OF VISION.
AFTER having considered, in detail, the various funcAfter having considered, in detail, the various func- tions which depend upon the contractility of the muscular fibre, we must now proceed to those which originate in the sensibility of the nervous system. These may be di vided into the two classes of physical and intellectual : the first comprehending those which, immediately origi nate in the operation of some external or physical agent ; the second, in the powers of the nervous system re-acting upon each other.
The physical functions of the nervous system are what we usually denominate the external senses, which are ge nerally, although not very correctly, classed under five heads. They are distinguished from all other faculties, both by the causes which produce them, and the instru ments by which the causes operate ; both the cause and the instrument being, in each case, appropriate, and not applicable to any other function. No organ, except the eye, can give us any conception of light, nor, except the ear,. of sound ; the first is produced by an agent of a subtile nature, that acts exclusively on the eye; the se cond by undulations in the air, the operation of which is equally restricted to the car. The primary effects which are produced upon these organs have received different names, but, perhaps, the most correct and comprehensive term is perception of impressions, implying both an effect produced by an external agent, and the transmission of the effect to the sensorium. We shall begin by consi dering the sense of vision, as it constitutes the most nu merous and important class of the perceptions of impres sions ; we are also the best acquainted with the action of the exciting cause, and of the mechanism of the instru ment by which it operates.
In giving an account of vision, it will not be necessary for us to consider the nature and properties of light, as this subject is amply treated of in other articles. We shall, therefore, begin by describing the structure of the eye, the uses of its different parts, and the manner in which light acts upon it, so as to produce vision ; and we shall afterwards enter upon the consideration of what have been termed the acquired perceptions of sight, and the the associations which are formed between the sight and the other classes of physical sensations. The cyc is an
optical instrument, consisting of three orders of parts. The most important is a transparent sphere, or lens, through which the rays of light pass, and are conveyed to a focus, producing an image of the object, similar to that in the camera obscura. The next point is to transmit the impression to the sensorium ; for this purpose it falls upon a nervous expansion, termed the retina, which is connect ed with the optic nerve, and this with the brain. In the third place, there are many auxiliary parts, by which the eye is protected from injury,—Is preserved in its pi ()per form,—is furnished with the necessary secretions, and enabled to perform all its various motions.
The ball, or globe of the eye, consists of what have been termed the three humours; these are, the crystal line, the aqueous, and the vitreous. The crystalline is a firm, transparent body, having the shape of a double con vex lens, which is placed perpendicularly in the eye, be hind the pupil. Its average density has been found to be intermediate between that of glass and of water. It ap pears to be composed principally of albumen ; it is soft ened by maceration in water, and is then divisib:e into a number of layers. Between the crystalline and the cor nea, which constitutes the fore part of the eye, there is a small space which is filled with the aqueous humour, a substance composed almost entirely of water, holding in solution a small quantity of albumen and some salts: this alone, of the different parts of the eye, is properly entitled to the appellation of humour. Behind the crystalline, and occupying the greatest part of the ball or globe, is the vitreous humour,—a transparent substance, nearly of the consistence of the white of the egg, and composed of nearly the same materials.