Page:Collier's New Encyclopedia v. 02.djvu/173

LEFT BBAIN 151 BBAIN connected tissue called velum interposi- tum, with its vascular margin, we ex- pose the two lateral and the third ven- tricles of the cerebrum, the former occu- pying the hemispheres, the latter lying between them and continued backward through a narrow channel (the aque- duct of Sylvius) into the fourth ven- tricle, which lies behind the pons Varolii and the medulla. Projecting into the third and lateral ventricles are rounded masses of gray matter, the corpus stri- atum and optic thalamus, called the basal ganglia. The whole cerebrum is sur- rounded by a thin, convoluted envelope of gray matter about a quarter of an inch thick. Within this lies the cent7-um ovale, composed of white nerve fibers passing in all directions and difficult to unravel. Connecting the two hemi- spheres is the corpus callosum. Under- neath are the septa lucida and the for- nix. Into the ventricles project the ovoid optic thalamus and the caudate nucleus of the corpus striatum. A wedge-shaped mass of gray matter, the lenticular nu- cleus of the corpus striatum, is separated from the first two nuclei by a band of white fibers. The internal capsule is composed of two parts, an interior and posterior limb, which meet each other at an obtuse angle (the knee). Directly outside the lenticular nucleus is the white external capsule, separated by the claustrum, a thin band of gray matter, from the island of Reil. Some of the fibers of the claustrum ovale connect the frontal and occipital lobes, others con- nect the basal ganglia with the cortical gray matter. An important group, the corona cortex, passes from the internal capsule to the whole of the cortex. That part of the corona radiata entering the occipital lobes is called the optic radia- tion of Gratiolet, who considered it to be the central expansion of the optic nerve. The. optic lobes consist of anterior and posterior pairs of rounded eminences of gray matter situated close to the optic thalami and underlying the pineal gland, a cone-shaped organ thought by Des- cartes to be the seat of the soul. Re- search justifies the belief that this is the remains of the pineal eye; the third organ of sight, formerly found in the lower animals. The optic lobes are closely connected with the optic nerves, part of these ending in the anterior pair, and to the third and fourth nerves, whose nuclei of origin lie just under- neath them in front of the aqueduct of Sylvius. The crura cerebri are formed of fibers passing up from the medulla cord and from the cerebellum to the cer- ebrum. Removing the cerebral lobe from an animal deprives it of volition and intelligence and a similar experi- ment on man, or an imperfect develop- ment of the cerebrum, results in im- becility and idiocy, and the races that have the heaviest cerebra and the most fully developed convolutions have the most intelligence. Among animals the degree of intelligence increases with the increase in size of the cerebrum rela- tively to the other parts of the brain. The cerebellum, possesses a median and two lateral hemispheres which have been subdivided into lobes. Its parts are arranged in thin laminae or folia with deep intervening fissures. These lam- inae have a central core of white matter with a thin covering of gray matter. The cerebellum has three pairs of ped- uncles: (a) Superior, which pass up- ward and across the middle line toward the opposite cerebral hemisphere but ending under the optic lobe; (b) middle, which is that part of the pons Varolii which enters indirectly into connection with fibers from the opposite cerebral hemisphere; (c) inferior (the restiform body), which are connected with the me- dulla and spinal cord. There is also a close connection with the auditory nerve and semicircular canals of the ear. Medulla Oblongata. — This is the low- est and most dependent division of the brain. It is conical in form, with the base toward the pons Varolii, the nar- row end toward the spinal cord, with each side of the middle line of the an- terior pyramids crossing the other where the medulla passes into the spinal cord. External to them is the ovoid projection. The olivary body of the dorsal surface is the lozenge-shaped fourth ventricle, which is bounded below with the two in- ferior and above by the two superior peduncles of the cerebellum. Destruc- tion of the anterior part of the medulla causes a tendency to fall forward. Loss of its posterior part causes a tendency to fall backward; and of one lateral lobe to rotate toward the side injured. Stim- ulation of any of the parts causes move- ments of the head, eyes, and limbs such as would counteract the disturbance of the equilibrium by the destruction of the parts. The medulla is the great seat or center for the functions of organic life, as it gives origin to all the cranial nerves except the first four numbers. These cen- ters are the center for respiratory move- ments, under the extremity of the fourth ventricle; (2) for the restraint and ac- celeration of the heart; (3) for the con- trol of the blood pressure including the diabetic center, which is simply a vaso- motor center for the liver; (4) the center for swallowing; (5) center for the move-