THE VISUAL APPARATUS 213 



developed from a tubular outgrowth from the roof of the diencephalon (the 

 - pineal organ or epiphysis, p. 162) ; this extends dorsalward from the brain 

 through a special foramen in the skull to reach the skin in the center of 

 the top of the head. The functions and evolutionary significance of this 

 eye are shrouded in mystery. 



Summary. The retina is developed as a lateral outgrowth 

 from the early neural tube and throughout life retains its char- 

 acter as a part of the brain, the "optic nerve" being really a cor- 

 relation tract comparable with the lemniscus systems. The rods 

 and cones of the retina are the photoreceptors and also the neu- 

 rons of the first order in the optic path. The "optic nerve" con- 

 tains neurons of the third order from the retina to the thalamus 

 and midbrain, and also centrifugal fibers from the midbrain to 

 the retina. In lower vertebrates the fibers of the optic path 

 decussate completely in the optic chiasma, but in those mammals 

 whose fields of vision overlap there is an incomplete decussation 

 so as to ensure the representation of the field of vision of one side 

 completely in the opposite cerebral hemisphere. Those fibers 

 of the optic tract which terminate in the midbrain effect various 

 kinds of reflex connections, while those which terminate in the 

 thalamus effect cortical connections. The parietal or pineal 

 eye of some fishes and reptiles is apparently functional as an 

 organ of vision which was developed quite independently of 

 the lateral eyes. 



LITERATURE 



In this chapter we have not attempted to present a systematic descrip- 

 tion of the structure of the eye or of the functions of the retina and theories 

 of vision. For the details of these questions reference must be made to the 

 larger text-books of anatomy, physiology, and physiological psychology. 

 A few general works are cited below, together with some special researches 

 to which reference has been made in the preceding text : 



VON BECHTEREW, W. 1909. Die Funktionen der Nervencentra, Jena, 

 Bd. 2, pp. 996-1103. Idem, 1911, Bd. 3, pp. 1554-1583, 1883-1964. 



COLE, L. J. 1907. An Experimental Study of the Image-forming Powers 

 of Various Types of Eyes, Proc. Amer. Acad. Arts and Sciences, vol. xlii, 

 No. 16. 



HARRIS, W. 1904. Binocular and Stereoscopic Vision in Man and Other 

 Vertebrates, with Its Relation to the Decussation of the Optic Nerves, the 

 Ocular Movements, and the Pupil Light Reflex, Brain, vol. xxvii, pp. 106- 

 147. 



LADD, G. T., and WOODWORTH, R. S. 1911. Elements of Physiological 

 Psychology, New York. 



MAST, S. O. 1911. Light and the Behavior of Organisms, New York. 



