738 SIGHT. 



interfered with ; and it has been urged that nervous gray matter like mus- 

 cular substance develops acidity during activity as well as upon death, the 

 acidity being probably due in each case to some form of lactic acid. And 

 just as it has been suggested that the dilatation of the minute arteries of a 

 skeletal muscle, accompanying or following the contraction of the muscle, 

 is brought about by the acid generated during the contraction causing a 

 relaxation of the muscular coats of the minute arteries, so it has been sug- 

 gested that a similar acidity, the product of nervous activity, similarly leads 

 in nervous tissue to a dilatation of the vessels of the part. The existence of 

 special vasomotor mechanisms would, however, afford a more satisfactory 

 explanation of these and other phenomena ; in spite of the negative results 

 so far obtained, the matter is obviously one needing further investigation. 

 Meanwhile we have abundant evidence that, however brought about, the 

 flow of blood through the brain, and probably through particular parts of 

 the brain, is varied in accordance with the needs of the brain itself and the 

 events taking place elsewhere in the body. 



CHAPTER III. 



SIGHT. 



614. A RAY of light falling on the retina gives rise to what we call a 

 sensation of light ; but in order that distinct vision of any object 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 eifected ; 

 these we may call briefly the dioptric mechanisms. We shall then have to 

 inquire into the laws according to which rays of light impinging on the 

 retina give rise to sensory impulses, and those according to which the im- 

 pulses thus generated giVe rise in turn to sensations. Sere we shall come 

 upon the difficulty of distinguishing between the unconscious or physical 

 and the conscious or psychical factors. And we shall find our difficulties 

 increased by the fact, that in appealing to our own consciousness we are apt 

 to fall into error by confounding primary and direct sensations with states 

 of consciousness which are produced by the weaving of these primary sen- 

 sations with other operations of the central nervous system, or, in familiar 

 language, by confounding what we see with what we think we see. These 

 two things we will briefly distinguish as visual sensations and visual judg- 

 ments ; and we shall find that both in vision with one eye, but more 

 especially in binocular vision, visual judgments form a very large part of 

 what we frequently speak of as our sight. 



615. [The eyeball is of as pheroidal shape. It consists of two seg- 

 ments of different-sized spheres. The larger segment is situated posteriorly, 

 and constitutes about five-sixths of the walls of the eyeball. From its free 

 margin projects the smaller segment, which is that of a smaller sphere. The 

 posterior segment is composed of a whitish, opaque, firm wall, consisting of 

 three coats or tunics the sclerotic, the choroid, and retina. The anterior 

 segment is continuous with the sclerotic coat. (Fig. 157.) It is a trans- 

 parent, elastic, convex organ, called the cornea. The cornea consists of three 

 layers an anterior and posterior elastic lamina, having between them a 



