The (1795) Croonian Lecture on Muscular Motion.

By Everard Home, Esq. F. R. S. From Philosophical Transactions of the Royal Society of London vol. 86 (1796), pp. 1ff.


In the Croonian Lecture which I had the honour of laying before the R. S. last year, I endeavored to prove, that the adjustment of the eye to different distances could take place independent of the crystalline lens; and that when this was the case, it appeared to arise from a change in the curvature of the cornea. I propse in the present lecture to prosecute the inquiry; and it will be found in this, as well as in the former, that I have received the most essential assistance from Mr. Ramsden, who continues to interest himself in the investigation, and has made all the optical experiments. As this was a new mode of explaining the adjustment of the eye, and differed from the theories that have been previously formed on the subject, it was thought right to consider it with caution, to pay attention to all the objections that could be made to it, and to put it to the test of such experiments as appeared likely to refute or confirm our former observations.

It readily suggested itself, that if the convexity of the cornea was increased to a certain degree, it could be measured by means of an image reflected from its surface, and viewed in an achromatic microscope, with a divided eye-glass micrometer. To ascertain whether the quantity of increase of the convexity of the cornea, in the adjustment of the eye, could in this way be ascertained, the following experiments were contrived, and made by Mr. Ramsden. Our former experiments had sufficiently proved the unsteadiness of the human eye; the first trials on the present occasion were therefore made on convex mirrors, as these artificial corneas could be more readily managed, and such previous experiments would enable us to apply the same instruments with more facility to the eye itself.

Two convex mirrors, one 4/10 of an inch focus [98.43 D], the other 5/10 [78.74 D], had their flat surfaces made rough, and blacked, to prevent an image being seen from both surfaces, which was found to be the case when this precaution waw omitted. One of these mirrors was struck on a piece of wood directly opposite to a window, at 12 feet distance from it; a board 3 feet long, and 6 inches broad, was placed perpendicularly against the sash of the window, and its image reflected from the mirror on the object-glass of an achromatic microscope, with a divided eye-glass micrometer. The 2 images were separated by means of the divided eye-glass, till their surface of contact, which appears like a black line, was tendered as small as possible. When this effect was produced on the images from the mirror of 4/10 of an inch focus [98.43 D], that mirror was removed, and the other put into its place; the contact of the 2 images, which before appeared like a line, had now acquired considerable breadth; corresponding exactly to the difference between the convexities of the mirrors.

Having in this way made trial of the instruments, and arranged all the necessary circumstances, the head of a person was so placed as to bring the eye into the same situation as the mirror, and made steady by the apparatus described in our former experiments. Under these circumstances the image reflected from the cornea was measured by the micrometer. Mr. Ramsden made an experiment with this instrument on my eye. In the first trials, when the eye was fresh, there was a perceptible change in the micrometer, but extremely small; this was not however seen afterwards, and the eye very soon became so much fatigued that it was necessary to desist. He found that every time the eye adapted itself to different distances, it was necessary to move the object-glass of the microscope farther from, or nearer to, the cornea.

This experiment was repeated on 4 different days; and in each experiment, on the first trial, the result was a change in the micrometer, but in all the subsequent trials it could not be detected. We were induced to conclude, that the effect on the micrometer might arise from the head being moved forwards, as we found, in making experiments with the mirror, that this effect could be produced by such motion; but had it arisen from that cause, it should more frequently have occurred, and rather after the head and eye were tired, than on the first trials. It was suppposed to arise from the action of the muscles of the head, but that should have produced a contrary appearance. The effect produced on the micrometer therefore did not seem to depend on external circumstances, but to arise from a change in the cornea; it was however too small to admit of any conclusions beng drawn from it. The same experiment was made on several young persons; but we found it necessary, that whoever was the subject of the experiment should understand perfectly what was meant to be done, otherwise the conclusions could not be depended on; for if the eye does not see the near object iwht a very defined outline, it is not accurately adjusted to it; and the length of time they keep their eye on the near object without making any complaint of being fatigued, was greater, we knew from our own observation, than it was possible to do it, had the object been seen with the necessary degree of distinctness.

