Page:The American Cyclopædia (1879) Volume III.djvu/200

 191 BRAIN first group is composed of the fifth pair for its sensory portion, and of the third, fourth, and sixth for its motor portion ; secondly, we have the facial and glossopharyngeal nerves com- FIG. 8. Human Brain, viewed from below, a. Anterior lobe of cerebrum. 6. Middle lobe of cerebrum, c. Pos- terior lobe of cerebrum, d. Cerebellum, e. Medulla ob- longata. f. Tuber annulare. 1. Olfactory nerves. 2. Optic nerves. 3. Motorea oculorum. 4. Pathetici. 5. Trifa- ciai. 6. Abducentes oculorum. 7. Facial. 8. Auditory. 9. GlosBopharyngeal. 10. Pneumogastric. 11. Spinal ac- cessory. 12. Hypoglossal. bined ; and lastly, the par vagum and spinal accessory form the third group ; the hypoglos- sal may be considered as the first of the true spinal nerves. For further details on this sub- ject the reader is referred to the works of Cams, Oken, Owen, and other writers on philosophical anatomy. The nature of the nervous force, the functions of the nerves, and the general physiology and pathology of the subject, will be treated under NEBVOUS SYS- TEM ; only a brief summary can be given here. Without question the various operations of the mind are associated with the cerebral convolu- tions; perception, memory, the power of ab- straction, imagination, &c., possess, as instru- ments of action, the folds of gray matter ; as Cuvier says, these parts are the sole recep- tacles in which the various sensations may be as it were consummated, and become per- ceptible to the animal. Mechanical injury to the convolutions and the central white sub- stance occasions no pain nor disturbance of the motive powers; in many diseases of the brain and its membranes convulsions accom- panied by pain occur, but this depends on a change produced in the striated and optic bodies, and through them propagated to the motor and sensitive nerves. On removing the hemispheres animals are thrown into a state of deep sleep, retaining their muscular power, yet apparently incapable of a single mental nervous action, voluntary or sensory. When the mem- branes are inflamed, especially the pia mater, the mental faculties are always disturbed ; in the delirium of fevers, in delirium tremens, &c., the circulation of the convolutions seems to be disturbed. The convolutions, then, are the centre of the intellectual actions; being con- nected with the striated and optic bodies (which have been regarded as the centres of volition and sensation), the intellectual centre may either excite or be excited by them. When the convolutions are insufficiently supplied with blood, the deficient nutrition occasions de- ranged phenomena of thought and a rapid de- velopment of ideas, which, being ill or not at all regulated by the will, assume the forms of deli- rium and insanity, just as disease of the nerves of vision and hearing may produce unnatural sights and sounds. As in every muscular action some portion of the muscular tissue is wasted, to be supplied by the general nutrition of the body, so every thought is doubtless accom- panied by some change in the nervous centre. Concussion of the brain from a fall or blow, or condensation of its substance by a clot of blood, checks the organic changes of the surface, and interrupts the joint actions necessary for con- sciousness. Gall, the founder of phrenology, assigned to certain convolutions certain facul- ties of the mind, moral feelings, and instinctive propensities. This theory has since his time been pursued with the zeal which must natural- ly attach itself to any science which professes to read the mental tendencies from external signs. In regard to phrenology, it can only be remark- ed here that, while it is undoubtedly true that the energy of a nervous centre bears a certain relation to its size, the stress laid by its followers on the temperaments shows that they consider the quality of the brain an important element in the development of nervous power. During sleep the nervous centres obtain the rest neces- sary to repair the waste of daily activity ; in this state the brain refuses or is slow to convey impressions from without. In deep sleep we are unconscious, and may be motionless ; as the sleep becomes lighter, consciousness begins to return, and mental changes take place, con- stituting dreams of various kinds. Man per- forms many actions instinctively, without the intentional adaptation of means to ends, just as the bee makes its cell, or the bird its nest; children are born and live for some time with- out cerebral hemispheres, who perform the acts of sucking and swallowing perfectly well ; re- move the hemispheres in an animal, and it will eat if food be placed in the mouth, though it will not go to seek it ; many idiots will do the same. In what part of the brain resides the power presiding over these actions? At the base of the bruin, concealed by the hemispheres, is a series of ganglia, the origin of the nerves of special sense, as well as the striated and op- tic bodies into which all the fibres connecting the hemispheres with the medulla oblongata pass; these nerves have therefore their own nervous centres, distinct in function from other parts of the brain. In fishes these ganglia are very large, and the hemispheres comparatively small, sometimes smaller than a single pair, the