their nuclei, which are also occasionally coarser 

 and of darker outline. 



About two years back, while examining some sections of 

 the spinal cord of the Calf and of other young animals, pre- 

 pared according to my own method, I was surprised at 

 finding that the whole of the ivhite columns were studded with 

 nucleated cells which adhered to the sheaths of the primitive- 

 fibres, and occupied the spaces between them. Now, there 

 is reason to believe that in the adult these cells become 

 developed into filamentous tissue; for in the ox and other 

 full-grown animals I found that the cells had disappeared, 

 but that their nuclei were still present between the primitive- 

 fibres.* In a short exposition which I gave, last summer, at 

 the University of Edinburgh, of some of the most important 

 points in the anatomy of the spinal cord, I alluded to the 

 presence of these nucleated cells in the white columns. It 

 appears, however, that Messrs. Lister and Turner were not 



* For further information on this subject, see my late Researches, read 

 before the Royal Society in 1S58, and published in the first part of (lie 

 ' Philosophical Transactions' for the present year. I may also take this 

 opportunity of calling attention to an Outline of the Anatomy of the Spinal 

 Cord, published in No. iii, 1858, of Scale's 'Archives of Medicine,' and 

 written for the use of those who have not time for reading the more detailed 

 treatises. 



