That this grey nerve-tissue is really the organ of thought has been firmly established by numerous experiments both in man and the lower animals. Injuries to it, or diseases in it, invariably affect what is called the mind; while considerable portions of the white matter may be removed without affecting the thinking and perceptive powers. A certain amount of it is indispensable for the existence of intellect; the more there is of it as the brain increases in size and the convolutions become deeper, the greater is the intellect; when these fall below certain dimensions intellect is extinguished and we have idiocy. The average brain of the male white European weighs 49½ ounces, of the negro a little under 47. The maximum brains which have been accurately weighed and measured, are those of Cuvier and Daniel Webster, the weight of the former being 64⅓ ounces, and the capacity of the latter being 122 cubic inches; while the average capacity of the Teutonic race, including English, Germans, and Americans, is 92 inches, of the negro 83, and of the Australian and Hottentot 75. The brain of the idiot seldom weighs over 23 ounces, and the minimum weight consistent with a fair degree of intelligence is about 34 ounces. The mechanism by which correspondence is kept up between the living individual and the surrounding universe is very simple—in reality, as simple as that of any ordinary electric circuit. In the most complex case, that of man, there are a number of nerve-endings, or small lumps of protoplasm, embedded in the tissues all over the body, or highly specialised and grouped together in separate organs such as the eye and ear, from which a nerve-fibre leads direct to the brain, or to the spinal cord and so up to the brain. These nerve-endings receive the different vibrations by which outward energy presents itself, which propagate a current or succession of vibrations of nerve-energy along the nerve-fibre. This nerve-fibre is a round thread of protoplasm covered by a white sheath of fatty matter which insulates it like the wire of a submarine telegraph coated with gutta-percha. This nerve-wire leads up to a nerve-centre, consisting of two corpuscles of protoplasm: the first or sensory, a smaller one, which is connected by branches with the second, a much larger one, called the motor, from which a much larger nerve-fibre or wire proceeds, which terminates in a mass of protoplasm firmly attached to a muscle. Thus, a sensation is propagated along the sensory nerve to the sensory nerve-centre, whence it is transmitted to the motor-centre, which acts as an accumulator of stored-up energy, a large flow of which is sent through the large conductor of the motor-nerve to the muscle, which it causes to contract and thus produces motion. It is thus that the simpler involuntary actions are produced by a process which is purely mechanical. In the more complex cases, in which consciousness and will are involved, the process is essentially