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the effect of the irritation caused by the section. To this dilatation succeeded contraction of the pupil, sometimes sooner, sometimes later, but which contraction was also only transitory. In the neighbourhood of the hypoglossal nerve, therefore, it is not to be doubted that the iridal sympathetic has a second centre-centrum cilio-spinale superius (anterius). .
Centre for the Optic and Oculo-motor Nerres.-By Flourens it was shown that removal of the corpora quadrigemina is followed by blindness. Destruction of them on one side only induced blindness on the opposite side. Flourens also observed contraction of the sphincter iridis of one, and even of both, sides when the corpora quadrigemina were irritated. These observations have been confirmed by Hertwig, Longet, and others.
In his experiments on the corpora quadrigemina, Budge has obtained the following results, in reference to the iris and sight. Suppose each of the corpora quadrigemina be divided into an outer and an inner half, the whole of the outer half of one of the anterior pair may be taken away without motion of the iris of the opposite or corresponding side being thereby necessarily destroyed. In white rabbits, which are well adapted for the experiment on account of their irritable eyes, Budge has seen that both pupils become smaller in the light, although on one side the whole outer half of one of the anterior of the corpora quadrigemina was removed to the very bottom.
The inner side of the corpora quadrigemina, on the contrary, stands in close relation to the iris. Thus, in a rabbit in which Budge destroyed it, complete insensibility to light on the part of the sphincter of the opposite side was remarked. On the side of the wound, the iris continued to react as usual.
The sight, on the contrary, was not abolished. Whether after complete extirpation of the corpora quadrigemina of both sides total blindness results, Budge cannot from his own experience say, as the animals experimented on were never in a condition, after the operation, to admit of any opinion being formed on the point.
Centre for the Iridal Fibres of the Fifth Nerve.-By section of the spinal marrow on one side, between the atlas and dentata, Budge found contraction of the pupil temporarily ensue; in a manner, he supposed, similar to what happens when the fifth nerve is cut.
Section of the spinal marrow on one side, at the point of the calamus scriptorius, is followed by loss of sensation in the domain of the fifth of the same side, along with contraction of the pupil.
When the inner part of the medulla oblongata was alone divided, the corpus restiforme being untouched, Budge found that sensation in the face aud eye was not abolished, and that there was not much contraction of the pupil.
Although further research is necessary in order to determine accurately the origin of the portio major of the fifth, and particularly that of the ophthalmic branch, it may, Budge thinks, still be conjectured that the ti brils (motor) having relation to the pupil, spring in the spinal marrow above (before the second cervical nerve, and that the rest of the fibrils join them in the corpora restiformia and locus coeruleus.
Influence of Light on the Pupil.-- Three effects-viz., special sensation, common sensation, and motion-are produced by light entering the eye. We perceive the light, we have a feeling of pain or the opposite, and the size of the pupil is altered.
By the perception and sensation, ideas and impulses are often awakened. The motion of the iris may therefore be either directly excited by the light, or only indirectly called forth through the ideas and impulses.
The reaction of the iris to light is not quite the same in warm and in cold blooded animals. Immediately after section of the optic nerve in mammals and birds, the pupil is no longer affected by light. In a rabbit, the optic nerve within the skull having been exposed, both eyes were tested to see if the pupil of each was equally affected by the light. One optic nerve was then divided, and the two eyes again tested, when it was found that the brightest light produced no effect on the pupil of the side operated on, whilst the pupil of the uninjured side remained obedient to light as before. In pigeons, the removal of the cerebral hemispheres with the optic tubercles does not alter the action of light on the pupil. After separation of the optic nerve from the corpora bigemina, however, the iris immediately becomes immovable to the brightest light. The iris of the opposite side reacts as usual.
A second condition on which the susceptibility of the iris to light depends, is the integrity of the corpora quadrigemina in mammals and corpora bigemina in birds. According to the experiments of Flourens, Hertwig, Longet, Magendie, &c., when the anterior of the corpora quadrigemina in mammals, or the corpora bigemina in birds, were removed on one side, the iris of the opposite side was no longer obe. dient to light, while that of the same side was less so than before. When the anterior pair of the corpora quadrigemina or the corpora bigemina were extirpated, complete immobility of both pupils resulted. Lastly, in complete paralysis of the oculo-motor nerve, whether from disease or section, light has no influence on the pupil. When the fifth nerve is cut the iris is often motionless, but again becomes obedient to the light.
