A TextBook of Physiology Author:Michael Foster Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: chapter{Section 4chapter{Section 5chapter{Section 6CHAPTER III. SIGHT. SEC. 1. ON THE GENERAL STRUCTURE OF THE EYE, AND ON THE FORMATION OF THE RETINAL IMA... more »GE. § 702. Ix dealing with the braiu we have been incidentally obliged to deal with some of the facts connected with the senses; but we must now study the details of the subject. And, for the very reason that it is the most highly developed and differentiated sense, it will be convenient to begin with the sense of sight; we shall find that the study of it throws more light on the simpler and more obscure senses than the study of them throws on it. A ray of light entering the eye and falling on the retina gives rise to what we call a sensation of light; but in order that distinct vision of any object emitting or reflecting rays of light may be gained, an image of the object must be formed on the retina, and the better defined the image the more distinct will be the vision. Hence in. studying the physiology of vision, our first duty is to examine into the arrangements by which the formation of a satisfactory image on the retina is effected; these we may call briefly the dioptric mechanisms. We shall then have to inquire into the laws according to which rays "f light impinging on the retina give rise to nervous impulses, and into the laws according to which the sensory impulses thus generated, which we will call visual impulses, give rise in turn to visual sensations. Here we shall come upon the difficulty of distinguishing between the events which are of physical origin, due to changes in the retina and optic fibres, and those which are of psychical origin, due to features of our own consciousness; for many of our conclusions are based on an appeal to consciousness. We shall find our difficulties further increased by t...« less