may be removed and yet distinctly seen. If the object be brought nearer to the eye than 200 feet, the accommodative power must come into play; the nearer the object the more energetic must by the contraction of the ciliary muscle and the consequent increase in the convexity of the lens. At a distance of five inches, however, the power of accommodation reaches its maximum; this is termed the punctum proximum, and indicates the nearest point at which an object may be seen distinctly. The distance between these two points is the range of accommodation.

Optical Defects. —Astigmatism is a condition of the eye which prevents vertical and horizontal lines from being focused at the same time, and is due to a greater curvature of the cornea in one meridian than another.

Spherical aberration is a condition in which there is an indistinctness of an image from the unequal refraction of the rays of light passing through the circumference and the center of the lens; it is corrected mainly by the iris, which cuts off the marginal rays, and only transmits those passing through the center.

Chromatic aberration is a condition in which the image is surrounded by a colored margin, from the decomposition of the rays of light into their elementary parts.

Myopia, or short-sightedness, is caused by an abnormal increase in the antero-posterior diameter of the eyeball, or by a subnormal refracting power of the lens; it is generally due to the first cause; the lens being too far removed from the retina, forms the image in front of it, and the perception becomes dim and blurred. Concave glasses correct this defect by preventing the rays from converging too soon.

Hypermetropia, or long-sightedness, is caused by a shortening of the antero-posterior diameter, or by an excessive refractive power of the lens; the focus of the rays of light would, therefore, be behind the retina. Convex glasses correct this defect by converging the rays of light more anteriorly.

Presbyopia is a loss of the power of accommodation of the eye to near objects, and usually occurs between the ages of 40 and 60; it is remedied by the use of convex glasses.

The Iris.—The iris plays the part of a diaphragm, and by means of its central aperture the pupil regulates the quantity of light entering the interior of the eye; by preventing rays from passing through the margin of the lens it diminishes spherical aberration. The size of the pupil depends upon the relative degree of contraction of the circular and radiating fibers;

the variations in size of the pupil from variations in the degree of contraction depend upon different intensities of light. If the light be intense, the circular fibers contract and diminish the size of the pupil; if the light diminishes in intensity, the circular fibers relax and the pupil enlarges.

Point of Most Distinct Vision.-While all portions of the retina are sensitive to light, their sensibility varies within wide limits. At the macula lutea, and more especially in its most central depression, the fovea, where the retinal elements are reduced practically to the layer of rods and cones, the sensibility reaches its maximum. It is at this point that the image is found when vision is most distinct. The macula and fovea are always in the line of direct vision. From the macula toward the periphery of the retina there is a gradual diminution in sensibility and a corresponding decline in the distinctness of vision. In those portions of the retina lying outside the macula the indistinctness of vision depends not only on diminished sensibility, but also upon inaccurate focusing of the rays.

Blind Spot. Although the optic nerve transmits the impulses excited in the retina by the ethereal vibration, the nerve fibers themselves are insensitive to light. At the point of entrance of the optic nerve, owing to the absence of the rods and cones, the rays of light make no impression. This is the blind spot. As this spot is not in the line of vision, no dark point is ordinarily observed in the field of vision, that circular space before a fixed eye within which objects are perceptible.

The rods and cones are the most sensitive portions of the retina. A ray of light entering the eye passes entirely through the various layers of the retina and is arrested only upon reaching the pigmentary epithelium in which the rods and cones are imbedded. As to the manner in which the objective stimuli, light, and color so-called, are transformed into nerve im pulses, but little is known. It is probable that the ethereal vibrations are transformed into heat, which excites the rods and cones. These, acting as highly specialized end organs of the optic nerve, start the impulses on their way to the brain, where the seeing process takes place. As to the relative function of the rods and cones, it has been suggested, from the study of the facts of comparative anatomy, that the rods are impressed only by differences in the intensity of light, while the cones in addition are impressed by qualitative differences or color.

Accessory Structures.-The muscles which move the eyeball are six in number: the superior and inferior recti, the external and internal recti, the superior and inferior oblique muscles. The four recti muscles, arising from the apex of the orbit, pass forward and are inserted into the sides of

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the sclerotic coat; the superior and inferior muscles rotate the eye around a horizontal axis; the external and internal rotate it around a vertical axis.

The superior oblique muscle, having the same origin, passes forward to the inner and upper angle of the orbital cavity, where its tendon passes through a cartilaginous pully; it is then reflected backward and inserted into the sclerotic just behind the transverse diameter. Its function is to rotate the eyeball in such a manner as to direct the pupil downward and outward.

The inferior oblique muscle arises at the inner angle of the orbit and then passes outward and backward, to be inserted into the sclerotic. Its function is to rotate the eyeball and direct the pupil upward and outward.

By the associated action of all these muscles, the eyeball is capable of performing all the varied and complex movements necessary for distinct vision.

The eyelids, bordered with short, stiff hairs, shade the eye and protect it from injury. On the posterior surface, just beneath the conjunctiva, are the Meibomian glands, which secrete an oily fluid; it covers the edge of the lids, and prevents the tears from flowing over the cheek.

The lachrymal glands are ovoid in shape, and situated at the upper and outer part of the orbital cavity; they open by from six to eight ducts at the outer portion of the upper lids.

The tears, secreted by the lachrymal glands, are distributed over the cornea by the lids during the act of winking, and keep it moist and free from dust. The excess of tears passes into the lachrymal ducts, which begin by two minute orifices, one on each lid, at the inner canthus. They conduct the tears into the nasal duct, and so into the nose.

THE SENSE OF HEARING.

The Ear, or Organ of Hearing, is lodged within the petrous portion of the temporal bone. It may be, for convenience of description, divided into three portions, viz.: I. The external ear. 2. The middle ear.

internal ear or labyrinth.

3. The

The External Ear consists of the pinna or auricle and the external auditory canal. The pinna consists of a thin layer of cartilage, presenting a series of elevations and depressions; it is attached by fibrous tissue to the outer bony edge of the auditory canal; it is covered by a layer of integument continuous with that covering the side of the head. The general