BRAIN 195 optic. In man the instinctive propensities are in a measure superseded by intelligence, but they may act independently of it. The real nervous centres for motion and sensation are those situated at the base of the brain, and not the hemispheres; as far as mere animal life and motion are concerned, the latter are not essential; a vast proportion of animated creatures (all the invertebrata) have no trace of them ; they are added in man for the in- tellectual and moral nature. The instinctive and emotional actions are excited through the special ganglia, following directly upon sensa- tion, without any process of thought ; they are sometimes stronger than the voluntary actions; e. g., we are often compelled to laugh at some- thing ludicrous though we have the strongest motives not to do so. Long-continued habit will often make us perform actions instinctively, as it were, which at first required an effort of the will : for instance, in an old snuff-taker, who had been seized with epilepsy, irritation of the nose with a feather to restore consciousness produced a contraction of the right forefinger and thumb to take a pinch. These emotional actions may be excited by mental operations. Whenever the feelings get the better of the reason, the sensory ganglia are excited at the expense of the hemispheres, and the individual is for the time being morally insane, even though these emotions may point in the right direction ; fanatics of all classes, in this way, are really insane, generally monomaniacs. These instincts may also be in opposition to the reason, and then the more a man follows them the closer does he approach the brutes. Com- parative anatomy teaches that the cerebellum is largest in those animals which have the great- est variety of motions ; injury or removal of this organ causes no pain nor convulsions, but de- stroys the power of combining properly the vol- untary motions. Man, though inferior to many animals in particular kinds of movements, far surpasses them in the number and complexity of their combinations ; the act of walking brings into action almost every muscle of the trunk and extremities, and is superior to all other modes of exercise. In man the cerebellum attains its highest development. Inflammation of its mem- branes, and even its almost complete destruc- tion by slow disease, has little effect on the in- telligence, but the motive powers are disturbed. In intoxication the energy of the cerebellum is first destroyed, and afterward the intelligence and consciousness, leaving the subject for the time little better than dead, motionless, and in- sensible. The distinct operation of these va- rious centres is made obvious by many condi- tions of the body, in which one or more are inactive. In deep sleep, the hemispheres, the sensory ganglia, and the cerebellum are more or less completely at rest, but the medulla ob- longata and the spinal cord must, as always, be wide awake ; in dreaming, the hemispheres are partially active; in somnambulism, a step nearer to wakefulness, the hemispheres are awake, and also the cerebellum, so that the movements are well adapted to the thoughts. It is well known that in this state persons have walked over dangerous places which they could never have passed in open day; there is an evident loss of control over the thoughts, which are more influenced by external impressions than in dreaming, so that the somnambulist may answer questions properly ; that there is not full com- mand over the senses, the dangerous accidents occurring in this condition fully prove; the events of this state may not be remembered in the waking hours, but may be taken up again by the memory the next night, constituting complete "double consciousness." A condi- tion remarkably analogous to somnambulism is the mesmeric sleep or trance ; a nervous habit of body predisposes to both. Overworking the brain exhausts the body ; wear and tear of the brain, like wear and tear of the muscles, require periodic and long intervals of rest; from want of attention to this fact, many a bright intellect has faded into imbecility and insanity. The primary ganglia of the verte- brate brain are three in number, and they are developed into the anterior or cerebrum, the posterior or cerebellum, and the median or quadrigeminal bodies. In fishes, the lowest vertebrates, the medulla is large, with the pyramidal and restiform bodies, but without the olivary ; the brain looks like a series of ganglia developed on the superior surface of the cord, two pairs and a single one : 1, the olfactory lobes, analogous to the hemispheres in man, from which the nerves of smell arise ; 2, behind these, the optic lobes, generally con- sidered analogous to the tubercula quadrigem- ina, in some fishes larger than the other parts of the brain ; from these arise the optic nerves, and the third, fourth, and sixth pairs; 3, be- hind these, the imperfectly developed cerebel- lum generally, but of large size in the sela- chians. In reptiles the brain well fills the cranial cavity, and the preponderance of the spinal cord is less ; the olfactory lobes, now obviously the hemispheres, are increased in size, with an internal cavity, and a commissure ; the second cerebral mass and its cavities are smaller ; the cerebellum is small in the lower orders, but with lateral appendages and external striss in the higher. In birds the brain and spinal cord are no longer on the same plane ; the brain is the larger, and the ganglia are more above and less behind each other ; the hemispheres are larger than the other parts, are united by com- missures, and contain true lateral ventricles in which is a tubercle resembling a corpus stri- atum; the optic lobes are small, separated, with smaller cavities ; the cerebellum is par- ticularly large, with evident lateral lobes and external striaa. In mammals the brain is much larger than the cord ; the cerebral hemispheres are of large size, with marked convolutions in the higher orders, with a corpus callosum, lateral ventricles with anterior, descending, and (in the monkey) posterior horns, optic and
Page:The American Cyclopædia (1879) Volume III.djvu/201
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