Iapetus is the eighth satellite of Saturn. It has a period of seventy-nine days, and an orbitalradius of 2, 225, 000 miles. Like the earth's moon, it always turns the same face towards its primary. Research Iapetus
The gorilla (Troglodytes Gorilla) is the largest anthropoid ape attaining a height of about 5.5 feet and is found in west Africa. Although a gentle and vegetarian animal, myths prevailed from the mid- 19th century until the mid-20th century of it attacking and eating the natives.
The erect position is more readily assumed by the gorilla than by most of the other anthropoid apes, owing to the shape of the sole of the foot, which is not inverted, and is shorter and broader;
but the ordinary gait is on all-fours. It has a ferocious-looking cast of features, due to the prognathism of the jaws, the extremely prominent supra-orbital ridges, and retreating forehead. Gorillas make a sleeping-place somewhat like a hammock, connecting the branches of a tree by means of the long, tough, slender stems of climbing plants, and lining it with dried fronds of palms or long grass. This abode is constructed at different heights from the ground, but there is never more than one such nest in a tree. The gorilla, like the chimpanzee, has thirteen ribs, whereas man and the orang have twelve. The gorilla and chimpanzee also have eight bones in the carpus or wrist, while the others have nine. The bones of the arm are much longer than in man, and the upper arm is longer than the forearm; the leg bones are shorter than in man. In the proportion of its molar teeth to the incisors and in the form of its pelvis it approaches somewhat closely the human form. The Phoenician navigator Hanno found the name in use in the 5th century BC in West Africa. Research Gorilla
The angular vein runs down the side of the nose. It is formed by the junction of the frontal vein, which runs down the middle of the forehead, and the supra-orbital vein, which lies near the frontal vein. Research Angular Vein
The corrugator supercilii (Coiter's muscle) is a small narrow muscle at the inner extremity of the eyebrow, just beneath the occipito-frontalis and occularis palpebrarum muscles. It originates from the nasal prominence and the orbital portion of the orbicularis oculi muscle and inserts in the skin of the eyebrow. This muscle is innervated by the facial nerve (VII cranial nerve) and supplied by the facial artery. The corrugator supercilii draws the eyebrows downward and inward, producing wrinkles on the forehead. It is often called the ' frowning muscle'. Research Corrugator Supercilii
The ethmoid bone is a bone of very spongy substance, somewhat irregularly cubical in shape, lying at the root of the nose, between the two orbits or eye-sockets, and forming part of the bony wall of both. The segment of the ethmoid bone which forms part of the inner wall of the orbital cavity is called the orbital plate, while another segment, forming the roof and back part of the septum of the nasal cavity is called the perpendicular plate. Two irregularly shaped, shell-like projections from the ethmoid bone are called the superior and middle turbinate (conchae), and form ledges on the inside wall of the nasal cavity. Research Ethmoid Bone
The eye is the organ of vision of animals, consisting in man of the globe of the eye, the muscles which move it, and of its appendages, which are the eyelids and eyebrows, and the lachrymal apparatus. The walls of the globe of the eye are formed principally of two fibrous membranes; one white and opaque - the sclerotic (from the Greek skleros meaning hard) - which envelops two-thirds of the globe posteriorly; and the other transparent, and resembling a horny plate, whence its name, cornea (from the Latin. corneus, meaning horny). The sclerotic is a tough fibrous coat, and is the part to which the phrase 'white of the eye' is applied. In the front of the globe the sclerotic is abruptly transformed into the transparent portion (the cornea), which is circular, and which forms a window through which one can see into the interior.
