The brain is the mind. When the brain is diseased the mind is diseased. When the brain is sick the mind is sick.
But the brain is part of the body. We see, hear, smell, feel, and taste with the brain. The nerves of the toes and fingers are connected with the brain; they are like twigs on a tree, of which the brain is the root. The same blood which circulates through the heart and limbs circulates through the brain.
It is only a figure of speech to speak of the mind and the body as distinct from each other. The mind and the body are one.
A wound in any part of the body—a burn, a stab, a lash—is felt in the brain. When the body suffers from illness or fatigue, the brain suffers also. When a limb is paralysed, the real paralysis is in a part of the brain. When the brain is paralysed the man can neither move nor speak, nor think nor feel. When the heart is weak the brain does not get enough blood, and the mind is languid, or syncope sets in ana the man dies.
We do not need a prophet nor a doctor to tell us that sickness affects the mind. We know that dyspepsia, gout, or sluggish liver makes us peevish, stupid, jealous, suspicious, and despondent.
We know that illness or weariness turns a sweet temper sour, makes a patient man impatient, a grateful man ungrateful. We know how trying are the caprices and whims of an invalid, and we commonly say of such, "he cannot help it: he is not himself to-day."
But we do not know, as doctors know, how searching and how terrible are the effects of some diseases on the brain. Dr. Lydston, in The Diseases of Society, says:
The old adage, mens sana in corpore sano, is too often forgotten. Especially is it ignored by the legislator and penologist. A normal psychic balance and a brain fed with blood that is insufficient in quantity or vicious in quality are physiologic incompatibles. The nearer we get to the marrow of criminality, the more closely it approximates pathology.
That is to say that the sound mind depends largely on the sound body; that a brain fed with diseased blood, or with too little blood, cannot work healthily and well; and that the more we know of crime the closer do we find its relation to disease.
I quote again from Dr. Lydston: