Blind boy re

when the rays of light from an object fell upon the optic nerve, the object became invisible. He fastened upon a wall about the height of his eye a small round paper, to serve as a fixed object of vision. He placed another paper on the side towards his right eye, a little lower than the first paper and about two fect from it. He then placed himself directly opposite to the first paper, with his left eye shut, and drawing back by little and little he found when he was about ten feet from the wall that the second paper totally disappeared.* He conceived that it disappeared because the rays from it fell upon the optic nerve. This conclusion has been generally acquiesced in, and indeed is very probable; though it is possible that the deficiency of vision is owing to the rays falling upon the hole in the retina, near the optic nerve, discovered by Soemmering in the human eyes, and by Home in the eye of the ape. From this discovery, Mariotte drew as a conclusion that the retina was not the medium of vision, as had been hitherto supposed, but the choroid coat. This opinion occasioned a controversy between Mariotte and Pequet, which is continued in several papers published successively in the Philosophical Transactions. Pequet's opinion is now universally adopted; indeed every thing known, respecting the nervous system, leads irresistibly to that conclusion. A nerve is absolutely necessary for the exercise of all the senses, and the retina is an expansion, according to the common opinion, of the optic nerve.

The second paper on vision contained in the Transactions is of great celestored to sight. brity, and throws a great deal of light on the nature of sight. Accordingly, it is perpetually referred to by metaphysical writers. It is a paper by Cheselden, giving an account of the observations made by a young gentleman, who had been born blind, and who was couched when between 13 and 14 years of age.† Before the operation he could distinguish certain colours, as black, white, and scarlet, when they were placed in a strong light, but he was incapable of perceiving the shape of any thing. When he first saw, he thought that all objects touched his eyes, as what he felt did his skin; and only learned very slowly and gradually to judge of distances. He thought no objects so agreeable as those which were smooth and regular; though he could form no judgment of their shape, or guess what it was in any object that was pleasing to him. At first he thought that pictures were nothing else than party-coloured planes, or surfaces diversified with variety of paint; but, about two months after he was couched, he discovered all at once that they represented solids. He was very much confounded at the discovery, expecting that the pictures would feel like the things which they represented, and was amazed when he found those parts, which, by their light and shadow appeared round and uneven, felt only flat like the rest, and asked which was the lying sense, feeling or seeing? Being shown

Phil. Trans. 1668. Vol. II. p. 668.

† Phil. Trans. 1728. Vol. XXXV, p. 447.

his father's picture in a locket, at his mother's watch, and told what it was, he acknowledged a likeness, but was vastly surprised; asking, how it could be that a large face could be expressed in so little room; saying, it should have seemed as impossible to him as to put a bushel of any thing into a pint. At first he could bear but very little light, and the things which he saw he thought extremely large; but on seeing things larger, those first seen he conceived less; never being able to imagine any lines beyond the bounds which he saw. The room which he was in he said he knew to be but part of the house, yet ye could not conceive that the whole house could look larger. Being afterwards couched in the other eye, he said that objects at first appeared large to this eye, but not so large as they did at first to the other; and looking on the same object with both eyes, he thought it looked about twice as large as with the first couched eye only, but not double, that they could anywise discover.

There is a third paper relating to vision, in the Philosophical Transactions, giving an account of a number of scales lying over each other in that part of the sclerotic coat of the eyes of birds which is next the cornea.* The author of the paper, Mr. Pierce Smith, considers the observation as new, and claims the discovery as his own. His account however, in both particulars, is inaccurate. Malpighi had long ago made the observation, and published an account of it; so that there was nothing new in the observation of Mr. Smith. But ignorance is not the only charge that may be brought against the author of this paper. The observation which he claims in it was not his own originally. He derived his knowledge of this particularity in the eyes of birds from a preparation of Dr. Barclay, at that time a Student of Medicine, in Edinburgh; and the dissections which he describes were subsequent to his examination of this preparation. As Mr. Smith's paper contains in fact nothing new, the injury which he did to Dr. Barclay by publishing his dissection as his own. was in reality trifling. But his obvious intention was to acquire credit by appropriating to himself what he conceived to be the discovery of another, and a fellow student. On that account his paper deserves to be stigmatized. VI. ORGANS OF MOTION. Dr. William Croone, one of the original Mem- Organs of mo bers of the Royal Society, and a philosopher and physician of considerable eminence, left behind him, at his death in 1684, the plan of an annual lecture on muscular motion before the Royal Society. This plan was put in execution by his widow. The first lecture was read in 1738, by Dr. Alexander Stuart, Physician to the Queen;t and has been continued ever since. These annual lectures, for a considerable number of years, have been regularly published in the Philosophical Transactions. They have been drawn up by the most eminent

* Phil. Trans. 1795. Vol. LXXXV. p. 263.

† Birche's History of the Royal Society, Vol. IV. p. 310.

T

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cular motion.

physiologists, who were Members of the Society, and contain a great collection of very curious and important facts, respecting the muscles and their motions. But few of the lecturers have attempted to investigate, far less to explain, the nature of muscular motion. Indeed the subject appears to be altogether beyond the reach of our faculties. We know that the motion of many of the muscles is obedient to the will; that when a muscle produces motion it becomes shorter, and at the same time swells out and becomes stiff. The degree of contraction produced is not great, and particular pains have been taken to make it as small as possible. When a muscle acts constantly, as the heart and diaphragm, the contraction which it undergoes is never great. When a muscle contracts very forcibly, it feels fatigued, and unless it be relieved by speedy relaxation, it is apt to lose its tone, and cannot be exerted again without pain rest, however, gradually restores it to its wonted energy. As to the cause of muscular motion, no less than six different hypothetical explanations have been offered; but all of them so absurd, or so hypothetical, that barely to mention them is sufficient. Hypothetical 1. The action of a peculiar fluid secreted in the brain, passing along the nerves, causes of mus- and therefore called the nervous fluid, was long a favourite hypothesis of physiologists. It was the opinion of Dr. Stuart, who read the first Croonian lecture on muscular motion. But this opinion, which in fact explained nothing, and the truth of which it was impossible to prove, has been long laid aside by the greater number of medical meu. 2. Dr. Hartley endeavoured to prove, that the intention of the will was communicated from the brain to the muscle, by means of a vibration produced in the nerves. This opinion was adopted by Dr. Priestley, who published a kind of abstract of the worst part of Dr. Hartley's Treatise on Man; and endeavoured, by means of it, to demonstrate the principles of materialism. This opinion is exceedingly absurd, and never had many advocates. Supposing we were to concede all that those who maintain it demand, namely, a vibration in the nerves; we are as far as ever from understanding how this vibration produces muscular motion. But the fact is, that nerves are the worst substances possible to be thrown into vibrations, and seem scarcely susceptible of it. They are soft substances, destitute of elasticity, and of every requisite for vibration. But though neither of these hypotheses seems capable of being maintained, we must admit that the power which the muscles have to produce motion, is somehow derived from the brain ; because, whenever the nerves going to a muscle are cut or tied, all muscular motion is destroyed. 3. Another hypothetical explanation of muscular motion is, that it is owing to a quantity of electric matter, transmitted from the brain to the muscles by the intervention of the nerves. This hypothesis is as little satisfactory as the two preceding; because, if it were granted, the motion of the muscle would still be as inexplicable as ever; and because the transmission of electric matter along the nerves in animal bodies is inconsistent with the