A mucousmembrane, the conjunctiva, so named because it unites the eye to the lid, spreads over the anterior portion of the globe, and then folds back on itself and lines the internal surface of the, eyelids. On the internal surface of the sclerotic is a vascular membrane called the choroid. This is essentially the blood-vessel coat of the eyeball. The front part of the choroid terminates about the place where the sclerotic passes into the cornea in a series of ridges, the ciliary processes. The circular space thus left in front by the termination of the choroid is occupied by the iris, a round curtain, the structure seen through the cornea, differently coloured in different individuals. In its centre is a round hole, the pupil, which appears as if it were a black spot. The iris forms a sort of transverse partition dividing the cavity of the eyeball into two chambers, a small anterior chamber filled with the aqueous humour, and a large posterior chamber filled with vitreous humour. The iris consists of a framework of connective tissue, and its posterior surface is lined by cells containing pigment which gives the colour to the eye. In its substance are bundles of involuntary muscular fibres, one set being arranged in a ring round the margin of the pupil, the other set radiating from the pupil like the spokes of a wheel. In a bright light the circular fibres contract and the pupil is made smaller; but in the dark these fibres relax and cause the pupil to dilate more or less widely, thus allowing only that quantity of luminous rays to enter the eye which is necessary to vision.
Just behind the pupil is the crystalline lens, resembling a small, very strongly magnifying glass, convex on each side, though more so behind. The greater or less convexity of the surfaces of the lens determines whether the vision is long or short. The internal surface of the choroid, or rather the pigmentary layer which covers it, is lined by the retina or nervous tunic upon which the objects are depicted that we see.
The ocular globe is put in motion in the orbit by six muscles, grouped two by two, which raise or lower the eye, turn it inward or outward, or on its antero-posterior axis. In these movements the centre of the globe is immovable, and the eye moves round its transverse and vertical diameters. These three orders of movements are independent of each other, and may be made singly or in combination, in such a manner as to direct the pupil towards all points of the circumference of the orbit.
Each eye is furnished with two eyelids, moved by muscles, which shield it from too much light and keep it from being injured. They are fringed with short fine hairs called eyelashes; and along the edge of the lids is a row of glands similar to the sebaceous glands of the skin. The eyebrows, ridges of thickened integument and muscle, situated on the upper circumference of the orbit and covered with short hairs, also regulate to some extent the admission of light by muscular contraction. In reptiles, some fishes (sharks, etc), in birds, and in some mammals a third eyelid or nictitating membrane is present, and can be drawn over the surface of the eye so as to clear it of foreign matters, and also to modify the light.
The lachrymal apparatus is composed of, firstly, the lachrymal gland, which lies in a depression of the orbitalarch; secondly, of the lachrymal canals, by which the tears are poured out upon the conjunctiva a little above the border of the upper lid; thirdly, the lachrymal ducts, which are destined to receive the tears after they have bathed the eye, and of which the orifices or lachrymal points are seen near the internal commissure of the lids; fourthly, the lachrymal sac, in which the lachrymal ducts terminate, and which empties the tears into the nasalcanal.
The tears, by running over the surface of the conjunctiva, render it supple and facilitate the movements of the globe and eyelids by lessening the friction. The influence of moral or physical causes increases their secretion, and when the lachrymal ducts do not suffice to carry them off they run over the lids.
The retina renders the eye sensible of light, and we may therefore consider it as the essential organ of vision. The function of the other portions is to converge the luminous rays to a focus on the surface of the retina, a condition necessary for distinct vision and the clear perception of objects. The visual impressions are transmitted from the retina to the brain by means of the optic nerve. The two optic nerves converge from the base of the orbit toward the centre of the base of the skull, where there is an interlacement of their fibres in such a manner that a portion of the right nerve goes to the left side of the brain, and a part of the left nerve to the right side; this is called the chiasma or commissure of the optic nerves. The principal advantage of having two eyes is in the estimation of distance and the perception of relief. In order to see a point as single by two eyes we must make its two images fall on corresponding points of the retinas; and this implies a greater or less convergence of the optic axes according as the object is nearer or more remote.
According to one estimate, four-fifths of everything we know reaches the brain through our eyes. The eyes transmit constant streams of images to the brain by electrical signals. The eyes receive information from light rays. The light rays are either absorbed or reflected. Objects that absorb all of the light rays appear black, whereas those that reflect all the light rays appear white. coloured objects absorb certain parts of the light spectrum and reflect others. When you look at something, the light rays reflected from the object enter the eye. The light is refracted by the cornea and passes through the watery aqueous humor and pupil to the lens. The iris controls the amount of light entering the eye. Then the lens focuses the light through the vitreous humor onto the retina, forming an image in reverse and upsidedown. Light- sensitive cells in the retina transmit the image to the brain by electrical signals. The brain perceives the image the right side up.
