206 Basic Structure of Vertebrates 



vague and "done" all in tones of gray. When an eye is focused for 

 optimum vision of an external object, the image is centered on the 

 fovea. There are, accordingly, at least three reasons why the fovea 

 should be the region of keenest vision: it is in the optic axis; rays of 

 light may reach its receptors without passing through the maximum 

 thickness of the ganglionic layers; and, presumably, adequate recog- 

 nition of colors is correlated with presence of cones. The rods appar- 

 ently serve for recognition of different intensities of illumination but 

 without distinction of colors. 



Accessory Optic Structures 



The muscles which move the eye in the orbit have been described 

 in connection with the account of the cranial nerves (see p. 162). 

 Eyelids, present in most terrestrial vertebrates, are folds of skin above 

 and below the eye (Fig. 176). The skin of the external surface of the 

 fold is similar to that of the adjacent region of the head. The internal 

 layer consists of the very thin conjunctiva which continues over the 

 cornea. Lacrimal ("tear") glands may be present within the lids, 

 and small oil-glands along the edges of the lids serve to prevent over- 

 flow of the watery lacrimal secretion. 



The "Inverted" Retina 



An extraordinary feature of the retina is that it seems to be func- 

 tionally inside-out. A sense-cell at the surface of the body has its 

 receptor-pole at the outer surface and its conductor-pole at its deep 

 end. The receptor-pole is directed toward the source of the stimulus. 

 The rods and cones are so placed that their conductor-poles are directed 

 toward the source of the stimulus. The entering light-rays are further 

 embarrassed by the necessity of penetrating a considerable thickness 

 of nervous tissue before they may arrive at even the "wrong" end of 

 the receptor. In these respects, the vertebrate retina seems poorly 

 contrived. It is commonly referred to as an "inverted retina." 



A reason for this inversion may be found by taking into account 

 the embryonic history of a rod or cone (Fig. 172). It may be imagined 

 that a certain ectodermal cell in the anterior middorsal region of an 

 early embryo is a prospective rod or cone with its potential receptor- 

 pole directed outward as it should be in a sense-cell. After infolding 

 of the dorsal ectoderm to form the neural tube, the receptor-pole of 

 that cell will point into the lumen of the tube (Fig. 172A). If the cell 

 happens to lie at the center of the optic outgrowth, it will later be 

 found with its receptor-pole pointing into the cavity of the primary 

 optic vesicle (B). Following the infolding which produces the optic 

 cup, the cell will lie with its receptor-pole against the prospective 



