the cortical; so that, if we suppose its functions to consist in receiving, retaining, and communicating vibrations, it will be rendered peculiarly unfit for these functions, from the compression here mentioned; i.e. the animal will be indisposed to sensation and motion, agreeably to observation.
There are many other arguments which might be brought to shew, that during sleep, and sleepy distempers, the brain is particularly compressed, if it were necessary. But the instance of the Parisian beggar above noted, is most to this purpose. This person had a perforation in his skull, which did not ossify; whence, by external pressure upon that part, the internal regions of the brain might be affected; and it was constantly observed, that, as the pressure increased, he grew more and more sleepy, and at last fell into a temporary apoplexy.
In young children, there seems to be a constant moderate pressure of the skull upon the brain. For the brain is of a great relative magnitude in them; and by its endeavour to expand itself, it keeps the sutures from uniting too firmly, till such time as it is arrived at its full growth. It must therefore be compressed in return, by the re-action of the skull. And this may be considered as a circumstance, which concurs to render young children more apt to sleep than adults. When old persons are sleepy, it is a morbid affection, and may arise either from an hydropical disposition, whereby the turgescence of the neighbouring parts compresses the medullary substance; or from a defect of nutrition in this substance, which renders it soft and compressible in a preternatural degree. If the venal sinuses, and other blood-vessels, of the brain, be by any accident preternaturally distended, and continue so for a considerable time, they will scarce ever recover their pristine tone and dimensions; and this so much the more, as the person approaches to old age.
For the same reason, as the medullary substance within the skull and vertebrae is compressed during sleep, that of the ganglions, plexuses, and trunks of the nerves, in other parts of the body, will be compressed also, though in a less degree. For this substance has no blood or gross fluids within it, and is far the softest of all the parts of the body; and the membranes, which invest all the parts of the body, perform the same office to them, in a less degree, as the skull does to the brain, i.e. check their distention. The surrounding membranes must therefore compress the soft medullary substance in the ganglions, plexuses, and trunks of the nerves, during sleep, on account of the rarefaction of the humours at that time; whence, according to the doctrine of vibrations, sensory ones can neither ascend freely from the external organs to the brain, nor motory ones descend into the limbs; i.e. the animal will be insensible and inactive, as it is found to be in fact.
Is it not probable, that, as sleep comes on, the opposite sides of the ventricles of the brain approach towards each other, on
