The =cerebrum= or brain proper has two parts or hemispheres, roughly oval in shape, each of which has five lobes separated by fissures, the frontal, parietal, occipital, and temporo-sphenoidal lobes, and the central lobe or _island of Reil_ at the base of the brain. The chief fissures are the _longitudinal fissure_, the _fissure of Sylvius_ at the base of the brain, and the _fissure of Rolando_ between the frontal and parietal lobes. There are also five serous cavities called _ventricles_, the two lateral and the third, fourth, and fifth ventricles, of which the first two, one in either hemisphere, are the most important. Around these cavities is the _brain substance_, which is made up of two tissues, the white and the gray, the latter forming the outer part of the brain to the depth of 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 the brain. At the base of the brain are also the exits of the twelve cranial nerves.