Page:The American Cyclopædia (1879) Volume III.djvu/195

BRAIN 189 birds, ^4^ ; in reptiles, 13 1 81 ; and in fishes, In some apes, rodents, and singing birds the weight of the brain bears a higher proportion to that of the body than it does in man, even as high as -fa in the blue-headed titmouse ; the increase, however, is not in the cerebrum, the seat of intellect, but in the sensory ganglia, the seat of the instinctive actions. The size of the brain is not in proportion to the physical development of the body, either in animals or man ; the horse has a brain inferior in weight to the smallest adult human brain ; that of a whale 75 feet long was found to weigh not quite twice as much as that of a man. Even in men there is no fixed relation between the size of the body and of the brain ; a small man may have a large brain, and vice versa. Men of great intellectual power have generally, if not always, possessed large brains ; the brain of Cuvier, the great French naturalist, weighed between 59 and 60 oz. ; that of the French sur- geon Dupuytren, 58 oz. ; those of Napoleon and Daniel Webster, an ounce or two less. The qual- ity of the brain, however, is quite as important as the quantity, so that a large brain does not of necessity constitute a great man. According to Tiedemann, the female brain, though absolutely smaller than that of the male, is larger when compared with the size of the body. The brain reaches its highest development anatomically at the age of 20 years, which it maintains until 60, after which, in most persons, it begins to de- crease in size, with a corresponding decline in the mental powers. There do not appear to be any striking differences between the brains of the various races of man. For the topograph- ical and pathological anatomy of the brain, an examination from the hemispheres downward is the most practicable method ; but for physio- logical anatomy, it is more advantageous to make the examination from below upward, by which method the student proceeds from the simple to the more complex, following the di- rection of the fibres of the medulla oblongata to their ultimate distribution in other parts of the brain. The medulla oblongata is the upper en- larged portion and direct continuation of the spinal cord, extending from the plane of the oc- cipital foramen about an inch upward to the tuber annulare ; through this the brain is brought into communication with the other vital organs, and it is therefore the naiud vital, " the link which binds us to life." As its size is proportionate to that of the nerves which proceed from it, it is much larger in some lower animals than in man. Like the spinal cord, it consists essentially of anterior and posterior columns; it may be anatomically dis- tinguished from the cord by the decussation or crossing of some of the anterior fibres. In front are the " anterior pyramids," separated by a median fissure ; external to these are the oval protuberances, the " olivary bodies ;" more external, and forming the lateral and great part of the posterior portions, are the "restiform bodies," separated from each other in the mid- dle by two slender columns, the " posterior pyr- amids." The anterior pyramids or fibres ex- tend from the antero-lateral columns of the cord to the cerebral hemispheres, passing through the tuber annulare, the corpora striata, and the optic thalami, contributing to form the lower portion of the erus cerebri; in the tuber annulare these fibres are crossed at right angles by others belonging to it, and are interlaced with them; on tracing them downward, the greater part connect themselves with the middle or lateral columns of the opposite side, while a few are continued down on the same side into the anterior columns of the cord, and others, the "arciform fibres," curve round the olivary bodies and ascend to the cerebellum, not pass- ing to the cord ; the anterior pyramids are en- tirely of a fibrous structure. The arrangement of these fibres is highly interesting in explain- ing the phenomena of disease of the brain ; since, owing to the decussation of the fibres in the medulla oblongata, an injury of the right side of the brain will produce paralysis on the left side of the body, and nice versa ; while a lesion of one lateral half of the spinal cord, below the point of decussation, causes paralysis of the same side of the body. The restiform bodies consist of fibrous strands enclosing a gray nucleus, and pass upward into the crura cerebelli; below they are chiefly continuous with the posterior spinal columns, and partly with the posterior part of the middle columns; as the fibres ascend they diverge, leaving be- tween them the fourth ventricle, and pass into the corresponding hemisphere of the cere- bellum, connecting this latter with the spinal cord; the cerebellar columns also communi- cate by a band of arciform fibres, according to Solly, with the anterior spinal columns; the gray nucleus, or " restiform ganglion," seems to be the ganglionic centre of the pneu- mogastric and a part of the glossopharyngeal nerves. The posterior pyramids can hardly be distinguished from the restiform bodies ex- ternally; but their columns, bounded by the median fissure and by a very slight groove, establish a connection between the sensory tract of the crura cerebri and the posterior columns of the cord ; their gray nuclei are the ganglionic centres of the auditory nerves. The olivary bodies, continuous inferiorly with the anterior or motor columns of the cord, and affording attachments to the motor fibres of the first and second cervical nerves, enclose a gray nucleus, and send their fibres forward to the motor tract of the crus cerebri, and backward to the quadrigeminal bodies; the nucleus, or corpus dentatum, seems to be connected with the hypoglossal or motor nerve of the tongue, and also with the glossopharyngeal, one of the sensory nerves of this organ. The medulla is not only a transmitter of fibres from the spinal cord, but is a nervous centre itself; with it are connected the nerves of respiration and de- glutition, which are quite independent of the cerebral hemispheres, and beyond the control