#632

The Brain — is wider than the Sky —
For — put them side by side —
The one the other will contain
With ease — and You — beside —

The Brain is deeper than the sea —
For — hold them — Blue to Blue —
The one the other will absorb —
As Sponges — Buckets — do —

The Brain is just the weight of God —
For — Heft them — Pound for Pound —
And they will differ — if they do —
As Syllable from Sound —

— Emily Dickinson (1830-86)

The Brain

The adult human brain weighs, on average 1350 grams (about three pounds), and is about the size of your two fists pressed together. Our closest living cousins — the chimpanzees — have brains only one-third as large, while blue whales have brains five-times larger than ours. More significant, however, is not the absolute weight of a brain, but the brain/body weight ratio; here we find that the human brain is six-times as large as what would be expected from the ratio found in other mammals.

Brains are biologically expensive: with only 3% of the body’s weight, they consume 17% of the total calories — this caloric requirement suggests a close relationship between the growth in human brain size and our diet during the course of our evolution.

Neurons — the cells that comprise about one-half the bulk of the brain — come in over 200 varieties, and there are about 86 billion of these cells in the brain. The interconnections among these neurons are estimated at 1000 trillion.