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it arises here; the upper, because, though it arises along with the recti from the back of the orbit, yet, after passing forwards and becoming tendinous at the upper and inner corner of the orbit, it traverses a pulley-like loop of ligament, and then turns downwards and outwards to its insertion. The action of the oblique muscles is somewhat complicated, but their general tendency is to roll the eyeball on its axis, and pull it a little forward and inward.
24. The eyelids are folds of skin containing thin plates of cartilage, and fringed at their edges with hairs, the eyelashes, and with a series of small glands called Meibomian.
The front view of the right eye dissected to show, Orb., the orbicular muscle of the eyelids; the pulley and insertion of the superior oblique, S.Ob., and the inferior oblique, Inf.Ob.; L.G. the lachrymal gland.
Circularly disposed fibres of striped muscle lie beneath the integuments of the eyelids, and constitute the orbicularis muscle which shuts them. The upper eyelid is raised by a special muscle, the levator of the upper lid, which arises at the back of the orbit and runs forwards to end in the lid.
The lower lid has no special depressor.
25. At the edge of the eyelids the integument becomes continuous with a delicate, vascular, and highly nervous mucous membrane, the conjunctiva, which lines the interior of the lids and the front of the eyeball, its epithelial layer being even continued over the cornea.
The numerous small ducts of a gland which is lodged in the orbit, on the outer side of the ball (Fig. 79, L.G.), the lachrymal gland, constantly pour its watery secretion into the interspace between the conjunctiva lining the upper eyelid and that covering the ball. On the inner side of the eye is a reddish fold, the caruncula lachrymalis, a sort of rudiment of that third eyelid which is to be found in many animals. Above and below, close to the caruncula, the edge of each eyelid presents a minute aperture (the punctum lachrymale), the opening of a
A front view of the left eye, with the eyelids partially dissected to show lachrymal gland, L.G., and lachrymal duct, L.D.
small canal. The canals from above and below converge and open into the lachrymal sac; the upper blind end of a duct (L.D., Fig. 80) which passes down from the orbit to the nose, opening below the inferior turbinal bone (Fig. 40, h). It is through this system of canals that the conjunctival mucous membrane is continuous with that of the nose; and it is by them that the secretion of the lachrymal canal is ordinarily carried away as fast as it forms.
But, under certain circumstances, as when the conjunctiva is irritated by pungent vapours, or when painful emotions arise in the mind, the secretion of the lachrymal gland exceeds the drainage power of the lachrymal duct, and the fluid, accumulating between the lids, at length overflows in the form of tears.
I. IN explaining the functions of the sensory organs, I have hitherto confined myself to describing the means by which the physical agent of a sensation is enabled to irritate a given sensory nerve; and to giving some account of the simple sensations which are thus evolved.
Simple sensations of this kind are such as might be produced by the irritation of a single nerve-fibre, or of several nerve-fibres by the same agent. Such are the sensations of contact, of warmth, of sweetness, of an odour, of a musical note, of whiteness, or redness.
But very few of our sensations are thus simple. Most of even those which we are in the habit of regarding as simple, are really compounds of different sensations, or of sensations with ideas, or with judgments. For example, in the preceding cases, it is very difficult to separate the sensation of contact from the judgment that something is touching us; of sweetness, from the idea of something in the mouth; of sound or light, from the judgment that something outside us is shining, or sounding.
2. The sensations of smell are those which are least complicated by accessories of this sort. Thus, particles of musk diffuse themselves with great rapidity through the nasal passages, and give rise to the sensation of a powerful odour. But beyond a broad notion that the odour is in the nose, this sensation is unaccompanied by any ideas of locality and direction. Still less does it give rise to any conception of form, or size, or force,
If a man had
or of succession, or contemporaneity. no other sense than that of smell, and musk were the only odorous body, he could have no sense of outness— no power of distinguishing between the external world and himself.
3. Contrast this with what may seem to be the equally simple sensation obtained by drawing the finger along the table, the eyes being shut. This act gives one the sensation of a flat, hard surface outside oneself, which appears to be just as simple as the odour of musk, but is really a complex state of feeling compounded of— (a) Pure sensations of contact.
(6) Pure muscular sensations of two kinds, the one arising from the resistance of the table, the other from the actions of those muscles which draw the finger along. (c) Ideas of the order in which these pure sensations succeed one another.
(d) Comparisons of these sensations and their order, with the recollection of like sensations similarly arranged, which have been obtained on previous occasions.
(e) Recollections of the impressions of extension, flatness, &c. made on the crgan of vision when these previous tactile and muscular sensations were obtained.
Thus, in this case, the only pure sensations are those of contact and muscular action. The greater part of what we call the sensation is a complex mass of present and recollected ideas and judgments.
4. Should any doubt remain that we do thus mix up our sensations with our judgments into one indistinguishable whole, shut the eyes as before, and, instead of touching the table with the finger, take a round lead pencil between the fingers, and draw that along the table. The "sensation" of a flat hard surface will be just as clear as before; and yet all that we touch is the round surface of the pencil, and the only pure sensations we owe to the table are those afforded by the muscular sense. In fact, in this case, our sensation" of a flat hard surface is entirely a judgment based upon what the muscular sense tells us is going on in certain muscles.
A still more striking case of the tenacity with which we adhere to complex judgments, which we conceive to be pure sensations, and are unable to analyse otherwise
than by a process of abstract reasoning, is afforded by our sense of roundness.
Anyone taking a marble between two fingers will say that he feels it to be a single round body; and he will probably be as much at a loss to answer the question how he knows that it is round, as he would be if he were asked how he knows that a scent is a scent.
Nevertheless, this notion of the roundness of the marble is really a very complex judgment, and that it is so may be shown by a simple experiment. If the index and middle fingers be crossed, and the marble placed between them, so as to be in contact with both, it is utterly impossible to avoid the belief that there are two marbles instead of one. Even looking at the marble, and secing that there is only one, does not weaken the apparent proof derived from touch that there are two.1
The fact is, that our notions of singleness and roundness are, really, highly complex judgments based upon a few simple sensations; and when the ordinary conditions of those judgments are reversed, the judgment is also reversed.
With the index and the middle fingers in their ordinary position, it is of course impossible that the outer sides of each should touch opposite surfaces of one spheroidal body. If, in the natural and usual position of the fingers, their outer surfaces simultaneously give us the impression of a spheroid (which itself is a complex judgment), it is in the nature of things that there must be two spheroids. But, when the fingers are crossed over the marble, the outer side of each finger is really in contact with a spheroid; and the mind, taking no cognizance of the crossing, judges in accordance with its universal experience, that two spheroids, and not one, give rise to the sensations which are perceived.
5. Phenomena of this kind are not uncommonly called delusions of the senses; but there is no such thing as a fictitious, or delusive, sensation. A sensation must
A ludicrous form of this experiment is to apply the crossed fingers to the end of the nose, when it at once appears double; and, in spite of the absurdity of the conviction, the mind cannot expel it, so long as the sensations last,