interest to us now, which do not run directly to the visual cortex of the brain where we"see""the images, but instead go into the mid-brain H. These are the fibers which measure the average light and make adjustment for the iris; or, if the image looks foggy, they try to correct the lens; or, if there is a double image, they try to adjust the eye for binocular vision. At any rate, they go through the mid- brain and feed back into the eye. At K are the muscles which run the accommoda tion of the lens and at L another one that runs into the iris the iris has two muscle systems. One is a circular muscle L which, when it is excited, pulls in and closes down the iris; it acts very rapidly and the nerves are directly connected from the brain through short axons into the iris. The opposite muscles are radial muscles so that, when the things get dark and the circular muscle relaxes, these radial muscles pull out. Here we have, as in many places in the body, a pair of muscles hich work in opposite directions, and in ry such which control the two are very delicately adjusted, so that when signals are sent in tighten one, signals are automatically sent in to loosen the other. The iris is a eculiar exception: the nerves which make the iris contract are the ones we have already described, but the nerves which make the iris expand come out from no one knows exactly where, go down into the spinal cord back of the chest, into the ODO thoracic sections, out of the spinal cord, up through the neck ganglia, and all the way around and back up into the head in order to run the other end of the iris in fact, the signal goes through a completely different nervous system, not the central nervous system at all, but the sympathetic nervous system, so it is a very of making We have already emphasized another strange thing about the eye, that the light-sensitive cells are on the wrong side, so that the light has to go through several vers of other cells before it gets to the receptors -it is built inside out! So some Fig 36-4.The of the features are wonderful and some are apparently stupid connections Figure 36-4 shows the connections of the eye to the part of the brain which is from the eyes to the most directly concerned with the visual process. The optic nerve fibers run into a certain area just beyond D, called the lateral geniculate, whereupon they run out to a section of the brain called the visual cortex. notice that some of the fibers from complete. The optic nerves from the left side of the right eye run across the optic chiasma B, while the ones on the left side of the left eye come around and go thi same way. So the left side of the brain receives all the information which comes from the left side of the eyeball of each eye, i.e., on the right side of the visual field, while the right side of the brain sees the left side of the visual field. This is the manner in which the information from each of the two eyes is put together in order to tell how far away things are. This is the system of binocular vision The connections between the retina and the visual cortex are interesting. If a spot in the retina is excised or destroyed in any way then the whole fiber will die and we can thereby find out where it is connected. It turns out that, essentially, the connections are one to one-for each spot in the retina there is one spot in the visual cortex-and spots that are very close together in the retina are very close together in the visual cortex. So the visual cortex still represents the spatial arrangement of the rods and cones, but of course much distorted. Things which are in the center of the field, which occupy a very small part of the retina, are expanded over many, many cells in the visual cortex. It is clear that it is useful to have things which are originally close together, still close together. The most remarkable aspect of the matter, however, is the following. The place where one would think it would be most important to have things close right in the middle of the eld. Believe it or not, the our visual field as we look at something is of such a nature that the information from all the points on the right side of that line is going into the left side of the brain and information from the points on the left side is going into the right side of the brain, and the way this area is made, there is a cut right down through the middle so that the things that are very close together right in the middle are very far apart in the brain! Somehow, the information has to go from one side of the brain to the other through some other channels, which is quite surprising