In giving my opinion of the causes of Night-Mare, I feel still more embarrassed for proper terms to express my ideas, than in explaining its nature. I have already said, that I conceived the ancient physicians to be well acquainted both with its nature, cause, and mode of cure; it might be well to take some notice here of their opinions, especially as to the causes which produce the disease, and the means by which it may be removed, as these two things depend in a great measure on one another. In order to render these opinions understood by those who have never read any of the ancient writers of medicine, from a mistaken idea of their not being worth the reading, I must premise, that they considered the nerves to be the conductors of a certain subtile fluid, by means of which the muscles are made to contract, and all the animal functions are performed; this they denominated the _animal spirits_, and were of opinion that it was secreted from the blood by the brain. This secretion they supposed, like all the other secretions, could only be pure, when the blood from which it was formed was healthy and proper; and that the goodness and purity of this latter fluid depended on its being elaborated by healthy organs out of proper materials. Consequently, that good blood could never be formed out of an unwholesome diet, nor even out of the most wholesome food badly digested. This induced them to pay great attention to the state of the digestive organs, and to the diet of patients, and their observations led to these conclusions; 1st. That the half digested, or ill digested food, furnished a smaller proportion of good blood than nature required, but a large proportion of excrementitious, half digested matter: 2ndly. That some portion of this matter would be taken up by the lacteal vessels into the circulation, and deprave the blood, and consequently all the secretions, more or less: 3dly. That different kinds of humors resulted from this condition of the digestive organs; the principal of which, and the only one we have here to deal with, is the _pituitous_ humor, of which they enumerated several kinds, and to the presence of which they attributed most of the diseases of the human body. That kind of _Pituita_, to which most of the ancient physicians attributed the production of Night-Mare and deranged sleep, was called the _Pituita Acida_, which was found to be generated in great abundance in the stomachs and intestines of all patients subject to Night-Mare: this was supposed to enter into the circulation, and to cause some derangement in the functions of the nerves and animal spirits, by obstructing the free motion of the latter, and rendering the former bad conductors. They had various opinions respecting the manner in which this obstruction was effected, and also as to the part where it took place; some making the brain the seat of this affection, others the lungs and diaphragm. But without entering into the