498 HISTORY OF THE HUMAN BODY 



become specialized in form and function so that they respond 

 directly to the stimulus of light vibrations. To this essential 

 part, which, with a pigmented outer layer, are the only parts 

 derived from the brain, accessory organs are added from two 

 sources to complete the formation of the eyeball; the lens, 

 formed from the ectoderm of the outer surface; the chorioid 

 and sclerotic coats and the vitreous body from the surrounding 

 connective tissue. Aside from the eyeball itself there are 

 many external accessory parts, such as muscles and glands, 

 conjunctiva and eyelids, which come from several sources and 

 aid in the movement and protection of the sensitive organ. 



To begin with the essential sense-organ, that is, the retina, if 

 we follow the in- and outpushings of its layer of origin from 

 the beginning, it is clear that the original external surface lines 

 the lumen of the neural tube and eventually forms the outer 

 retinal surface, that is, the surface turned toward the pigmented 

 tapetum. Now in all cases it is the primarily external surface 

 that becomes specialized to receive external stimuli, and it is 

 also the original outer or superficial end of the sensory cells 

 that develop the specially modified flagella and other parts. It 

 thus happens that the terminal cells of the sense of vision are 

 not only the outer ones of the retina, which is several cell- 

 layers in thickness, but also that their free ends, bearing the 

 terminal rods and cones, point in the same direction, namely, 

 towards the interior of the head and away from the source of 

 light. Moreover, since a sensory nerve must approach its 

 terminal cells from their physiological inner side, this arrange- 

 ment compels the optic nerve first to penetrate the entire retina 

 and attain the interior of the eyeball, and there spread out its 

 separate fibers, which severally become recurved and pass back 

 again through the retina to supply the terminal cells. Finally, 

 in order that the image, received through the pupil and focused 

 by the lens upon the physiological outer side of the retina, may 

 reach the terminal rods and cones, all the intervening parts, the 

 nerve fibers and the various layers of retinal cells, have to be 

 perfectly transparent; and, furthemore, the terminal rods and 

 cones must needs be buried in ihe pigment of the tapetum in 