Finding from these experiments, that the change in the convexity of the cornea was not to be seen distinctly in the micrometer, it became an object to ascertain the degree of change which could in this way be distinctly determined. For this purpose 2 mirrors were ground, and prepared in the same way as those used in the preceeding experiment; their radii were exactly ascertained by measuring the tools in which they were finished off; the one was 4/10 of an inch focus [98.43 D], the other 408/1000 [96.50 D], the difference between the size of the images reflected from their surface was just visible in the micrometer; and from their remaining fixed, the experiment could be made with every advantage; but it did not appear probable that the same difference would have been visible had the mirror not been perfectly at rest. A smaller change could not therefore be detected in the eye; and when we consider the disadvantages under which an experiment of this nature must be made on the human eye, from the unsteadiness of that organ, the short time it remains adjusted (a part of which is lost in bringing it within the focus of the microscope), and also from the motions of the head; it is not unreasonable to suppose that a change might take place in the cornea, to the same extent, without being distinctly seen.

To give an idea of the short time that a part can remain nicely adjusted by muscular action, I shall point out an experiment which any one may make on himself: let him take a glass spirit level, and rest one end of it on a table, supporting the other with his hand, and endeavour to keep the air bubble in the middle; if the hand is very steady the bubble may be kept nearly in its place, but not exactly so; it will undulate, its motion corresponding with the actions of the muscles; making up for want of steadiness by short motions in contrary directions.

From these experiments the change in the curvature of the cornea could not be more than 1/125 part of an inch [0.20 mm], as any greater quantity would probably have been distinctly seen in the micrometer; this however is still more than was ascertained by our former experiments, which made it to exceed 1/800 part of an inch [0.03 mm]. This change in the cornea, on the first view of the subject, appeared sufficient to account for the adjustment of the eye; and when the lens is removed it probably may be sufficient; but the refractions at the cornea are so much changed by those at the lens, as considerably to lessen their effect in fitting the eye for seeing near objects, and make this small increase of convexity inadequate to such an effect. Finding this to be the case, it became necessary to examine the eye with attention, to see in what way the full effect was most likely to be produced. For this purpose the following experiments were made on the human eye, to determine whether the axis of vision could be elongated by any uniform pressure applied to its coats.

The experiments were made in the following manner: an eye of a dead subject was carefully removed from the socket, before any change could be produced in consequence of death, and its different diameters were measured by a pair of caliper compasses. As soon as these were determined, a hole was made in the centre of the optice nerve, and a pipe fixed into it, through which air could be thrown into the cavity of the eye, so as to distend its coats. While distended in a moderate degree, by compressing with the hand a small bladder, containing air and quicksilver, attached to the pipe, the same diameters were measured again, and compared with those which were taken while in the natural state. These experiments were made by Mr. Mutltlebury and Mr. Williams, two very intelligent and dilligent students in surgery, who were filling situations that gave opportunity of making such experiments. They measured the diameters in these 2 states, and marked them on paper, without ascertaining their difference, so that there could be no fallacy in the measurement from any pre-conceived opinion; and I have every reason to believe there was none from inattention.

A = The eye of a boy 6 years old, 45 minutes after death
B = The eye of a man 25 years old, 1 hour after death
B = The eye of a man 50 years old, 20 minutes after death

Measurements given in 20th parts of an inch.

                        Transverse      Axis from       Axis of
                        diameter        optic nerve     vision

A  Natural state        17-1/2          17-1/2          17-1/2
   Distended state      17-1/4+         17-1/4+         18

B  Natural state        17-3/4          17-3/4          17
   Distended state      17-1/2          17-1/2          17-1/2

C  Natural state        19              19              18-1/2
   Distended state      19              19              18-1/2


Measurements converted into mm.

                        Transverse      Axis from       Axis of
                        diameter        optic nerve     vision

A  Natural state        22.23           22.23           22.23
   Distended state      21.90+          21.90+          22.86

B  Natural state        22.02           22.02           21.59
   Distended state      22.23           22.23           22.23

C  Natural state        24.13           24.13           23.49
   Distended state      24.13           24.13           23.49

From these experiments it appears, that the diameters of the eye do not always bear the same proportion; sometimes the transverse diameter is the longest, in other eyes it is of the same length as the axis of vision; but when the coats are distended, the transverse diameter is diminished, and the axis of vision is lengthened. This change, however, does not take place at all ages, for at 50 it was not met with.

In those experiments the pressure was made in the most unfavourable way for producing the greatest degree of elongation in the axis of vision; it was however the least exceptionable mode for ascertaining that such an effect could take place; when the pressure is made laterally and from without, the elongation must be still greater; and the action of the straight muscles is the most advantageous that could be imagined for that purpose. This lateral pressure will not only elongate the eye, and increase the convexity of the cornea, but it will produce an effect on the crystalline lens and ciliary processes, pushing them forward in the same proportion as the cornea is stretched. This is necessary for two reasons; viz. to preserve the cavity containing the aqueous humour always of the same size, and to keep the cornea and lens at the same distance from each other. The ciliary processes, as they form a complete septum between the vitreous and aqueous humours, must be moved forward, together with the lens, when the cornea is rendered more convex, and when the cornea recovers itself they are thrown back into their former situation. In order to effect this with the nicety that is required, the ciliary processes are probably possessed of a muscular power.

