VOL. XXXVIII.] PHILOSOPHICAL THANSACTIUNS. 587 



From what has been said on this subject, it seems as plain as the nature of 

 such a physical demonstration will admit of; 1. That the universal cause of 

 sleep, is a paucity of animal spirit. 2. That this defect will arise from what- 

 ever exhausts, wastes or evaporates them when produced; as labour or exercise; 

 or from whatever absorbs them, as a great quantity of crude chyle, recently and 

 suddenly admitted into the blood, in the time of, or soon after, a plentiful 

 meal ; or whatever can fetter or reunite them with the grosser parts of the 

 blood, as much as brandy or spirituous fermented liquors and opiates. All these, 

 either by evaporating and wasting them, or by hindering their production or 

 secretion, bring on that paucity of spirits spoken of, and sleep or some degree 

 of sleepiness, as a necessary consequence. 



Yet it will be still true on the same foot of reasoning, that where the blood 

 is extremely depurated, and the secretions and excretions from it already per- 

 fectly performed, as in long fasting, the whole mass of blood is become only fit 

 for the secretion of spirits ; has no crudity or impurity in it, to absorb or fetter 

 the spirits already produced ; and no crude chyle admitted to answer that end : 

 in such a case opiates can have no effect, the spirits cannot be absorbed, fettered 

 or restrained, where the qualities of the mass of blood do not concur to that 

 effect. 



Another concurring cause of the inefficacy of opiates in the case of fasting, 

 is, that all the natural parts, as those of the primae viae, which serve for 

 digestion, are at rest, for want of the weight and stimulus of food, and also of 

 the gall in the case referred to, to keep up their peristaltic motion ; therefore, 

 few or none of the spirits being spent on those parts, there is a greater supply 

 sent to the animal organs of sensation and voluntary motion ; and indeed in 

 such a case even the vital parts for respiration and circulation act but very slug- 

 gishly, for want of a recruit of blood and fluids proper to excite their functions: 

 hence also the supply of spirits to the organs of sensation and voluntary motion, 

 is by so much the greater ; and the possibihty of restraining their secretion, for 

 the reasons above assigned, impracticable by any power of opium, without the 

 accession of a fresh recruit of chyle. 



Hence also those who have any considerable defect in the natural and vital 

 functions, or in either of them, by obstructions of the viscera, are generally 

 bad sleepers, or watchful ; and in such opiates have but little effect to procure 

 rest; with this great disadvantage, that by impeding the secretions, they in- 

 crease the obstructions ; though in many cases, where the viscera are sound, 

 they must be acknowledged to be excellent medicines. 



What has been said will also sufficiently account for the anodyne power of 

 4 F 2 