laws which electricity follows in its passage through bodies. To obviate this difficulty, some physiologists have supposed that the electric matter belonging to animal bodies follows peculiar laws of its own. When this opinion is explained, it means that a nervous fluid is secreted in the brain, having properties analogous to the electric fluid; but differing in the way in which it passes through bodies. This opinion, having nothing whatever to support it, is unworthy of examination. A fourth opinion was proposed by Mayow in his Treatise on Respiration. According to him, the spiritu-sulphureous particles of the blood, and the nitroaërial particles imbibed from the air by respiration, are thrown together into every muscle previous to action. These two substances, when they come in contact, occasion an effervescence, which swells up the muscle and occasions motion. This opinion, not being intelligible, is not capable of discussion. 5. Dr. Morton, in a paper published in the Philosophical Transactions about the year 1751, ascribed muscular motion to a sudden heat generated in the blood by the will. This would not serve to explain the phenomena, though it were admitted; and it is well known that no such sudden generation of heat in muscles takes place in muscular motion. The cold blooded animals, as fish, possess muscular motion as perfectly as quadrupeds, yet a very moderate heat, when applied to them, is found sufficient to destroy life. 6. The last explanation of muscular motion, with which I am acquainted, was proposed by Dr. George Fordyce in a Croonian lecture on muscular motion, which he read before the Royal Society in the year 1788.* All motions, according to him, are produced by attractions or repulsions. There are various kinds of attractions known to philosophers, as the attraction of gravitation, of electricity, of magnetism. Now muscular motion is produced by a particular species of attraction between the parts of a muscle, which makes them approach nearer each other. As this attraction is confined to the living state of the muscle, Dr. Fordyce distinguishes it by the name of the attraction of life. This attraction, according to him, is inscrutable, like the ultimate attractions which regulate the motions of inanimate matter; but we may determine the laws according to which it acts. And this, according to him, should be the sole object of our inquiries, when we turn our attention to muscular motion. This explanation of Dr. Fordyce, we are afraid, is nothing else than the introduction of a new word, of which we do not know the meaning. An attraction confined to animal matter, dependent upon the will, and capable of being suspended at pleasure, has no analogy to gravitation, or any other attraction belonging to inanimate matter, which has been dignified with a name. Even if its existence were taken for granted, we should have no notions of the nature of muscular motion, unless we knew how it is connected with the brain and nerves, and how it is influenced by the will. Muscular motion

*Phil. Trans, Vol. LXXVIII. p. 23.

on the eye.

in fact resolves itself into the nervous energy, which is obviously inscrutable in the present imperfect state of our faculties.

The greater number of physiologists have not attempted to explain the nature of muscular motion; but have satisfied themselves with endeavouring to point out some of its laws, or with noticing some peculiarities belonging to the Experiments muscles of certain parts of the body. One of the most interesting topics made choice of is the eye, a subject intended by Mr. John Hunter for a lecture on muscular motion. He unfortunately died before his experiments were completed; but the subject was continued by Mr. Everard Home in several repeated lectures, with a great deal of ingenuity and success. He was assisted in his experiments by Mr. Ramsden and Sir Henry Englefield. Mr. Hunter had conceived that the crystalline lens of the eye is muscular, and that the adjustment of the eye to different distances is performed by the action of these muscles. Leuwenhoeck had long before observed that the lens was fibrous. Mr. Hunter had found in the eye of the cuttle-fish a lens in which this fibrous structure was very conspicuous. These fibres Mr. Hunter supposed acted as muscles. But when Mr. Home tried the experiment on an eye deprived of the lens by extraction, it was found to possess the power of adjustment as completely as a perfect eye. Hence it was obvious that Mr. Hunter's opinion was inaccurate. Upon examining the subject experimentally, it was found that the cornea changed its shape in order to adjust the eye; and Mr. Home concluded that the adjustment was accomplished partly by the action of the cornea, partly by the elongation of the axis of vision, and partly by a change of place in the lens itself.

Specific gravity of men.

It may not be improper to notice here a curious paper published in the Transactions for 1757 by Mr. Robertson.* His object was to determine the specific gravity of living men. For this purpose he constructed a vessel into which (when containing water) men might be immersed, and he determined the specific gravity by the bulk of water displaced, or, which is the same thing, by the rise of the water on the side of the vessel. Ten trials were made in this way on ten labouring men belonging to the ordinary of Portsmouth yard. They were all thin and varied in size from 6 feet 2 inches to five feet 3 inches. The following table exhibits the specific gravities of each of these, arranged according to the height of each; the specific gravity of rain water being reckoned 1.000.