THE NEBVOUS SYSTEM. 195 



general. Nevertheless, I do not look upon these relations of 

 size as altogether of little consequence, and in particular does 

 the attenuation of the fibres, where they extend through grey 

 substance (vid. sup., § 112), appear to me to be important, as 

 also their diminution at their origins and terminations. It is, 

 however, difficult to perceive the physiological import of these 

 facts. Were it the case, that in the nerve-fibres the axis- 

 cylinder alone was the conducting, and the medullary-sheath, an 

 insulating substance, and could it be proved that the medullary 

 sheaths were wanting in the attenuated portions, the peculiar 

 activity of the nerve-fibres in these situations (the transverse 

 conduction in the spinal cord, the acutcness of sensibility at 

 the terminations, &c.) would be satisfactorily explained. It is 

 well known that such a notion has already been entertained by 

 various writers, and its conception has usually proceeded upon 

 the idea that a close alliance or identity exists between elec- 

 tricity and the nervous force, and the medullary sheath abound- 

 ing in fatty matter, has from this point of view been regarded 

 as aninsulator. But (1.) it is anything but demonstrated, that 

 the nerves possess no other active force but electricity; and (2.) 

 there is nothing to indicate an absence of the medullary sheath, 

 and a free condition of the axis-fibres in many peripheral extre- 

 mities of the nerves (skin, muscles), and in those portions of 

 the central organs (spinal cord) in which a transverse conduc- 

 tion is evident. The question always remains, whether the 

 medullary sheath, although not altogether, yet at all events 

 partially, may not insulate more or less, according to its thick- 

 ness. Since, however, this membrane is wanting not only in 

 many terminations of nerves, where an insulated conducting 

 faculty might not be required, but also in other situations, as in 

 the Invertebrata and the nerves of Petromyzon generally, as well 

 as in the processes of the nerve-cells which certainly act as nerves, 

 in the central organs of the higher animals, and in the finest 

 nerve-fibres in those situations (brain), the notion that such is 

 its effect in the dark-bordered nerves loses all ground of 

 support. It would seem to me, that the medullary sheath 

 represents nothing more than a protective soft envelope for the 

 tender central fibre. This mode of explanation also, renders it 

 intelligible, why it is, that in dark-bordered nerves, where the 

 medullarv sheath is thin or wanting, and the central fibre is in 



