As the ordinary person approaches middle life, he becomes able to see better at a distance than near to. This _presbyopia_, as it is called, which is practically far-sightedness, is due to a partial loss of the power of accommodation in the lens, the result of a general loss of elasticity in the parts. Another very common defect is _astigmatism_, a failure of the rays to focus upon a point, owing generally to a flattening in the surface of the cornea. _Color perception_ is also an important function of the eye. The waves of hyperluminous ether when of a certain rate of vibration give the sensation of heat and when their vibrations are more rapid they give the sensation of light. Each of the primary colors of the spectrum gives off a pretty definite number of light rays which travel through the air and enter the eye, the number of rays determining the color thrown upon the retina and the velocity determining the intensity of the color. Occasionally when light is passing through into the eye it is broken up as in a prism and the person gets a sensation as of all sorts of colors, _chromatic aberration_. Total or partial absence of sensitiveness to color is called _color blindness_. It is commonest in the form of inability to distinguish between red and green and is probably due to a defect in the retina. 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.