COMMENTARY. In the ancient system of physiology, the heart was considered as the seat of the Vital powers, its office being the preservation of the innate heat of the body. The philosopher, Aristotle, had pointed out the connexion between heat and vitality, and had taught that the heart, as being the centre of heat, is the prime organ in the animal frame. Hence, as his commentator, Averrhoes, remarks, it is the _primum movens et ultimum moriens_. Galen, however, maintained with Hippocrates, that the animal frame is a circle, having neither beginning nor end, and that, consequently, it has no prime organ. He taught that the brain does not, properly speaking, derive its powers from the heart, nor the heart from the brain; but that these organs are mutually dependent upon one another, the heart being indebted to the brain for supplying the parts concerned in respiration with muscular energy, and the brain being indebted to the heart for its vital heat, without which it could not continue to be the vehicle of sensibility and motion. (De Placitis Hippocrat. et Platon. pluries.) We have mentioned in the preceding Section, that the ancient physiologists looked upon respiration as being a process similar to combustion. See Galen (de Usu Respirationis), Alexander Aphrodisiensis (Probl. i, 16.) The heart, then, was supposed to convey heat to all parts of the body, by means of the animal spirits incorporated with the blood in the arteries. Respecting the contents of the arteries, two hypotheses divided the ancient schools of medicine. The first was that of the celebrated Erasistratus, who maintained, that the arteries do not contain a fluid, but merely certain airs or vapours. The other hypothesis was that of Galen, who keenly attacked this, as he did most of the tenets of Erasistratus, and endeavoured to prove, by experiment, observation, and reasoning, that the contents of the arteries is blood, mixed, indeed, with a certain proportion of heat and airs, but in every respect a fluid, little different from that contained in the veins. It was also part of his system, that the right cavity of the heart attracts blood from the liver, and conveys it to the left, from which it is diffused all over the body by the arteries. He taught that, at every systole of the arteries, a certain portion of their contents is discharged at their extremities, namely, by the exhalents and secretory vessels; and that at every diastole a corresponding supply is attracted from the heart. He decidedly inculcates, in opposition to Asclepiades, that it is the expansion or diastole of the artery which occasions the influx of the blood, and not the influx of the blood which occasions the expansion of the artery; or, in other words, that the systole is the function of the heart, and the diastole its return to its natural state. (De Diff. Puls. iv, 10.) Though he demonstrated the anastomosis of arteries and veins, he nowhere hints