In another work we have thus explained the ancient theory of the action of opium upon the human frame. “In order to understand properly the ideas entertained by the ancients respecting the _modus operandi_ of opium, it will be necessary to say a few words in explanation of their opinions upon certain points of physiology. Aristotle taught that the prime cause of all the operations of life is mind, and that the prime instrument by which it performs them is heat, which, therefore, he denominates the _co-cause_ (συνάιτιον). He illustrates his meaning by comparing the mind to the artificer, and heat to the wimble or saw by which he performs his work. Having remarked, no doubt, that the heart is the warmest part of the body, he appears to have considered it as the spring which turns the whole machinery of the animal frame, the brain and nerves deriving their origin and influence from it. (I need scarcely mention how well these ideas accord with the ingenious hypothesis lately advanced by M. Serres.) Many facts, indeed, seem to point out the supreme importance of the heart. It is, as the ancients remarked, the _primum movens et ultimum moriens_; and, along with its accessory organ, the lungs, it is evidently the part which, in the higher classes of animals, renders them independent of the many variations of heat and cold to which they are subjected. It is this wonderful organ which, under the guidance of the principle of life, preserves the heat of the body unaltered in all the different gradations of temperature, from more than 100 degrees above the boiling, to as many below the freezing point of the thermometer. It seems, in fact, a real Prometheus that steals the fire from heaven. The connexion between heat and the vital actions is very apparent also in the inferior animals, who are not provided with such an apparatus for preserving an equability of temperature. Thus the zoophyta, insecta, et vermes, with the loss of heat, lose also sensibility and muscular energy, which they recover again when their heat is restored. In this case it is evident that heat is the cause (or at least the _co-cause_) of the vital actions, and not the vital actions of heat. It has always appeared to me a striking fact, illustrative of the great influence of heat over the vital actions, that the strength of all animals is, bulk to bulk, proportionate to the degree of their animal heat.