perhaps half an inch, and the white matter forming the rest. The outer or gray part is called the cortex and is largely made up of nerve cells. It might be called the active part of the brain. The white part consists largely of nerve fibers which are given off from the nerve cells and are carried down into the spinal cord.
The surface of the brain is convoluted, the ridges being separated by deep furrows or sulci, by which means a great extent of gray matter is secured. The furrows contain fluid from the subarachnoid spaces and vary in number and depth according to intelligence. While the convolutions are not uniform in all brains, the principal ones are constant.
Both the brain and the spinal cord are covered by three membranes, the dura mater, the arachnoid, and the pia mater. The dura mater is dense and fibrous and lines the interior of the skull, being firmly adherent to it at many points. In fact, it constitutes the internal periosteum of the cranial bones. The arachnoid is a delicate serous membrane, with two layers, lubricated to prevent friction, which divides the space between the dura mater and the pia mater, bridging over the convolutions and enclosing the subdural and subarachnoid spaces which are connected with lymphatics and contain a serous secretion, the cerebro-spinal fluid. This fluid forms an elastic water cushion, on which the brain rests, and prevents concussion. The pia mater is vascular, containing blood-vessels, lymphatics, and nerves, and is closely attached to the surface of the brain, dipping down into all the sulci.
At the base or under surface of the brain are some very important structures. The olfactory bulbs lie beneath the frontal lobe and projecting back is the olfactory tract, through which the olfactory nerves come from the brain. Back of the olfactory tract is the optic commissure where the optic nerves coming from the brain cross each other. And back of the commissure again is the optic tract, where the optic nerves emerge from