Contraction of the pupil by light, it is generally acknowledged, is not owing to direct action on the iris or its sphincter in mammals and birds. In these animals an essential condition for the action of light on the pupil, is that the path from the retina to the iris, through the optic nerve, to the anterior of the corpora quadrigemina, and thence to the oculo-motor nerve, be not interrupted. It has lately been, however, asserted by a Dutch physiologist, Ruiter, that he has observed light act on the pupil of the dog after death. And Dr. Brown-Séquard also affirms that he has observed contraction of the pupil after death in mammifera, and even in man, excited by light. Brown-Séquard admits, however, that the movements of the pupil during life are not due to the direct action of light on the iris.
In frogs and fishes it has been discovered that light by its direct action on the iris excites the sphincter iridis.
Influence of the Will on the Activity of the Iris.-Cases have been related in which it was alleged that the motion of the iris was subject to the will. In reference to this, Budge relates the following observation. Professor Beer, of the University of Bonn, is able in the same light to contract or dilate his pupil at will. This change in the size of the pupil, however, he brings about only through certain ideas. When, for example, he thinks of a very dark space, the pupil dilates. When, on the contrary, he thinks of a very light place, the pupil contracts. He finds it more difficult to induce contraction than dilatation Budge has met with several other persons who can dilate the pupil in consequence of such ideas, but not another who can contract it also. From such cases we must conclude, not that the motion of the iris is voluntary, but that the idea of a sensation can bring forth motions, as well as the actual sensation itself.
Professor Allen Thomson, of Glasgow, has lately published, in the Glasgow Medical Journal,' some remarks on the case of Dr. Paxton, of Kilmarnock, who possesses an unusual power of contracting and dilating the pupil, alleged to be voluntary and independent of any effort at adjustment of the eye.
Dr. Paxton showed Dr. Thomson the motions of his iris, “alternately contracting and dilating the pupil to a great extent with apparent ease at will.” And he informed Dr. Thomson
“That although in producing the motions of contraction or dilatation of the pupil, he did not actually make an effort of adjustment, or attempt to fix the eye alternately on a near and distant object, yet the effort to make either of these motions seemed to him, as it were, very similar to the motions for adjustment.”
Dr. Paxton further stated to Dr. Thomson, in proof of his possessing a greater than usual power of moving the iris independently of adjustment, that he
* Can fix the eyes upon a near object, and, while steadily looking at it, dilate the pupil without any effort for adjustment for distant vision, and while continuing to look at a distant object, he can still further dilate the pupil and contract it at will, without any attempt at adjusting the eye for near vision."
In short, as Dr. Paxton himself informs us in a letter, “he can alternately dilate and contract the pupil with as much facility as he oan open and shut his hand,” and that without the slightest mental effort at adjustment. This he can do also more rapidly than the pupil oan adjust itself for near and distant vision,
The pupil, Dr. Paxton says, has the ordinary action under the influence of light and shade, but he can always at will dilate it, whether the eyes be exposed to light or shade. It is by dilating be must always begin the movements in question. By a slight effort of what appears to him to be relaxation, he dilates the pupil, and when the pupil is dilated, he can, by a slight effort of bracing up, contract it. Furthermore, Dr. Paxton says that it is not by raising up any idea in the mind, such as thinking of light and shade, that he calls forth the movements of his pupils; but by distinct efforts, and that he is always conscious, both by the state of vision and by the sensation in the eye, whether the pupil is in its normal condition or not.
Though it is thus evident that Dr. Paxton has the power volantarily to dilate and contract the pupil in some manner or other, it is not clear that he has the power to do so directly in the same manner that he can will the opening and shutting of his hand. The power which Dr. Paxton has of bringing on a temporary dimness of sight may have something to do with the dilatation of the pupil; whilst, on the other hand, the bracing-up effort (which is most likely exerted on the muscles of the eyeball) by which distinct vision is brought back, may be the immediate cause of the contraction of the pupil.
Action of Belladonna on the Iris.-Two principal opinions have prevailed in regard to the mode in which dilatation of the papil is caused by belladonna. According to the one, the sphincter pupillæ is paralysed, and the dilator being thereby no longer restrained, freely contracts. According to the other, the dilator pupillae is directly excited. A third opinion might be mentioned, according to which belladonna acts by both paralysing the sphincter and exciting the dilator.
