(4) _The Muscles_.--It has been observed clinically that if the muscles are impaired by long disuse, or by a disease such as myasthenia gravis, then the range of production of both heat and motion is below normal. This is in agreement with the experimental findings that anesthetics, curare, or any break in the muscle-brain connection causes diminished muscular and heat production. (5) _The Thyroid_.--In myxedema one of the cardinal symptoms is a persistently subnormal temperature and, though prone to infection, subjects of myxedema show but feeble febrile response and readily succumb. This clinical observation is strikingly confirmed by laboratory observations; normal rabbits subjected to fear showed a rise in temperature of from one to three degrees, while two rabbits whose thyroids had been previously removed and who had then been subjected to fright showed much less febrile response. Myxedema subjects show a loss of physical and mental energy which is proportional to the lack of thyroid. Deficiency in any of the organs of the kinetic chain causes alike loss of heat, loss of muscular and emotional action, of mental power, and of the power of combating infections--the negative evidence thus strongly supports the positive. By accumulating all the evidence we believe we are justified in associating the brain, the adrenals, the thyroid, the muscles, and the liver as vital links in the kinetic chain. Other organs play a role undoubtedly, though a minor one. Studies in Hydrogen Ion Concentration in Activation of the Kinetic System Having established the identity of some, at least, of the organs which constitute the kinetic chain, we endeavored to secure still further evidence regarding the energy-transforming function of these organs by making studies of the H-ion concentration of the blood, as one would expect, _prima facie_, that the normal reaction would be altered by kinetic activation.[*] [*] The H-ion observations were made in my laboratory by Dr. M. L. Menten. H-ion concentration tests were made after the application of the adequate stimuli by which the function of the kinetic organs had been determined, and we studied also the effect upon the acidity of the blood of strychnin convulsions after destruction of the medulla; of deep narcotization with morphin before anesthesia; of deep narcotization with morphin after the H-ion concentration had already been increased by fear, by anger, by exertion, by injury under anesthesia, or by anesthesia alone.