nose, and eyes. However, the disorder of the medullary substance is great and general in great mental uneasinesses.
As to sneezing; no one can doubt its being automatic. And it is reasonable to expect, that the muscles actually concerned in it, viz. those of inspiration, and the erectors of the head and neck, should be affected by vivid sensations in the pituitary membrane. It seems also to me, that the muscles which stop the passage through the nose, ought to be contracted first, i.e. during the inspiration, as being nearer to the seat of irritation; and afterwards relaxed during expiration, partly by their having exhausted their own power, partly by the contraction of their antagonists, which are irritated also. The contrary happens, but for the same general reasons, in the action of deglutition, as has been already observed. And there is a remarkable coincidence of the efficient and final causes in both these instances.
In speaking of the sources of motory vibrations above, Prop. XVIII. I supposed, that just before the motory vibrations excited by the irritation of membranes took place, the sensory ones in them were checked by the general contraction of their fibres, in all their directions. And I mentioned sneezing, as affording an instance of this. For the sensation, which causes it, disappears the instant before the inspiration; and if this be not strong enough, i.e. if the muscles do not receive the vibrations from the pituitary membrane with sufficient freedom, it returns again and again, being increased by this reciprocation, till at last it causes sneezing. It seems agreeable to this account, that the passage of air, cold absolutely or relatively, through the nose, will often occasion sneezing; and through the mouth, yawning. For cold air must contract the membranes along whose surfaces it passes.
When sneezing rouses from a stupor, it may be supposed to excite the usual degree and kind of vibrations in the medullary substance of the brain, by such a moderate concussion of it, as lies within the limits of nature and health.
The short, quick, alternate inspirations and expirations, by which we distinguish smells in perfection, are in men, totally or nearly, a voluntary action, derived partly from common respiration, partly from sneezing, the prospect of pleasure and convenience concurring to it, and modelling it, as in other cases. It seems also, that in brutes this action must pass from its pure automatic state to some degree of a voluntary one.
In what manner and degree deglutition is voluntary, has been considered already.
Sneezing is checked for a time by attention, surprise, and all
