As to the most important organ of all--the brain--every one is conscious of its impaired efficiency under emotional strain, and laboratory researches show that the deficiency is accounted for by actual cell deterioration; so the individual who day by day is under heavy emotional strain finds himself losing strength slowly-- especially do his friends note it. By summation of stimuli his threshold becomes lowered until stimuli, which under normal conditions would be of no effect, produce undue responses. "The grasshopper becomes a burden," and prolonged rest and change of environmental conditions are necessary for restoration. If in a long emotional strain the brain is beaten down; if the number of "low-efficiency" cells increases, the driving power of the brain is correspondingly lessened and therefore the various organs of the body may escape through the very inefficiency of the brain to produce in them forced activity. On the other hand, if the brain remains vigorous, the kidneys may take the strain and break down; if the kidneys do not break, the blood-vessels may harden; if the blood-vessels are not affected, the thyroid may become hyperplastic and produce Graves' disease; if the thyroid escapes, diabetes may develop; while if the iron constitution of the mechanism can successfully bear the strain in all its parts, then the individual will break his competitors, and their mechanisms will suffer in the struggle. This whole train of deleterious results of body activation without action may be best observed and studied in that most emotional of diseases--exophthalmic goiter. In this disease the constantly stimulated distance ceptors dispossess the contact ceptors from the common path, and drive the motor mechanism to its own destruction, and the patient has the appearance of a person in great terror, or of a runner approaching the end of a Marathon race (Figs. 16 and 48 to 54). Exophthalmic goiter may result from long emotional or mental stress in those cases in which the thyroid takes the brunt of the strain upon the mechanism. As adrenalin increases blood-pressure, so thyroid secretion increases brain activity, and increased brain activity in turn causes an increased activation of the motor mechanism as a whole.