Inside the cranium or brain-case is the brain, small and composed of ganglia which are smooth at the surface and contain little gray matter. At the posterior end of the brain is the thickened end of the spinal cord, called the medulla oblongata. Next overlapping this is the cerebellum, always single. Before this lie the largest pair of ganglia, the optic lobes or midbrain, round, smooth, and hollow. From the under side of these, nerves run to the eyes with or without a chiasma or crossing. In front of the optic lobes and smaller than them is the cerebrum or forebrain, usually of two ganglia but sometimes (in the sharks) united into one. In front of these are the small olfactory lobes which send nerves to the nostrils. The sense organs are well developed. The sense of touch has in some fishes special organs for its better effectiveness. For instance certain fin-rays in some fishes, or, as in the catfish, slender, fleshy, whip-like processes on the head, are developed as feelers or special tactile organs. Other fishes, the sucker and loach for example, have specially sensitive lips and noses with which they explore their surroundings. The sense of taste does not seem to be well developed in this group. Taste-papillæ are often present in small numbers on the tongue or on the palate. The sense of smell is good. The olfactory organs, one on each side of the head, are hollow sac-like depressions, closed at the rear. In most cases each sac has two openings or nostrils. The sense of hearing is not very keen. The ears are fluid-filled sacs buried in the skull, and without external or (except in a few cases) internal opening. Fishes are far more sensitive to sudden jars or sudden movements than to any sound. They possess what is generally believed to be a special sense organ not found in other animals. This is the lateral line which extends along the sides of the body and which consists of a series of modified scales (each one with a mucous channel) richly supplied with nerves. The eyes are usually large and conspicuous. They differ mainly from the eyes of other vertebrates in their myopic spherical crystalline lens, made necessary by the density of the medium in which fishes live. There are usually no eyelids, the skin of the body being continuous but transparent over the eyes. Being near-sighted, fishes do not discriminate readily among forms, their special senses fitting them in general to distinguish motions of their enemies or prey rather than to ascertain exactly the nature of particular things.