This section is from the book "The Human Body: An Elementary Text-Book Of Anatomy, Physiology, And Hygiene", by H. Newell Martin. Also available from Amazon: The Human Body.
The Psychic Nerve Centres lie in the fore-brain, and mainly in the gray matter of its convolutions. If by any accident the cerebral hemispheres of a pigeon should be so injured as to be destroyed and all the rest of its body left intact, the animal could still control its muscles so as to execute many movements, but it would give no sign of consciousness. Left to itself it would stand still until it died; corn and drink placed before it would arouse in it no idea of eating; it would die of starvation surrounded by food. Yet it could move all its muscles, and if food should be placed in its mouth it would swallow it. If its tail should be pulled it would walk forward; if it should be put on its back it would get on its feet; if it should be thrown into the air it would fly until it struck against something on which it could alight; if its feathers should be ruffled it would smooth them with its bill.
What fibres are found in the posterior spinal nerve-roots ? What in the anterior?
Name the main varieties of nerve-centres. What is done by automatic centres? What by reflex? What is the characteristic property of the psychic centres?
Where are the psychic nerve-centres located? What may be observed in a pigeon whose cerebral hemispheres have been destroyed?
The difference between a pigeon in this state and an uninjured pigeon lies in the absence of the power of forming ideas or initiating movements. It has no thoughts, no ideas, no Will. We cannot predict what an uninjured pigeon will do under given circumstances: we can say beforehand what the pigeon with no cerebral hemispheres will do; it is a mere machine or instrument, which can be played upon. In such a pigeon the excitation of any given sensory nerve or nerves excites unconscious nerve-centres which set certain muscles at work, and the result of any one stimulus is always the same invariable movement. The animal exhibits no evidence of possessing any consciousness ; it has no desires or emotions; it is like a piano which while untouched is silent, but when a given key is struck emits always the same note ; the pigeon without its cerebral hemispheres stays quiet while left to itself, and responds to any one given stimulus always in one invariable and predicable way.