It is convenient to divide our exposition into two parts, the first the dependence upon proper brain function and structure, and the second the dependence upon the proper health of other organs. For it is not true that mind and character are functions of the brain alone; they are functions of the entire organism. The brain is simply the largest and most active of the organs upon which the mental life depends; but there are minute organs, as we shall see, upon whose activity the brain absolutely depends. Any injury to the brain may destroy or seriously impair the mentality of the individual. This is too well known to need detailed exposition. Yet some cases of this type are fundamental in the exquisite way they prove (if anything can be proven) the dependence of mind upon bodily structure. In some cases of fracture of the skull, a piece of bone pressing upon the brain may profoundly alter memory, mood and character. Removal of the piece of bone restores the mind to normality. This is also true of brain tumor of certain types, for example, frontal endotheliomata, where early removal of the growth demonstrates first that a "physical" agent changes mind and character, and second that a "physical" agent, such as the knife of the surgeon, may act to reestablish mentality. In cases of hydrocephalus (or water on the brain), where there is an abnormal secretion of cerebro-spinal fluid acting to increase the pressure on the brain, the simple expedient of withdrawing the fluid by lumbar puncture brings about normal mental life. As the fluid again collects, the mental life becomes cloudy, and the character alters (irritability, depressed mood, changed purpose, lowered will); another lumbar puncture and presto!--the individual is for a time made over more completely than conversion changes a sinner,--and more easily.