Thus in the adult normal cerebrum all parts of the cortex are brought into close connection with one another and with the other half of the cerebrum; the connections with the cerebel lum and with the cord are established as well. The richness of association is an index of the education and intelligence of the individual. These cortical connections are not a helter skelter, hit-or-miss system, they are all care fully laid down, constituting the human brain one of the most remarkable "switchboards° ever made. Modern anatomy is busy unraveling all the fibre and bundles of fibre tracts, and it will not be many years before the map of the brain will be as well known as that of New York. When that time arrives there will be clearer knowledge of the mechanism through which normal effectual adaptation to the ex ternal world is attained. Such light is already shining more fully upon these and the darker problems of failure in adaptation through psychical misdirection of the brain mechanism or organic lesion of the machine which prevents adequate psychical expression.
In addition to the cortical ganglionic masses of cells, there are a number of similar masses of cells located within the substance of the brain mass. These are subsidiary stations, as it were, for many of the fibre tracts going to and coming from the cortex. These are the caudate and lenticular nuclei, the optic-thala mus, and a number of smaller ones.
The cerebellum, or little brain is situated behind and almost beneath the cere brum, which partly overlaps it. It is attached to the brain stem by penduncles and its connec tions with the cerebral centres and those of the cord are many and complex. In minute struc ture the cerebellum has a number of character istic features by which it may be recognized under the microscope, but fundamentally the nerve cells are similar, the interstitial connective tissue is the same in kind as in the cerebrum and the blood vessels, veins and lymphatics have similar properties. The chief function of. the cerebellum is that of adaptation to space rela tions.
Surrounding the entire brain mass and extending down over the spinal cord there are three coverings. These are an out side strong and thick dura mater, and two inside delicate membranes, the arachnoid and pia mater.
The brain is not a solid organ. It is really a flattened-out expansion of nervous tissue peculiarly grouped about a central cavity. This central cavity at one time was as simple almost as the space occupied by the graphite in a lead pencil, but in the adult brain there are lateral ventricles, third and fourth ventricles, all of which are too complicated to be described here. The ventricles contain a fluid, the cerebro spinal fluid, which also bathes the outside of the brain. The cavities of the brain are continuous with the central cavity of the spinal cord.
This modern conception of the brain as a complicated automatic switchboard may be elab orated to any amount of detail. If one should trace, the path of a single impulse from the outside world, be it one of sight, smell, taste, touch, pain, etc., one would trace it, say for pain — first from the point of con tact, for instance, of the finger, whence the special nerves of sense would carry it to the spinal cord; here it travels up definite tracts in the cord (for the upward paths for the pas sages of sensations and the downward ones of messages to act are as definitely known as are the railroads from New York to Chicago) ; from the cord it passes into the medulla, still in a well-defined path, where only it and its kind travel (about here the fibre tract crosses to the opposite side of the medulla) ; then through the pons, through the cerebral pe duncles, up to the secondary centres, to the cere bellum and the sensory area in the cortex, which is supposed to be situated just behind the motor area. As soon as the sensory impulse
reaches the cortex it is felt as pain and re ferred to the spot in the skin in contact with the irritant. Immediately on the perception of pain, so intimate are the connections of the sensory areas with the motor areas from these motor cells, a conscious impulse is flashed down another series of fibres, down the peduncles to the medulla (where the fibres also mostly cross to the opposite side), down the spinal cord, out on a motor nerve to the muscle to cause a muscular act of pulling the hand away from the harmful irritant. This is the long, conscious series. There ma also have been a shorter reflex cycle where the impulse passed to the spinal cord and an immediate motor connection was made that caused a quick jerking away of the hand, even before the perception of the sensation had taken place. This is the reflex cycle. See REFLEX AcTiox.
The study of the comparative anatomy and physiology of the nervous system is one of the most enchanting departments of human knowl edge. To trace the gradual development of this intricate and marvelously adjusted regulator of the entire body, from its simplest terms of "pro toplasm through the isolated gang lionic masses in such animals as the starfish, the gradual chaining of one mass to another as in i the worms and insects, thus bringing a certain relation of one part to another, up to the fusion of different ganglionic masses to form a chief mass, the brain, and secondary masses, the spinal cord — this is a story of so many chap ters and volumes that it cannot even be sketched here; but it is very certain that the gradual evolution of the nervous system shows the ex treme complexity of man's activities. Although throughout the entire series, nerve cells are alike, it is only in the great multiplicity of co ordinations and connections that man's brain differs from the nervous system of a jellyfish or a worm. It is only in the animal series beginning with Amphioxus that a distinct brain mass commences to be seen. But from this point forward the modification in form, size and complexity is gradual. While man has the most complex brain, he has not the heaviest brain, although in comparison with his size it is the heaviest. The brain of man is usually heavier than that of woman, although at birth and at the age of 14 the female brain is heavier. The average weight of the male brain is about 48 oz., of the female 43;4 oz. Taller and heavier persdns have usually heavier brains. Weight of brain, however, has no direct rela tionship with intelligence, as idiots' brains are known that have weighed as much as those of many of the ablest of men. Intellectual capac ity, as already said, consists in the great multi plicity of nerve cell connections. In which con nection it might also be added that the shape and size of the outside skull bears no constant relation to the shape and size of the inside brain. Cuvier's brain weighed 6454 ounces. Gambetta's only 39 ounces. While it is true that a number of celebrated men of recognized brain power have had large brains, there are many more of equal capacity whose brain weights have not been remarkable. (See