Physiology and histology of the Cubomedusæ / including Dr. F.S. Conant's notes on the physiology

by E. W. Berger

For illustration, Patten supposes that if red light only were admitted to the retinophora this would stimulate the fibrils near the broader end of the cone (but that all the fibrils of the retinidium would be stimulated a little) and that we would thus have the sensation of red light. Likewise, if violet light only were admitted, the fibrils at the narrower end of the cone would be stimulated, and we should have violet light. Similarly, if light including all the different wave lengths of the spectrum were admitted, all the lateral fibrils would be stimulated and the sensation of white light produced. The method of stimulation need not be that of a vibration of the fibrils.

Certain grave objections may be raised against such a theory, the most serious, perhaps, being the fact that no such fibrils as Patten has described have as yet been demonstrated for the eyes of those animals that we know have color vision. Yet, as a whole, the objections are perhaps no more serious than any that can be brought against other theories of color vision. What Patten’s theory does do,—it gives us a definite mechanical basis to work from, and if these fibrils should be demonstrated for the rods and cones of vertebrates, physiologists would then have a mechanical basis for color vision quite as they now have for hearing. As Patten says, the problem is primarily a mechanical one. However, the theory cannot well pass for more than a suggestion, a stimulus for future work, and in this lies its present value.

It is quite evident that my results for the retinal cells of Charybdea are, if any thing, a support to Patten’s theory. While I have not been able to demonstrate the fibrils that are the essential to Patten’s theory, yet I have demonstrated the axial fibers of the rods, and if these fibers should be continued as a nerve fiber to some central ganglion (as I believe is reasonable to suppose, see [p. 47]), I do not see how we can avoid the conclusion that these axial fibers of the prism and pyramid cells are somehow concerned in vision. In Patten’s theory these fibers would represent a conducting element, the real sensory element (fibrils perpendicular to these axial fibers) not having been demonstrated by me.

I have recently read in a short review of Patten’s theory[9] that the evidence we at present have points to the tips of the cones (vertebrate eye) as being the seat of the sensation of red. This would be exactly the converse of what Patten’s theory supposes. Whether or not this objection is a real one, future investigation only can determine.

Hesse[13] regards the axial fibers that he describes for the rods in worms as the primitive fibers of Apathy. In this I agree with him, regarding the axial fibers I have described as “Primitivfibrillen.” Further, I believe, if I understand Apathy rightly, that the fibrils described by Patten as extending laterally from the axial fibers correspond to Apathy’s “Elementarfibrillen.”

It is the long pigment cells that are the puzzling element. Since there can be little doubt but that these cells can project and retract their pigmented parts (as already described), it would seem that a part of their function is to check the diffusion of light in the vitreous body when exposed to strong light. This function would be quite analogous to that of the pigmented cells of the vertebrate retina, which in light become projected between the rods and cones. Similar observations have also been made on the compound eyes of arthropods by Herrick[10] and by Parker[7], who find that the distal retinula cells of Palæmonites project themselves distad in the dark, thus surrounding the vitreous cones with a cylinder of pigment, while (Parker) the pigment of the proximal retinula cells migrates centrad and the accessory cells move distad; in light the reverse takes place. Other observations of this kind are not wanting for crustacea, insects and arachnids. To my knowledge, the pigment changes that I have described are the first of their kind for medusæ.

I suggested while describing the capsule, that the lens might be adjustable. That the fibers of the long pigment cells extend to the lens is my principal reason for this. May these cells not represent ganglion cells and their distad fibers nerve fibers? That they are not sensory (i. e. are stimulated by light waves) seems to be suggested by their not having any axial fiber and in having several centrad processes. These facts suggest that they are not sensory but the center of a reflex mechanism.[h] When the sensory cells proper are stimulated, the impulses are conducted centrad into some nerve center (it may be the nerve tissue underlying the retina, or other nerve centers such as the two groups of ganglion cells in the upper part of the club, or the radial ganglia) from which center, again, impulses return over fibers leading to the long pigment cells causing them to project their pigment, and conducting the impulse to the lens, to produce a change in its adjustment. Since these cells are not so numerous as the prism and pyramid cells taken together, but in turn have a number of processes continued centrad (the sum of which processes approximates the number of sensory cells, prism and pyramid cells) it appears that these cells are admirably adapted to function in just such a mechanism as I have described,—each long pigment cell serving a number of its immediate neighbors.

Further, we may conceive each of the centrad processes of the long pigment cells as receiving a fiber from one of the sensory cells directly as well as indirectly, as just described. While I have been able to demonstrate only a single centrad process for the sensory cells (prism and pyramid cells), yet this does not exclude the possibility of a nerve fibril passing out from such a centrad process to one of the processes of the long pigment cells, and it seems possible that this constitutes the reflex mechanism. That nerve fibrils ramify in ganglion and sensory cells, and may even leave these cells to join those of other cells, has been well demonstrated by Apathy,[6] so that my finding only a single process of the visual cells leading centrad without giving off lateral fibers cannot be a serious objection. Again, fine nerve fibers coming off from the main centrad process of sensory cells in medusæ have been figured by other observers, among whom I mention the Hertwigs. Careful macerations at the seashore would probably demonstrate them for Charybdea.

Hesse thinks that the eyes of the Alciopidæ are adjustable. He describes what he supposes to be muscle fibers just exterior (distal) to the lens, and believes that a contraction of these fibers would have the effect of forcing the lens nearer the retina, or vice versa. His supposition, like mine, needs experimental verification. Hitherto the only instance known of accommodation in the eyes of invertebrates was that described by Beer[17] for Cephalopods.

The Proximal Complex Eye.—With four exceptions, the description and discussion given for the distal complex eye also holds good for the proximal complex eye ([Fig. 13]). The four exceptions are: the absence of a capsule to the lens; the absence of the long pigment cells; the absence of the pyramid cells; and the different relative position of the lens and retina. This eye, then, has a cornea continuous with the epithelium of the sensory club, a lens, in structure and probable origin quite like that described for the distal complex eye, and a retina of prism cells with axial fibers for the prisms. Since Conant[8b] has described this eye quite fully, and discussed Schewiakoff’s conclusions at length, I shall be brief. Suffice it to say, that Schewiakoff describes two kinds of cells (supporting cells and spindle-shaped visual cells) for the retina of this eye just as he described for the distal complex eye. The vitreous body he likewise describes as being homogeneous and with spaces for the visual rods (fibers) of the visual cells. It is evident that Schewiakoff has interpreted the structure of this eye from analogy with his results on the distal complex eye. Claus likewise has described two kinds of cells for the retina, and the vitreous body as homogeneous. Conant and myself find only one kind of cells in the retina of this eye. The pigmentation that Schewiakoff describes for the vitreous body I believe to have been dissolved in from the pigmented zone of the retina, for I find no regular pigmentation in the vitreous body. Haake’s observation, previously noted ([p. 42]), applies also to the proximal complex eye.