That the ciliary processes are muscular is a very generally received opinion, and in the course of this lecture I shall adduce some facts in favour of it; they will also tend to confirm the opinion of these processes being a sling, in which the lens is suspended, and rendered capable of a small degree of motion. The result of this inquiry, which has not been confined to the support of any particular theory, but carried on with the sole view of discovering the truth, appears to be, that the adjustment of the eye is produce by 3 different changes in that organ; an increase of curvature in the cornea, an elongation of the axis of vision, and a motion of the crystalline lens. These changes in a great measure depend on the contraction of the 4 straight muscles of the eye. Mr. Ramsden has made a computation, by which the degree of adjustment produced by each of these changes may be ascertained. This he has promised to render more correct; and also to institute a series of experiments by which the effects of the motion of the lens may be more accurately determined. From Mr. Ramsden's computation, the increase of curvature of the cornea appears capable of producing 1/3 of the effect; and the change of place of the lens, and elongation of the axis of vision, sufficiently account for the other 2/3 of the quantity of adjustment necessary to make up the whole.

Having explained the mode by which the axis of vision can be elongated, and the convexity of the cornea increased, in the human eye, for the purpose of its adjustment, I was desirous of applying these observations to the eyes of other animals, that I might see whether their different structures would admit of the necessary changes, for producting an adjustment to different distances in the same way. As many animals are known to have their vision distinct at very different distances, it appeared that much information might be gained by examining the structure of the eyes of those whose range of vision varies most from that of the human eye. Quadrupeds in general must have their eyes fitted to see very near objects, as many of them collect their food with their mouths, in which action the objects are brought very close to the eye. Birds are under the same circumstances in a still greater degree with respect to their food; but from their mode of life, they also require the power of seeing objects at a great distance. Fishes, from the nature of the medium in which they live, must have some other mode of adjusting the eeye, than that of a change in the cornea, as that substance is possessed of the same refractive power with the surrounding fluid.

To avoid confusion in so extensive a field of inquiry, I shall separately consider the peculiarities in the eyes of these different classes of animals, so far as they appear to be concerned in producing the adjustment to different distances. Quadrupeds have 3 modes of procuring their food; one by their fore-paws only, which they use like hands, as all the monkey tribe; the 2d, by their fore-paws and mouths, as the lion, and cat tribe; the 3d, by the mouth only, as all ruminating animals. These 3 different modes require the food being brought to different distances from the eye; and it is curious, that the muscles of the eye are different in all the 3 tribes. In the monkey tribe, the muscles of the eye are exactly the same as in the human. In the lion tribe, they are double in number, and the 4 intermediate muscles are lost in the sclerotic coat, at a greater distance from the cornea than the others. In the ruminating tribe, they are double in number, and the 4 intermediate muscles are lost in the sclerotic coat, at a greater distance from the cornea than the others. In the ruminating tribe, there are 4 muscles, as in the human eye; but there is also a muscle surrounding the eye-ball, attached to the bottom of the orbit, round the hole through which the optic nerve passes, and lost on the sclerotic coat immediately before the broadest diameter of the globe of the eye; the upper portion of this muscle is rather the longest, but not to the axis of the eye from the entrance of the optic nerve.

In quadrupeds in general, the ball of the eye is broader in proportion to its depth, than in the human subject; in the bull the proportion is 1-5/8 inch to 1-3/8. The cornea is larger and more prominent; its real thickness is hardly to be determined, since, as well as that of the human eye, it readily imbibes moisture immediately after death. When dried, it is thinner than the sclerotic coat in the same state. In ruminating animals, it appears externally of an oval form; it is not however really so, the cornea itself being circular, as in other animals; but a portion of it is rendered opaque, by a membrane which voers its external surface, and produces an oval appearance. This circular form of cornea is necessary, that when it is stretched it may form a regular curve. The ciliary processes, as in the human eye, are connected with the choroide coat; but they are larger, and are united at their origin with the iris. This structure of the eye in quadrupeds, so far as it differs from that of the human eye, appears calculated to increase the power of adjusting it to see near objects, and from the mode of life which these animals pursue, such additional powers appear necessary to enable them with ease to procure their food.

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