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Since the days of Gerlach's reticulum there has been a growing tendency among neurologists of all schools to lay great stress on the functional importance of the neuropil, or feltwork of finest non-medullated nerve terminations. It would appear
that here some of the most characteristic nervous reactions take place, and that the peripheral fibrillar networks are not less important. Just what these reactions are it is still too early to affirm with confidence, but the problem is being attacked from several sides and with a fair prospect of immediate success in some of its phases.
Anatomical interest centers on neurofibrillae and enough facts have already been gathered in to justify the prediction that we shall not have to wait much longer for an accurate knowledge of what the structure of the neuropil really is. Physiological experimentation, too, is daily adding new facts and developing new points of view. Undoubtedly both of these classes of evidence must be greatly enlarged before we shall be in a position to determine just how far the newer conceptions of nervous function can be cast in the mold given by the terminology of the neurone as current in the decade just closed. Certain it is that we are not yet ready to throw away that terminology; for even a contracted and defective mold is better than none so long as it turns out fruitful hypotheses and promotes clear analysis and accurate expression, provided only one does not make a fetish of it and in the end perhaps come to venerate its very defects. Practically, even the most striking of our latest physiological experiments on the functional differentiation of the nervous elements can still be expressed more conveniently in terms of the neurone doctrine than in any
This much, at least, is clear, that the nervous system is not made up of structural elements (neurones) in the same sense that a house is built up of bricks or even that the liver is made of cells. The functional unit of the nervous system is the conduction path or functional system of neurones, and for aught that we know to the contrary, the same neurone may be a member now of one functional circuit, now of another totally different. This is suggested, not only by the familiar anatomical connections of the associational centers and simpler reflex stations of the brain, but also by some more recondite phenomena, such as the vicarious functioning of one cortical area after injury to another.
Still more striking in this connection are the cases of substitution of function after peripheral nerve anastomosis, such as that recorded by Cushing and referred to in our last issue. After traumatic destruction of the facialis root and resultant paralysis, the central end of the spinal accessory nerve was sutured on to the peripheral facialis and a successful union effected. There resulted total permanent paralysis of the trapezius and sterno-mastoid muscles and almost perfect restoration of facial symmetry both at rest and (less perfectly) in the facial movements.
Experimental cross-suturing has long been practiced on lower animals and LANGLEY has recently been reporting in the Journal of Physiology a series of such operations, especially upon the cervical sympathetic. In a case reported upon in February of this year the fifth cervical nerve in a kitten was sutured to the cervical sympathetic and functional union resulted. After 187 days, stimulation of the fifth cervical nerve caused the usual effects produced by stimulation of the cervical sympathetic. Since the fifth cervical root contains no preganglionic sympathetic fibers it follows that certain somatic nerve fibers are capable in favorable circumstances of making functional connection with sympathetic nerve cells."
It is evident that such remarkable changes in peripheral connections must result in profound changes in the central conduction pathways, and that too probably without the loss of any central neurones. This plasticity of the central organ, then, seems to be functional largely, not merely regenerative or structural. And this again will have an application in any attempt to define the value of the neurone.
Some progress has been made with the functional analysis of the neurone. VAN GEHUCHTEN's law of the polarization of the neurone (the dendrite being cellulipetal, the neurite cellulifugal), while certainly not universally applicable, is nevertheless quite generally true in higher vertebrates. The selective action of certain drugs on parts of the neurone is well known. Thus, curare will paralyze the terminal arborization (motor endplate) of peripheral somatic motor neurones without destroying the functional integrity of the remainder of the neurone, and it is probable that nicotine acts in a similar way upon the terminal arborization of the pre-ganglionic sympathetic neurones. And, still more recently, LANGLEY has made it very probable that the difference between vaso-constrictor nerves and vaso-dilator nerves lies in the mode of the ending upon the unstriated muscle cells of the arteries of the post-ganglionic sympathetic neurones involved, and not upon the general character of these neurones or their central connections.
of much greater importance is the differentiation within the neurone of neurofibrils and a more fluid plasma-apparently a conducting substance and its nutrient stroma. And the further differentiation within the latter of Nissl bodies, we are taught, is a device for the distribution of a modified nuclear chromatin to facilitate rapid metabolism in the cytoplasm.
We hear much recently about neo-vitalism in biology. In the minds of some the mechanical theory of life is on the decline. It is asserted that physics and chemistry have not explained organic phenomena as was anticipated by the defenders of the mechanical theory fifty years ago. Many biological facts are not as yet capable of satisfactory explanation in terms of any known physical or chemical laws. To ask whether, when we have arrived at a complete knowledge of the organic world, the biological phenomena will be found to be explicable in terms of the laws of the inanimate world, is to miss the true significance of the problem. The truth is that both vitalism and the mechanical theory are undergoing transformation. Each is being interpreted in terms of the other. The reason that the mechanical theory today is inadequate to explain all the biological facts is that this theory was formulated upon the basis of too narrow a range of phenomena. If it is to remain the working hypothesis of the physiologist it must be allowed that development which, as itself an organic phenomenon, every working hypothesis exhibits. To take the mechanical theory as a rigid concept, as it was fixed by thought half a century ago, is logically as vicious as to push recklessly forward to an unwarranted vitalism. In so far as the neo-vitalism is a protest against the static character of this mechanical theory, it may well be that the truth lies, for the time being, in this swing of the pendulum towards vitalism. It behooves the defenders of the mechanical theory to look to the vitality of the mechanical theory itself.
There are no neurological researches which American students can claim as their own with greater propriety than those centering about the functional analysis of the peripheral nervous system. The recent phase of this movement may be said to date from the suggestion of Osborn in 1888 of the possibility of an anatomical correlation of certain components of the peripheral nerves and their end organs with corresponding centers within the brain, a correlation of which we had at that time only vague hints. This suggestion was taken up and first worked out in a concrete case for the cranial nerves by STRONG in 1895, and since that date has dominated most of the really valuable morphological work on the peripheral nerves; in fact it is safe to say that no investigators in this field who have neglected to take account of this point of view have been able fully to enter into their own data. Already some dozen researches have appeared in this country largely inspired by this point of view, which has, however, been generally ignored abroad save for the admirable studies of Cole of Liverpool.
We may, then, claim for the doctrine of nerve components as comparatively studied that it is distinctly an American contribution to neurological science. It is not necessary in this place to enter into an exposition of what that doctrine is, for this has been done in extenso in the address printed in our issue for last December. What we wish here to emphasize is that, apart from its great morphological value in determining homologies and critically defining the proper use of the cranial nerves in attacking such problems as the segmentation of the vertebrate head and its relation to the trunk, etc., perhaps its chief interest and value lie in the fact that it opens a very attractive avenue for the study of the physiological subdivision and interpretation of the entire nervous system, both central and peripheral.
In fact the whole point of this series of researches from the beginning has been the accurate demarcation of functional systems of neurones as the real units of the nervous system. Starting at the periphery where the functions of the terminal organs of the nerves are either well known or open to direct experimental determination, the conduction pathway is followed proximally into the brain and through its devious ramifications within that organ.
Ultimately when each such functional system is exhaustively known we shall have the anatomy and physiology of the central, as well as the peripheral nervous system well outlined and, when this knowledge is made comparative, the materials for a complete phylogeny of the nervous system.
The great problems of evolution, when finally solved, must be stated in functional terms. It is the problem of evolution to determine not merely what has been the history of the structural metamorphosis of organs and species, but what have been the dynamic factors which have shaped that metamorphosis, what influences of environment and internal organization have