some types of neurons may be found only in specific species.

As David Eagleman describes it in his wonderful book Incognito: Your brain is built of cells called neurons and glia — hundreds of billions of them. Each one of them is as complex as a city. . . . The cells [neurons] are connected in a network of such staggering complexity that it bankrupts human language and necessitates new strains of mathematics. A typical neuron makes about ten thousand connections to neighboring neurons. Given billions of neurons, this means that there are as many connections in a single cubic centimeter of brain tissue as there are stars in the Milky Way galaxy.

In its evolution from a more primitive nervous system, the brain, as an organ with ten or more billion neurons and many more connections between them must have changed and grown as a result of many accidents.

There are approximately two billion cells in the nervous system of a human being. The process of thinking in any individual is partly dependent upon the variable arrangement of these cells. Therefore no person can think exactly as another, any more than he can change his facial features to duplicate the facial features of another. If people would remember this fact and regulate their attitudes towards others to conform to it, it would encourage the exercise of tolerance, the human attribute most needed today.

The firing of individual nerve cells involved in perceptual and motor processing is modified by attention and decision making.

Every mental activity — contemplating a theorem, savoring a hot fudge sundae, dreaming of the boy next door — consists of neurons firing in a certain sequence.

In fact, we’re not smart and we have bodies — we’re smart because we have bodies.7 The heart has about 40,000 neurons that play a central role in shaping emotion, perception, and decision making. The stomach and intestines complete this network, containing more than 500 million nerve cells, 100 million neurons, 30 different neurotransmitters, and 90 percent of the body’s supply of serotonin (one of the major neurochemicals responsible for mood and well-being). This “second brain,” as scientists have dubbed it, lends some empirical support to the persistent notion of gut instinct.

If you or I learn something and want to transfer that knowledge to someone else, we can’t just send them a copy. But I can have 10,000 neural networks, each having their own experiences, and any of them can share what they learn instantly. That’s a huge difference. It’s as if there were 10,000 of us, and as soon as one person learns something, all of us know it. It’s a completely different form of intelligence. A new and better form of intelligence.

The human body has about eleven million sensory receptors.7 Approximately ten million of those are dedicated to sight. Some experts estimate that half of the brain’s resources are used on vision.8 Given that we are more dependent on vision than on any other sense, it should come as no surprise that visual cues are the greatest catalyst of our behavior.

The neurochemistry of the brain is astonishingly busy, the circuitry of a machine more wonderful than any devised by humans. But there is no evidence that its functioning is due to anything more than the 10¹⁴ neural connections that build an elegant architecture of consciousness.

The brain has millions of local processors making important decisions. It is a highly specialized system with critical networks distributed throughout the 1,300 grams of tissue. There is no one boss in the brain. You are certainly not the boss of the brain. Have you ever succeeded in telling your brain to shut up already and go to sleep?

Every one of our cells has the same DNA, of course, so what differentiates a nerve cell from a skin cell is the epigenome, the collective term for the control systems and cellular structures that tell the cell which genes should be turned on and which should remain off. And this, far more than our genes, is what actually controls much of our lives.

The linear size of a neuron varies widely from one nerve cell to the other, since some of these cells are contained in closely integrated large aggregates and have, therefore, very short axons, while others conduct pulses between rather remote parts of the body and may, therefore, have linear extensions comparable to those of the entire human body.

The point of a brain is that it's not one huge neural network with feedback, it has up to 50 to 60 identified areas, all of which have feedback and all of which are capable of knowledge storage. We've got a complex system and, within this complex system, we can start discovering what the mechanisms that support deliberation are. Consciousness must come out of these interactions.