Now we know the real meaning of the condition. Mouth breathing is possible, but it is inadequate. Insufficient respiration leads to insufficient oxidation of tissues, and to lowered vitality in all structures, and this is particularly notable in the brain, as well as in certain other higher structures. It is not because the individuals are lacking in intelligence that their mouths are open, but because the same reason that compels the open mouth also affects their intellectual activity. The blocking of nasal respiration lowers vital activity of all kinds. Hence the lowered intellectual vitality. The thick lips, which are supposed to be characteristic of a certain passionateness of nature, and which usually are associated with a lack of thorough control over animal inclinations, probably owe their significance to the fact that this special peculiarity of feature usually accompanies mouth breathing, and that the individual who labors under this deficient respiration, is likely to lack control to at least some degree. There is even a question whether the deficient oxidation is not likely to be much more notable in its effect upon the higher faculties than on the lower, and as a consequence the latter develop somewhat to the detriment of the former. These studies in physiognomy may, indeed, be correlated in many ways with distinct physical conditions instead of as formerly with the general constitution of the individual. For instance, large protruding eyes used to be said to be characteristic of nervous, timid, sensitive individuals, easily scared, and not well able to take up the harder parts of the battle of life. Now we know that this feature is usually associated with an excess of secretion of the thyroid gland, and that the nervousness is not a matter of character so much as it is due to the disturbance of internal metabolism consequent upon this interference with the proper function of an important organ. It might well be called a slight thyroid intoxication. In large amounts it produces all the symptoms of Graves' disease.