under voluntary control, because there does not seem to be any way to do it. We are already wired to a certain extent. This is an important point, because most of e earlier books on anatomy and psychology, and so on, do not appreciate or do not emphasize the fact that we are so completely wired already-they say that everything is just learned 36-3 The rod cells Let us now examine in more detail what happens in the rod cells. Figure 36-5 shows an electron micrograph of the middle of a rod cell(the rod cell keeps going p out of the field). There are layer after layer of plane structures, shown magnified at the right, which contain the substance rhodopsin (visual purple), the dye, or pigment, which produces the effects of vision in the rods. The rhodopsin, which is the pigment, is a big protein which contains a special group called retinene which can be taken off the protein, and which is, undoubtedly, the main cause of the absorption of light. We do not understand the reason for the planes, but it is very likely that there is some reason for holding all the rhodopsin molecules parallel. The chemistry of the thing has been worked out to a large extent, but there might be some physics to it. It may be that all of the molecules are arranged in some kind of a row so that when one is excited an electron which is generated, say, may Fig. 36-5. Electron micrograph of a run all the way down to some place at the end to get the signal out, or something This subject is very important, and has not been worked out. Itis a field in which both biochemistry and solid state physics, or something like it, will ultimately be used This kind of a structure, with layers, appears in other circumstances where /NNM photosynthesis. If we magnify those, we find the same thing with almost the same kind of layers, but there we have chlorophyll, of course, instead of retinene. The hemical form of retinene is shown in Fig. 36-6. It has a series of alternate double bonds along the side chain, which is characteristic of nearly all strongly absorbing rganic substances, like chlorophyll, blood, and so on. This substance is impossible Fig. 36-6. The structure of retinene human beings to manufacture in their own cells-we have to eat it. So we eat it in the form of a special substance, which is exactly the same as retinene except that there is a hydrogen tied on the right end it is called vitamin A, and if we do not eat enough of it, we do not get a supply of retinene, and the eye becomes what we call night blind, because there is then not enough pigment in the rhodopsin he rods at nigh The reason why such a series of double bonds absorbs light very strongly is also known. We may just give a hint: The alternating series of double bonds is called a conjugated double bond; a double bond means that there is an extra electron there, and this extra electron is easily shifted to the right or left. When light strike this molecule, the electron of each double bond is shifted over by one step. All the electrons in the whole chain shift, like a string of dominoes falling over, and though each one moves only a little distance(we would expect that, in a single atom, we could move the electron only a little distance), the net effect is the same as though the one at the end was moved over to the other end! It is the same though one electron went the whole distance back and forth, and so, in this manner we get a much stronger absorption under the influence of the electric field, than if ye could only move the electron a distance which is associated with one atom So, since it is easy to move the electrons back and forth, retinene absorbs light very strongly; that is the machinery of the physical-chemical end of it. 36-4 The compound (insect) eye et u return to biology. The human eye is not the only kind of eye. In the vertebrates, almost all eyes are essentially like the human eye. However, in the lower animals there are many other kinds of eyes: eye spots, various eye cups, and other less sensitive things, which we have no time to discuss. But there is one other highly developed eye among the invertebrates, the compound eye of the insect (Most insects having lal also have va eyes as well. )A bee is an insect whose vision has been studied very carefully. It is 36-6