The axon projection from the eye to the brain provides an important example. The neurons in the retina that convey visual information back to the brain are called retinal ganglion cells (RGCs). There are more than a million of them in humans, each one reporting on a different part of the visual field. Their axons converge on the optic nerve head at the back of the eye and travel together along the developing optic nerve toward the brain. Their main site of termination, in most vertebrates other than mammals, is the optic tectum—a broad expanse of cells in the midbrain. In connecting with tectal neurons, the RGC axons distribute themselves in a predictable pattern according to the arrangement of their cell bodies in the retina: RGCs that are neighbors in the retina connect with target cells that are neighbors in the tectum. The orderly projection creates a retinotopic map of visual space on the tectum (Figure 21–75).
Orderly maps of this sort are found in many brain regions. In the auditory system, for example, the neurons that project from the ear to the brain form a tonotopic map in which brain cells receiving information about sounds of different pitch are ordered along a line, like the keys of a piano. And in the somatosensory
Figure 21–75 The neural map from  eye to brain in a young zebrafish.
(a) Diagrammatic view, looking down on the top of the head. (B) Fluorescence micrograph. Fluorescent tracer dyes have been injected into each eye—red into the anterior part, green into the posterior part. the tracer molecules have been taken up by the neurons in the retina and carried along their axons, revealing the paths they take to the optic tectum in the brain and the map that they form there. (courtesy  of chi-Bin chien, from D.h. Sanes, t.a. reh and W.a. harris, Development of the Nervous System. San Diego, ca: academic press, 2000.) genitalia leg toesfoot intra-abdominalpharynx tongue teeth, gums, and jawlower lip lips upper lip facenoseeyethumbindexmiddleringlittlehandwristforearmelbowarmshoulderheadnecktrunkhip
Figure 21–76 A map of the body surface in the human brain. the surface of the body is mapped onto the somatosensory region of the cerebral cortex by using an orderly system of nerve cell connections to pair body sites with the brain sites that receive their sensory information. this means that the map in the brain is largely faithful to the topology of the body surface, even though different body regions are represented at different magnifications according to their density of innervation. the homunculus (the “little man” in the brain) has big lips, for example, because the lips are a particularly large and important source of sensory information. the map was determined by stimulating different points in the cortex of conscious patients during brain surgery and recording what they said they felt. (after W. penfield and t. rasmussen, the cerebral cortex of Man. New York: Macmillan, 1950.) system, neurons conveying information about touch map onto the cerebral cortex so as to mark out a “homunculus”—a small, distorted, two-dimensional image of the body surface (Figure 21–76).