CHAP, in.] SIGHT. 835 



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 fre- 

 quently speak of as our ' sight.' 



525. 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, 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 medul- 

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

 which, with scanty underlying mesoblast, is the rudiment of the 

 future skin. 



Veiy 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 hemispher- 

 ical cup with a double wall, one the hind or outer wall corre- 

 sponding 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 oblit- 

 erated. 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 mesoblas- 

 tic tissue, carrying blood vessels. This core of vascular meso- 

 blast, 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 devel- 

 opment carried forward through the centre of the cup. The 



