The brain is very richly supplied with blood. The main arteries enter the base of the brain, dividing and subdividing until they become of very small size; whilst the chief veins tend towards the surface of the hemispheres, where they enter into great cavities or sinuses, by which the blood is carried off to the great veins of the neck. The special feature of the arrange ments for the blood-supply of the brain is that only very small vessels ramify in the brain substance. The gray matter contains a much denser net-work of fine vessels than the white matter.
The average weight of the human brain is about 49 oz. for the male and 44 oz. for the female, the average male brain being thus about 5 oz. heavier than that of the female. In both sexes the weight of the brain increases rapidly up to the seventh year, then more slowly to between sixteen and twenty, and again more slowly to between thirty-one and forty, at which time it reaches its greatest size and weight. As age advances it diminishes in weight at the rate of about 1 oz. for each period
of ten years.
Other things being equal, the size and weight of the brain bear a general relation to the mental power of the individual. The brains of many eminent men have been found to be 8 to 14 oz. above the average weight, but these are notable exceptions. The brains of idiots are always small ; indeed, any weight under 30 oz. seems to be invariably associated with idiocy. The human brain is absolutely heavier than that of any other animal except the whale and the elephant. The brain of a whale, 75 feet long, weighed upwards of 5 lbs., whilst that of an elephant is from 8 to 10 lbs. The average proportion of the weight of the brain to the total weight of the body is greater in man than in most other animals, being about 1 to 36'5; but in some small birds, the smaller monkeys, and in sonic other animals the weight of the brain to that of the body is even greater than what it is in man.