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cils set on end. This is called the bacillary layer (bacillum, a small rod), or layer of rods, b. Third, between these is found a layer composed of granules and nucleated cells, g. This may be called for the present the granular and nuclear layer.

Further, it will be seen that these layers exist, all three, in every part of the retina except two spots. These are the spots where the optic nerve, o, enters, and the central spot, c, which is in the axis of the eye. Where the optic nerve enters, of course, no other layer can exist except the fibrous layer. In the central spot the fibrous layer is wholly wanting, and the granular and nuclear layer is almost wanting, so that the retina is here almost reduced to the bacillary layer. For this reason this spot forms a depression in the retina, and is often called the fovea or pit.

But the extreme importance of the retina requires that these layers be examined more closely. For this a much greater enlargement is necessary. Fig. 24 represents such enlargement. The fibrous layer, h, requires no further description ; but the granular and nuclear layer is seen to be composed of two distinct layers of small granules, d and f, and two layers of large nucleated cells, c and e, and a layer of very large nucleolated cells, g, from which go out branching fibers. These are multipolar cells, or ganglia. It is further seen that the bacillary layer is composed of two kinds of elements, viz., slender cylindrical rods and larger but shorter conelike bodies. These are called rods and cones. It is seen, still further (Fig. 24, B), that all these different elements of the retina are in continuous connection functionally, if not physically, with each other, and with the fibers of the optic nerve. They must be regarded, therefore, as nerve-fiber terminals.

The bacillary layer is of the extremest interest. It consists mostly of rods, but among these are distributed the larger cones, as in Fig. 25, A. On the extreme anterior margin of the retina there are no cones, but only rods. On the general surface the rods are more numerous than the cones (Fig. 25, A). As we approach the central spot the cones become more numerous, as seen in B. In the depression of the central spot (fovea centralis) we find only cones, and these are of much smaller size than those in other parts of the retina, as

Fig. 25.

B

BACILLARY LAYER, VIEWED FROM THE OUTSIDE SURFACE.-A, appearance of

usual surface; B, appearance of surface of the raised margin of centrul spot; C, surface of central spot.

seen in C. The rods are about to inch in length and 14000 inch in diameter. The cones are shorter and about three times thicker than the rods, except in the central depression, where they are nearly as small as the rods, being there only 7oou to totoo inch in diameter. In this spot, therefore, there are probably no less than one million cones in a square 1 inch.

Distinctive Functions of the Layers.—As the distinctive functions of the several sub-layers of the middle layer (granular and nuclear) are unknown, we will treat of only the three layers—inner, middle, and outer. The outer layer of rols and cones (bacillary) is undoultedly the true receptive layer, which corresponds to the iodized film of the sensitized plate of the camera. These rods and cones receive and respond to the vibrations of light; they co-vibrate with the undulations of the ether. The inner or fibrous layer conducts the received impression to the optic nerve; for each rod and cone is connected by a slender thread, continuous with nucleated cells of the granular layer and a fiber of the fibrous layer. The fibrous layer may, in fact, be regarded as a layer of conducting threads coming from the rods and cones, which threads are then gathered into a cord or cable, the optic nerve, which in its turn finally conducts the impression to the brain. The function of the middle layer is more obscure ; but nucleated nerve-cells, and especially multipolar cells, are always generators or originators of nerve-force. They evidently have an important function. They probably act as little nervecenters; and many unconscious, involuntary, or reflex acts of vision are probably performed by their means, without referring the sensation to the brain.

The manner in which the whole apparatus operates is briefly as follows: The light penetrates through the retina until it reaches the outer layer of rods and cones. These are specially organized to respond to or co-vibrate with the undulations of light. These vibrations are carried through. the connecting threads to the fibrous layer, then through the fibers of this layer to the optic nerve, then along the fibers of the optic nerve to the gray matter of the brain, where they finally determine changes which emerge into consciousness as the sensation of light.

That we have correctly interpreted the function of the layer of rods and cones is rendered probable not only by its very remarkable and complex structure, adapting it to responsive vibrations, not only by the fact that the rods and cones are fiber terminals (all sense impressions are on terminals), but also by the peculiar properties of two spots on the retina in which all the layers do not co-exist. Just where the optic nerve enters, as shown in Fig. 23, page 52, the bacillary layer is necessarily wanting, and it is the only spot in which this is the case. Now, this spot is blind (see page 81). Again, just in the axis of the globe, or what might be called the south pole of the eye, is the central spot or central pit. In this spot is wanting the fibrous layer and the whole of the middle layer, except some nuclear cells of the outer part (Fig. 24, c). The bacillary layer is here, therefore, directly exposed to the action of light. Now, this is the most sensitive spot of the retina. The distinctive functions of the rods and cones will come up later under color perception (page 81).

Visual Purple.—There has recently been found in the outer or receptive part of the rods (but not of the cones) a peculiar purple substance, which probably has some important but as yet imperfectly known function in vision, and is therefore called visual purple. It is bleached by light, and again restored by darkness. Photographic images (optograms) of objects may be taken on the purple retina, and by appropriate means may be fixed. These discoveries naturally excited hopes that the study of this substance would solve the mystery of sensation by reducing it to a chemical process. But these hopes have not been fulfilled; for it is now known that visual purple is not present in all animals, nor does it exist in the cones, and therefore is not found in the central spot of the human eye—which is nevertheless the most sensitive spot in the retina to both form and color, though not to simple faint light. It is therefore evidently not essential to the perception of either light or color. Very recently Parinaud * has made some acute ob

* Revue Scientifique, vol. iv, p. 134, August 3, 1895.

cones.

servations and ingenious experiments on the subject of the function of visual purple. It is wanting in nightblind animals, such as most birds and all snakes, and is abundant in nocturnal animals, such as most ruminants, all cats, and the owl among birds. According to Parinaud, its function is to produce great sensitiveness in the retina to simple faint diffused light, but not to form or color, and therefore is found in the rods but not in the

It is easily destroyed by light and reformed in darkness, and is therefore especially adapted to feeble light. Hence it is that in very faint light, but not in full light, by night, but not by day, we detect the presence of an object, though not its form, by indirect better than by direct vision. Direct vision is by the cones only, indirect vision by the rods mostly; and these are made very sensitive by the presence of the visual purple. This explains also the temporary night-blindness of one coming out of a brilliantly lighted room into the night. The restoration of night vision is the result of re-formation of visual purple destroyed by the brilliant light.*

SECTION II.-SPACE PERCEPTION.

We have now explained both the instrument for making an image and the structure of the retina or receiving

We proceed to show how these co-operate to produce the phenomena of vision. There is a certain peculiarity in the general function of the retina, optic nerve, and associated brain apparatus which must first be explained and clearly apprehended, in order to understand the phenomena of vision, for it lies at their very basis.

screen.

* The decomposition of visual purple is confined to the blue end of the spectrum. It is not affected by red. llence the relative brightness of llue, as compared with red or ze!low, ircreases with the faintness of the light.

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