In the mean time, I cannot but acknowledge many, or even most, things in the Boerhaavian doctrine concerning the structure and functions of the brain, to be beautiful, just, and useful. And it may even be that the doctrine of a glandular secretion, properly qualified, is not inconsistent with that of vibrations.
Sir Isaac Newton supposes the nerves, when singly taken, to be pellucid, because otherwise they could not be sufficiently uniform for the purpose of transmitting vibrations freely to and from the brain; the opacity of any body being, according to him, an argument that its pores are so large and irregular as to disturb and interrupt the vibrations of the æther. For the same reasons we must suppose the fibrils of the medullary substance of the brain to be pellucid, when singly taken. And this consideration may incline one to conjecture, that in palsies, the infinitesimal vessels of the fibrils of the brain, and capillaments of the nerves, are so obstructed, as to render these fibrils and capillaments white and opake, in the same manner as the hair in old age, or the cornea in an albugo.
Since the pia mater, with its blood-vessels, enters the interstices of the several folds of the brain, one may suspect, that it penetrates not only the cortical substance, but also the medullary, along with the several descending orders of vessels, and consequently that it divides and subdivides the medullary substance into various greater and lesser regions. One may affirm, at least, that such a distribution of the pia mater would be greatly analogous to that of the cellular membrane, through the system of muscles, their separate portions, fibres, and fibrils. But then we may reasonably suppose the pia mater to be so attenuated in these its processes, as that the medullary substance may still remain sufficiently uniform for the free propagation of vibrations. Or, if there be some little impediment and confinement in certain regions, on account of some exceedingly small discontinuity, arising from this intervention of the pia mater between certain regions, it may, as it seems to me, suit this theory rather better than an absolute and perfect continuity, as before supposed. It is reasonable also to think, that the nerves of different parts have innumerable communications with each other in the brain, in the ganglions, (which are, as it were, little brains, according to the opinion of Winslow,) and even in the plexuses; and that many phænomena, particularly those of the sympathetic kind, are deducible from these communications. But as it seems impossible to trace out these communications anatomically, on account of the great softness of the brain, we must content ourselves with such conjectures as the phænomena shall suggest, trying them by one another, and admitting for the present those which appear most consistent upon the whole, till farther light appears. The same, or even a greater, obscurity attends all inquiries into the uses of the particular shape and protuberances of the medullary substance of the brain.
