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figure, from Müller, shows the simple and compound colours, and those which are complementary of each other: "The three simple colours, red, yellow, and blue, are placed at the angles of an equilateral triangle, which are connected together

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red

violet

simple or homogeneous colours; and the complementary colours-of which the pigments,

yellow

blue

green

when mixed, would con

stitute a grey, and of which the prismatic spectra would together produce a white light-will be found to be placed in each case opposite to each other, but connected by a line passing through the centre of the circle. The figure is also useful in showing the further shades of colour which are complementary of each other. If the circle be supposed to contain every transition of colour between the six marked down, those which, when united, yield a white or grey colour, will always be found directly opposite to each other; thus, for example, the intermediate tint betwen orange and red is complementary of the middle tint between green and blue."

The blindness of the entrance of the optic nerve (punctum cœcum) is proved by Mariotte's famous experiment, which may be performed as follows: place both thumbs together at arm's length, shut the left eye, and with the right look fixedly at the left thumb; if the right be now moved slowly outwards, its image will not be perceived as it passes over the entrance of the optic nerve, but will afterwards re-appear. It has been said that if this insensible point existed, there would be a dark spot in the field of vision whenever we used but one eye; but

as we require nerves to tell us of cold, or the absence of heat, to be aware of the darkness, or absence of light, we would require nervous matter, which is here deficient. As the optic nerves do not enter the centres of man's parallel eyes, the blind spots never correspond.

By the action of the lenses, the picture is reversed on the optical screen, but the mind nevertheless perceives the object in its natural erect position. Müller obscurely says that there is no need of correction, as, if all objects . do really appear inverted, their relative position is unaltered. If, besides the vertical decussation-which, as we have before noticed, Prof. Power discovered in a few animals-there was an interchange of the fibres of the optic nerve in every other direction, it would afford a satisfactory explanation; and if the rays penetrate through the retina to the choroid, as Kölliker, Draper, and others believe, it may also explain the phenomenon.

Colour-blindness, or the inability to distinguish certain colours was first observed by Dalton, the great chemist, in his own person (hence the name Daltonism, often applied to this defect), and many cases have been since recorded where ludicrous mistakes occurred from the way in which tailors who suffered from it arranged the cloths they were fabricating.

From what has been stated, it will be remembered that for perfect vision the image must be perfectly distinct, must be sufficiently illuminated, and of sufficient magnitude and duration on the retina.

HEARING.

The organ of hearing in man consists of the external ear or pinna and meatus, the middle ear or tympanum, and the internal ear or labyrinth.

The Pinna is composed of a folded sheet of fibro-cartilage, covered with thin skin and a few muscular fibres, often named after the following prominences-the helix or outer rim, the antihelix, which the semilunar fossa

divides from the first named, the tragus, and antitragus. The lobe is composed of areolar tissue alone. The attrahens, attollens, and retrahens muscles, and a few ligamentous bands, bind the external ear to the skull. The meatus begins by the hollow, named concha, and leads forwards and inwards, with a slight concavity downwards for about an inch, the lower wall being longer, as the membrana tympani is set at an angle of 45° towards it. About half the canal is made by cartilage, which being deficient above and behind (the fissures of Sanctorini), may allow of some alterations in calibre. The entrance of insects or dust is prevented by vibrissæ, and by a sticky secretion, the ear-wax, which, if allowed to harden, is a frequent cause of deafness. The external ear collects and conducts sounds to the membrana tympani, and its remarkably moulded surface fits it to catch vibrations in whatever direction they proceed.

Many animals exceed man in the development of the pinna; and, as is well known, ear-trumpets, or the hand placed behind the ear, will aid its office.

The Middle Ear, or tympanum, is a bony cavity hollowed in the petrous portion of temporal bone, lined by ciliated epithelium, and filled with air, which the Eustachian tube carries up from the pharynx. Its outer wall presents the large opening in which the membrana tympani is set.

This membrane is oval, being rather larger above, and consists of an outer cuticular layer, an inner nonciliated epithelial layer, and a middle proper layer, composed of strong fibrous tissue, set into a groove in the bone. Toynbee describes two such layers, the outer radiating, the inner annular; and Sir E. Home described the radiating fibres as muscular. The late Professor Harrison dissected that of the elephant, and could not demonstrate its muscularity. He noticed that in this animal there was a perforation, which Rivinus regarded as essential. The handle of the malleus is attached to

the upper half of the membrane, and, aided by the denser external air, it renders it concave outwards; but in birds it is convex. Examined during life with the speculum, Sir W. Wilde compares the membrane to the tint of wet parchment, but rather more pink. Its func

Diagram of the External, Middle, and Internal Ear."

tion in hearing may be shown by the following experiment of Savart. He sprinkled fine sand over it, and held a vibrating body near it; it at once vibrated powerfully, and the sand assumed remarkable figures, remaining at what are called " nodal lines" where least vibration occurred.

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The opposite or inner wall of the tympanum has two openings, the fenestra ovalis leading into the vestibule, and the fenestra rotunda into the cochlea. They are closed also by membranes, and the oval one, which is superior and more than twice as large as the round one,

is closed by the base of the stapes. The promontory, caused by the first turn of the cochlea, lies between these openings, and behind them is the pyramid, a hollow funnel of bone, containing the stapedius muscle, which, issuing from the apex, is inserted into the neck of the stapes. Through a hole at the base of the pyramid the corda tympani nerve enters the cavity, and passing between the malleus and incus escapes by a slit in the Glasserian fissure. The aqueduct of Fallopius, which carries the facial nerve, slightly raises the bone above the pyramid; and lastly, there are several openings into the mastoid cells which are filled with air.

The anterior part of the cavity has 4 openings: 1. A canal in which the tensor tympani muscle arises, and passes to be inserted into the short process of the malleus. 2. A foramen at the Glasserian fissure, transmitting a ligament misnamed the laxator tympani, which is attached to the processus gracilis of the malleus. 3. The foramen for the exit of the corda tympani; and 4. The Eustachian tube, which leads upwards, backwards, and outwards from the pharynx just behind the inferior spongy bone, where its orifice is large enough to admit the point of the little finger, or the Eustachian sound. It is about 1 inches long, the lower being fibro-cartilaginous, and the remainder a small canal, in the angle between the petrous and squamous portions of temporal bone, parallel to the canal for the tensor tympani, and only divided from it by a thin plate, the processus cochleariformis. This tube is of use in conducting air to support the membrana tympani, in discharging mucus, and in carrying up, it was said, the individual's own voice; but a watch, if placed in the pharynx without touching any solid, is scarcely heard to tick.

The carotid artery and lateral sinus lie close to the tympanum, so that they are endangered in fractures traversing it, and in the ulceration which often follows scarlatina. The late Professor Porter saw more blood

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