STRUCTURE OF THE EYE. [BOOK in. 



ness we are apt to fall into error by failing to distinguish between 

 those affections of consciousness which are the primary and direct 

 results of the stimulation of the retina and those secondary, more 

 recondite, affections of consciousness to which the former, through 

 the intricate working of the central nervous system, give rise, 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 judgments ; and we shall find that 

 even in vision with one eye, though more especially in binocular 

 vision, visual judgments form a very large part of what we 

 frequently speak of as our " sight." 



703. In the structure of the eye we may distinguish two 

 parts : the one is the retina, in which visual impulses are gene- 

 rated ; the other comprises all the rest of the eyeball, for all the 

 other structures serve either as a dioptric mechanism or as a 

 means of nourishing the retina. This distinction is readily seen 

 when we trace out the early history of the eye. 



The first of the three primary cerebral vesicles ( 600), that 

 which is the forerunner of the third ventricle, buds out on each 

 side the stalked and hollow optic vesicle. The wall of this optic 

 vesicle, like that of the rest of the medullary tube, consists of 

 epithelium, and the cavity of the vesicle is at first continuous, 

 through the canal of the hollow stalk, with that of the medullary 

 tube. The whole is covered over by the layer of epiblast which, 

 with scanty underlying mesoblast, is the rudiment of the future 

 skin. 



Very soon the vesicle is doubled back upon or folded in upon 

 itself so that the originally more or less spherical hollow single- 

 walled vesicle is converted into a more or less hemispherical cup 

 with a double wall, one the hind or outer wall corresponding to 

 the hind half, and the other the front or inner wall to the front 

 half of the vesicle, the two walls of the cup coming eventually 

 into contact so that the cavity of the vesicle is obliterated. The 

 folding is somewhat peculiar, inasmuch as it takes place not only 

 at the front but also and indeed chiefly at the side, forming at the 

 side a cleft, the choroidal fissure, the edges of which ultimately 

 unite. We cannot enter into the details of the matter here, and 

 indeed only refer to the character of the folding in order to point 

 out that it involves the stalk as well as the cup. The stalk is. 

 first flattened and then doubled up lengthwise, a quantity of 

 mesoblastic tissue being thrust into the hollow of the fold ; and 

 eventually the originally hollow stalk becomes a solid stalk having 

 within it a core of mesoblastic tissue, carrying blood vessels. This 

 core of vascular mesoblast, the origin of the future central artery 

 of the retina, is continuous with a quantity of mesoblast which 

 enters into the hollow of the cup at the time of folding, and, as we 

 shall see, the central artery of the stalk is up to a certain stage of 

 development carried forward through the centre of the cup. The 



