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 eerebri 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 lamina: or folia with deep intervening fissures. These lam inaa 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, tilt 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 ments of the gullet and stomach; (6) movements of articulate speech; (7) for the suppression of the saliva.
Every part of the brain is exactly symmetrical with the part opposite. Twelve pairs of nerves proceed from the base of the brain, including the nerves for the organs of sight, hearing, taste, and of smell and of touch, also those for the muscles of the face, those for the cav ity of the mouth and for the larynx.
Sight.—Destruction of one lobe causes permanent blindness in one side of both eyes. In man, disease of the left angular gyrus produces what is called "word blindness." In this condition, one loses the power of reading words, although one sees the characters distinctly and may be even able to spell the word, and write it, and yet be unable to read what has been written.
Hearing.—The center of hearing seems to lie in the first temporal sphenoidal convolution in both sides. Partial de struction of this convolution on the left side causes the condition of "word deaf ness," that is, to hear sounds but have no sense of their meaning.
Taste and Smell.—Ferrier locates the sense of taste and of smell in the unciate gyrus on the inner surface of the tern poro-sphenoidal lobe.
Touch.—Some physiologists place these in the motor area. Fibers from the vari ous sense organs lead to the cortical areas through the posterior third of the internal capsule. If this is injured, loss of all forms of sensibility, hearing, sight, taste, etc. of the opposite side follows.
In man the brain weighs from two to four pounds; the average weight in male European adults being 49 to 50 ounces or about 1-35th of the weight of the body; in the dog the average weight is about 1-120th of the animal; in the horse 1-450th. The brain of females weigh five ounces less on the average than that of males. The ratio of the brain—weight to that of the body—is the same in both sexes; consequently the difference of weight in the brain of males and females is due to the lesser body weight in the woman and not to inferior cerebral de velopment. At birth, the proportionate weight of the brain to the body is greatest. It diminishes slowly to the 10th year, when 1:14 is the ratio. About the 40th year the brain is heaviest. After that it diminishes at the rate of an ounce every 10 years. The theory that the size and weight of a man's brain is in direct proportion to his intellect is discredited.
The brain attains its highest degree of development earlier than any other part of the body. The parts lying in :runt have functions connected with the intel lectual part of man's nature; while the parts lying nearer the back belong to the merely animal or organic nature. As the central organ of the nervous system the brain is sympathetically affected in nearly all cases of acute diseases.
Diseases of the Brain.—Diseases of the brain fall into two classes, accord ing as they exhibit mental characteris tics alone, or also anatomical disturb ances. To the former class belong hypo chondria, mania, etc. Among the latter may be mentioned meningitis, or inflam mation of the membranes of the brain, which seldom occurs without affecting the substance of the brain also, and thus giving rise to phrenitis, hydrocephalus, or water in the head, caused by pressure of water in the cavities of the brain. Consult "Diseases of the Nervous Sys tem," Jeliffe and White (1917).