CHAPTER XIV 



THE VISUAL APPARATUS 



THE eye is the most highly specialized sense organ in the 

 human body, and in other respects it occupies a very unique 

 position. The essential receptive part of the eye is in the retina. 

 But the retina is much more than this; it is really a part of the 

 brain, and the so-called optic nerve is a true cerebral tract. This 

 is evident from a consideration of the embryologic development 

 of the retina. 



In the early embryonic stages the neural tube expands laterally in the 

 position of the future thalamus, and from the upper part of this region a 

 "primary optic vesicle" is evaginated from the lateral wall on each side 



Optic cup 

 Optic stalk | Lens rudiment 



Cavity of forebrain 



Ectoderm forming lens 

 rudiment 



Optic vesicle becoming 

 cupped 



Fig. 97. Diagrammatic section through the head of a fetal rabbit to 

 illustrate the mode of formation of the primary and secondary optic vesicles 

 and of the lens of the eye. The right side of the figure is drawn from a more 

 advanced stage than the left side. (From Cunningham's Anatomy.) 



(Figs. 46, 47, 49, 97). The optic vesicle grows outward toward the skin 

 and assumes the form of a hollow sphere, whose cavity remains in com- 

 munication with that of the third ventricle by a hollow stalk (Fig. 97). 

 While the formation of the primary optic vesicle is in progress the overlying 

 ectoderm (outer skin) is thickened and finally invaginated to form the lens 

 of the eye, the optic vesicle collapses so that its cavity is obliterated by the 

 apposition of its lateral and medial walls, and a secondary cavity (the sec- 



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