The Destiny of Man | Page 8

you diminish the vividness of consciousness. A familiar example will make this clear. When you are learning to play a new piece of music on the piano, especially if you do not read music rapidly, you are intensely conscious of each group of notes on the page, and of each group of keys that you strike, and of the relations of the one to the other. But when you have learned the piece by heart, you think nothing of either notes or keys, but play automatically while your attention is concentrated upon the artistic character of the music. If somebody thoughtlessly interrupts you with a question about Egyptian politics, you go on playing while you answer him politely. That is, where you had at first to make a conscious act of volition for each movement, the whole group of movements has now become automatic, and volition is only concerned in setting the process going. As the delay involved in the perception and the movement disappears, so does the consciousness of the perception and the movement tend to disappear. Consciousness implies perpetual discrimination, or the recognition of likenesses and differences, and this is impossible unless impressions persist long enough to be compared with one another. The physical organs in connection with whose activity consciousness is manifested are the upper and outer parts of the brain,--the cerebrum and cerebellum. These organs never receive impressions directly from the outside world, but only from lower nerve-centres, such as the spinal cord, the medulla, the optic lobes, and other special centres of sensation. The impressions received by the cerebrum and cerebellum are waves of molecular disturbance sent up along centripetal nerves from the lower centres, and presently drafted off along centrifugal nerves back to the lower centres, thus causing the myriad movements which make up our active life. Now there is no consciousness except when molecular disturbance is generated in the cerebrum and cerebellum faster than it can be drafted off to the lower centres.[5] It is the surplus of molecular disturbance remaining in the cerebrum and cerebellum, and reflected back and forth among the cells and fibres of which these highest centres are composed, that affords the physical condition for the manifestation of consciousness. Memory, emotion, reason, and volition begin with this retention of a surplus of molecular motion in the highest centres. As we survey the vertebrate sub-kingdom of animals, we find that as this surplus increases, the surface of the highest centres increases in area. In the lowest vertebrate animal, the amphioxus, the cerebrum and cerebellum do not exist at all. In fishes we begin to find them, but they are much smaller than the optic lobes. In such a highly organized fish as the halibut, which weighs about as much as an average-sized man, the cerebrum is smaller than a melon-seed. Continuing to grow by adding concentric layers at the surface, the cerebrum and cerebellum become much larger in birds and lower mammals, gradually covering up the optic lobes. As we pass to higher mammalian forms, the growth of the cerebrum becomes most conspicuous, until it extends backwards so far as to cover up the cerebellum, whose functions are limited to the conscious adjustment of muscular movements. In the higher apes the cerebrum begins to extend itself forwards, and this goes on in the human race. The cranial capacity of the European exceeds that of the Australian by forty cubic inches, or nearly four times as much as that by which the Australian exceeds the gorilla; and the expansion is almost entirely in the upper and anterior portions. But the increase of the cerebral surface is shown not only in the general size of the organ, but to a still greater extent in the irregular creasing and furrowing of the surface. This creasing and furrowing begins to occur in the higher mammals, and in civilized man it is carried to an astonishing extent. The amount of intelligence is correlated with the number, the depth, and the irregularity of the furrows. A cat's brain has a few symmetrical creases. In an ape the creases are deepened into slight furrows, and they run irregularly, somewhat like the lines in the palm of your hand. With age and experience the furrows grow deeper and more sinuous, and new ones appear; and in man these phenomena come to have great significance. The cerebral surface of a human infant is like that of an ape. In an adult savage, or in a European peasant, the furrowing is somewhat marked and complicated. In the brain of a great scholar, the furrows are very deep and crooked, and hundreds of creases appear which are not found at all in the brains of ordinary men. In other words, the cerebral surface of such a man, the seat of conscious