564 



THE NERVOUS SYSTEM. 



four centimeters from the eye, and the head of a pin is held as 

 close as possible to the cornea, the pinhole becomes a source of 

 light, and the shadow of the pin, not the image for the pin is too 

 near the eye to form an image falls on the retina. This shadow 

 is an upright one, and yet the pinhead appears inverted, for the 

 reason that the brain has become accustomed to interpret all im- 

 pressions made upon the retina as corresponding to objects in 

 the opposite portion of the field of vision. In the illustration, 

 AB is the card with the pinhole, P the pin, and p' its upright 

 shadow on the retina. 



Accommodation (Figs. 341, 342). The eye possesses the capa- 

 bility of adjusting itself to seeing objects at varying distances ; 

 this is accommodation. If the entire optical apparatus of the eye 

 was rigid and immovable, it would be necessary, in order to obtain 



FIG. 341. Anterior quadrant of a horizontal section of the eyeball, cornea and 

 lens cut in the middle of their vertical diameter: a, substantia propria of the 

 cornea; b, Bowman's membrane; c, anterior corneal epithelium; d, Descemet's 

 membrane ; e, its epithelium; /, conjunctiva ; g, sclera ; h, iris ; i, sphincter muscle 

 of the iris ; j, pectinate ligament of the iris with the adjacent fenestrated tissue ; 

 k, canal of Schlemm ; I, longitudinal, m, circular fibers of the ciliary muscle ; 71, cil- 

 iary process ; o, ciliary portion of the retina ; q, canal of Petit, with the zonule of 

 Zinn (Z) in front of it, the posterior leaflet of the hyaloid membrane (P) behind 

 it ; r, anterior, s, posterior, -capsule of the lens ; t, choroid ; u, perichoroidal space ; 

 T, pigment epithelium of the iris; x, margin (equator) of the lens. (Landois). 



a clear image of an object, either for the individual to approach or 

 to recede from the object, or to cause the object to do the same with 

 reference to him, for only parallel rays, namely, rays coming from 

 objects at a distance of two to three meters or more, are brought to 

 a focus in the normal eye unless some change is brought about in 



