All the conditions of the bodily organs that determine the character of the function are not known, but all analogy shows that what in popular phrase is called _quality_ is one of them. Exactly what this is nobody knows, nor is it necessary for our present purpose that we should know; but when we talk of the good or bad quality of an organ, we certainly do not talk without meaning. We have an intelligible idea of the difference between that constitution, of an organ which insures the highest measure of excellence in the function and that which admits of only the lowest. In the brain, as in other organs, size is to some extent a measure of power. The largest intellectual and moral endowments no one expects to see in connection with the smallest brain, and _vice versa_, setting aside those instances of large size which are the effect of disease. The _relative_ size of the different parts of the brain may have something to do with the character of the function, but this is a contested point. Education increases the mental efficiency, no doubt, but it is too late in the day to attribute everything to _that_. So that we are obliged to resort to that indescribable condition called _quality_, as the chief source and origin of the differences of mental power observed among men. It is easier to say what this condition is not than what it is. It is not manifested to the senses by weight or color, dryness or moisture, hardness or softness. In these particulars all brains are pretty nearly alike. When the cerebral action stops and the man dies, we may find lesions visible enough to the sense,--vessels preternaturally engorged with blood, effusions of lymph, thickening of the membranes, changes of color and consistency,--but no one imagines these to be the cause and origin of the disturbance. Behind and beyond all this, in that intimate constitution of the organic molecules which no instrument of sense can bring to light, lies the source of mental activity, both healthy and morbid. There lies the source of all cerebral dynamics. Of this we are sure, unable, as we are, to demonstrate the fact to the senses.