Sometimes a hair follicle on the lid becomes infected and a sty is formed. Pink eye is conjunctivitis or inflammation of the conjunctiva. A Meibomian duct may become stopped and cause bulging, or there may be a sagging down or ptosis of the upper lid in certain diseases, as meningitis, apoplexy, and more especially syphilis. Rodent ulcer often begins by the eye or on the cheek.
CHAPTER V.
THE NERVOUS SYSTEM.
The nervous system, which regulates all the vital processes of the body, physical and chemical, and which is situated partly in the head and partly in the trunk, may well form the connecting link between the description of the head and that of the trunk. It has two divisions, the cerebro-spinal system and the sympathetic system. The former consists of the cerebrum or brain proper, the cerebellum or little brain, the pons Varolii, the medulla oblongata, the spinal cord, and the cranial and spinal nerves; the latter of a series of ganglia or aggregations of nerve centers. The brain, which includes the cerebrum, cerebellum, pons, and medulla, occupies the cranium and the spinal cord is contained within the bony framework of the spinal column. In the male the brain weighs about 49 ounces and in the female 44, while in an idiot it seldom weighs more than 23 ounces.
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.
