3 If the beasts can properly be said to see at all, " they see as we do when our mind is distracted and keenly applied elsewhere; the images of outward objects paint themselves on the retina, and possibly even the impressions made in the optic nerves determine our limbs to different movements, but we feel nothing of it all, and move as if we were automata."
The midbrain is represented chiefly by the optic lobes, the cortex of which alone is homologous with the corpora quadragemina of the mammals.
Their transverse dorsal connexion is the posterior commissure; otherwise the whole roof portion of the midbrain is reduced to a thin membrane, continuous with that which covers the Sylvian aqueduct, and this ventricle sends a lateral cavity into each optic lobe, as is the case in reptiles.
Forwards it covers, and has driven asunder, the optic lobes; backwards it hides the much shortened medulla oblongata.
Another cartilage or ossification, the posterior sclerotic ring, occurs within the walls of the posterior portion of the cup, and surrounds, especially in the Pici and in the Passeres, the entrance of the optic nerve.