Sensitivity, the ability to discriminate between grades of sensation or acuteness of perception is another thyroid quality.  Just as the thyroid plus is more energetic, so is he more sensitive.  He feels things more, he feels pain more readily, because he arrives more quickly at the stage when the stimulus damages his nerve apparatus.  The electric conductivity of his skin is greater, sometimes a hundred times greater, than the average.  Conversely the thyroid deficient type has a low discriminative faculty.  Galton has recorded that idiots hardly distinguish between heat and cold and that their sense of pain is so obtuse that some of the more idiotic seem hardly to know what it is.  Cretins may moan but never shed tears.

Energy and sensitivity in an individual should direct attention to the thyroid element predominating in his composition.  Lack of energy and insensitivity to the degree of thyroid insufficiency in their make-up.

MEMORY, JUDGMENT, AND POISE

In between sensitivity and energy, the sensation and the reaction, comes a passage of the stimulus through the gauntlet of the stored past experience of the individual known as memory.  Many hypotheses have been advanced by philosophers, psychologists and physiologists to explain the phenomenona of memory.  To conceive of memory materially at all one must admit some sort of memory trace as the basis for the persistence of memory.  This memory deposit facilitates the occurrence of the chemical reaction constituting the memory along the same path the next time.  Forgetting then consists in a disappearance of these memory traces or deposits.  Forgetting is greatest in the first hour after remembering, more than half of the memory trace being lost in that time.  Comparison of the curve of forgetting, and the curve of diffusion of a colloid like gelatine from its solution, into a surrounding medium, shows them to be exceedingly similar.  Forgetting may be explained by some such loss of the memory trace or deposit into the blood continually flowing by it.

The internal secretions influence the amount and duration of the memory deposits.  The thyroid appears to be essential to the laying down of the memory trace.  Cretins have poor memories on the retention side and so cannot learn.  The memory of thyroid insufficients is wretched.  In the extreme grades, the memory for recent occurrences becomes completely lost.  Iodine and thyroid increase the electric conductivity of the brain, so that the memory trace must be deposited more easily in those who have an excess of thyroid.  Removal of the thyroid produces a degeneration of nerve cells and their processes, and associative memory becomes difficult or impossible because conduction from cell to cell is interfered with.  If sufficient thyroid is fed in excess, brain conduction may be so facilitated that epilepsy may result upon slight irritation.