THE NERVOUS SYSTEM. 433 



pother of little consequence, and in particular does the attenuation of the 

 fibres, where they extend through gray 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 insu- 

 lating 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 acuteness of sensibility at the terminations, &c.) would be satisfac- 

 torily 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 electricity 

 and the nervous force, and the medullary sheath abounding in fatty 

 matter, has from this point of view been regarded as an insulator. 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 extremities of the nerves (skin, muscles), and in 

 those portions of the central organs (spinal cord) in which a transverse 

 conduction is evident. The question always remains, whether the medul- 

 lary sheath, although not altogether, yet at all events partially, may not 

 insulate more or less, according to its thickness. 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 gene- 

 rally, 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-borderved 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 expla- 

 nation also, renders it intelligible, why it is, that in dark-bordered 

 nerves, where the medullary sheath is thin or wanting, and the central 

 fibre is in a more free condition, the nerve-fibres are more readily excited 

 and able to communicate their conditions ; and as regards the pale nerve- 

 fibres, in this case they would essentially have the same functions as the 

 others, and the absence of the medullary sheath in them could either be 

 explained on the supposition, that they are less readily excitable, as in 

 the invertebrate animals, and the Cyclostomata, or because they occur 

 in situations where a protective tunic to the nerve-fibres is no longer 

 required, as in the retina, in the nasal mucous membrane, in the gray 

 substance, and in the electric organs, or even where its refractive power 

 upon light would be prejudicial to a certain object, as in the cornea. A 



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