In agreement with Bernard, Budge has found that after section of the oculo-motor nerve, full dilatation of the pupil does not take place, but that this effect is produced by belladonna as usual. Moreover, it is known that in cases of complete paralysis of the oculo-motor in the human subject, the pupil is usually only somewhat dilated, but becomes still more so by belladonna. This result is not consistent with the opinion that belladonna acts merely by paralysing the sphincter; but it is consistent with either of the two other opinions-viz., that belladonna acts by exciting the dilator, or by both paralysing the sphincter and exciting the dilator.
As was first observed by Kieser, belladonna does not cause dilatation of the pupil in birds. The explanation of this Budge thinks must be sought for either in the transversely striated structure of the muscular fibres of the bird's iris, or in the absence of an iridal sympathetic. These, however, are not the only alternatives, for supposing that belladonna causes dilatation of the pupil in man and the mammifera, not by paralysing the sphincter, but by exciting to contraction the dilator pupillæ, it might be said that the reason why belladonna does not cause dilatation of the pupil in birds is that there is no dilator pupillæ to be excited to action by the belladonna, the pupil in birds being dilated by elasticity coming into play on the cessation of the action of the sphincter.
In agreement with Biffi, Cramer, and Ruiter, Budge has found that after section of the sympathetic in the neck, and even after extirpation of the superior cervical ganglion, belladonna still exerts its dilating influence on the pupil, though in a less degree. Dr. Harley, * however,
• Additional Experiments on the Action of Atropine in Dilating the Pupil, in Edinburgh Medical Journal for February, 1857. See also a previous paper by Dr. Harley, in the same journal, for Nov. 1856; and two papers by Mr. Benjamin Bell, in the same journal, for July and December, 1856.
has recently made the important observation that, after section of the sympathetic in the neck, by continuing the application of the atropia the pupil becomes at last fully dilated. · If belladonna acted merely by paralysing the sphincter, we could not have such a result as this, seeing that the dilator, already so completely paralysed by section of its norve, would not be in a condition to act spontaneously on the cessation of the antagonism of the sphincter. The result, however, is consistent with the opinion that belladonna excites the dilator pupillæ, if we admit that the drug comes by ab. sorption to act on that muscle. The result is also not inconsistent with the supposition that belladonna acts both by paralysing the sphincter and exciting the dilator.
Budge has cut both the oculo-motor nerve and the sympathetic; nay more, he has cut all the ciliary nerves, together with the optic, and still found the pupil to dilate distinctly under the influence of atropia. This result is entirely consistent with the opinion that belladonna acts by exciting the dilator pupillæ, if we make the admission above stated
viz., that the atropia by being absorbed is brought into contact with that muscle. The result is at the same time not inconsistent with the opinion that belladonna acts both by paralysing the sphincter and exciting the dilator pupillæ.
Budge thinks, however, that his observations make it very probable that belladonna does not act on the nerves of the iris at all in dilating the pupil; but that, on the contrary, it directly paralyses the fibres of the sphincter muscle. And in favour of this opinion, he adduces the fact of the non-action of belladonna on the pupil of birds, as it is improbable, he thinks, that the oculo-motor in birds would be unaffected by the same agent which paralyses it in mammalia.
It has been above shown that the non-action of belladonna on the pupil of birds is susceptible of a different explanation from that of Budge, and is not therefore of any weight as regards the view of the question Budge has here broached—viz., whether belladonna acts directly on the muscular fibres, and not through their nerves. It would have been more reasonable, perhaps, in Budge to have concluded from his observations, that belladonna directly stimulates the dilator à conclusion to which Dr. Harley is disposed to come, from his observation above cited, that by continuing the application of atropia to the eye, in a case in which he had divided the sympathetic in a cat's neck, the pupil at last became completely dilated. The theory that atropia acts by paralysing the sphincter muscle through the third pair, Dr. Harley now considers overthrown by that observation. His other experiments he appears to believe bad disposed of the theory that atropia acts by exciting the dilator pupillæ through the sympathetic. What is now to be done? he asks. Both theories are upset, and yet the action of atropine is not explained.
"The only other view of the action of atropine in dilating the pupil," continues Dr. Harley, “with which I am acquainted, is that held by Ludwig and some other German physiologists—that the action of atropine depends on its directly stimulating the radiating muscle of the iris to contraction."