He felt it with an intensity far beyond the natural capacity of his primitive brain.

Such an enlargement might be called the first and most primitive brain.

The eyes were sightless in the black, and the other senses transmitted nothing extraordinary to the small, primitive brain.

Their internal anatomy includes a radula and a primitive brain.

So why not find out what was going on in their small, primitive brains?

But nothing works, except for autonomic functions run by parts of the brain so primitive that it's sometimes difficult to kill them.

Something connected in my primitive brain and instantly got lost again.

The trouble with the primitive brain is that it doesn't reason, or explain, it just feels.

Remove the hornworm's primitive brain, and it still trudges forward.

Religion was our primitive brains trying to make sense of the world, in the past.