To accommodate the eye to different distances the lens is capable of altering itself with great precision and rapidity. When we look at a near object the anterior surface of the lens bulges forward, becoming more convex the nearer the object; the more distant the object the more the lens is flattened. When the transparency of the cornea, the crystalline lens, or any of the humours, is destroyed, either partially or entirely, then will partial or total blindness follow, since no image can be formed, upon, the retina; but although all the humours and the cornea be perfectly transparent, and retain their proper forms, which is likewise necessary to distinct vision, yet, from weakness or inactivity of the optic nerve, or injury of the central ganglia with which it is connected, weakness of sight or total blindness may ensue. Defective vision may also arise from the crystalline lens being so convex as to form an image before the rays reach the retina (a defect known as short sight or myopia), in which case distinct vision will be procured by interposing a concave lens between the eye and the object of such a curvature as shall cause the rays that pass through the crystalline lens to meet on the retina; or the lens may be too flat, as is the case in old age, a defect which is corrected by convex lenses.
In the lower forms of life the organs of sight appear as mere pigment spots. Ascending higher, simple lenses or refracting bodies occur. Insects, crustaceans, etc, have large masses of simple eyes or ocelli aggregated together to form compound eyes - the separate facets or lenses being optically distinct, and sometimes numbering many thousands. In the molluscs well-developed eyes approaching in structure those of the highest animals are found; and in all vertebrate animals the organ of vision corresponds generally to what has been described, though they vary much in structure and adaptation to the surroundings of the animal. Research Eye
The frontal vein (frontalis vein), is one of the exterior veins of the head and face. It extends from the forehead to the root of the nose where it joins the supra-orbital vein near the eyeorbit and forms the angular vein. This vein carries blood from the venous plexus of the forehead and scalp to the facial vein. The blood then continues towards the heart and lungs to be reoxygenated. Research Frontal Vein
The inferior oblique is a thin narrow muscle that originates from the orbital surface of the maxilla and extends along the inner side of the eyeball. It helps move the eye sideways and rotates it slightly. The inferior oblique muscle works in conjunction with the other eyemuscles to move the eye. These muscles are anchored to the sclera near the cornea. The
inferior oblique muscle is innervated by the third cranial (oculomotor) nerve. Research Inferior Oblique
The levator palpebrae muscle (orbitopalpebralis) is a thin flat muscle with a triangular shape that is situated in the eyelid. It originates from the orbital portion of the lesser wing of the sphenoid bone and inserts in several places along the skin of the eyelid and the orbital walls. It is innervated by the oculomotor nerve. This muscle raises the eyelid, the direct opposite of the orbicularis palpebrarum muscle. These muscles are used together to blink and to close the eye during sleep. Research Levator Palpebrae Muscle
The maxilla is one of the primary facial bones and defines much of the face below the orbits and above the jawbone. The maxilla (with the ethmoid bone, frontal bone, and zygomatic bones) forms much of the lateral walls of the nasal cavity, the orbital cavity, and part of the cheek. The
maxilla also mounts the top row of teeth in a non-movable foundation, so that chewing is enabled by moving the mandibular teeth against them. The outer wings of the maxilla, where it meets the zygomatic bones, are called the zygomatic processes. The upper part of the
maxilla, which forms the lower, inner surface of the orbit cavity, is called the orbital surface of the
maxilla, while the exterior surface, just between the orbit and the nasal cavity, is called the frontal process of the
maxilla. The roof of the mouth is formed by the palatine bone, which is attached to the maxilla just behind the teeth. Research Maxilla
The Probert Encyclopaedia was designed, edited and programed by
Matt and Leela Probert
Abbreviations
Research Iapetus
