LESSON 89.] ORGANS OF SPECIAL SENSE. THE EYE. 287 



the entire surface of the ganglion, and ultimately yield up their con- 

 tents, by another twig (b) to the vein, which passes it into the jugu- 

 lar vein, and thence to the heart. 



1285. Such, then, are the peculiarities which characterize the vas- 

 cular system of nerves and ganglia, and by which they may be at all 

 times recognized, as no such distribution pertains to any other tissue. 



LESSON LXXXIX. 



OKGANS OF SPECIAL SENSE. THE EYE. 



1286. Want of space in this small volume will not permit a full 

 inquiry into the subjects which form the title of this lesson, but one 

 organ, from its almost universal distribution in the animal kingdom, 

 together with its importance, claims special attention the Eye. 



1287. As far as our knowledge of it extends, the eye is strictly 

 an optical instrument a camera and as capable of performing cor- 

 rectly in death, as in the living organ. To prove this fact it is only 

 necessary to remove, carefully, the posterior portion of the sclerotic 

 coat of the fresh eye of a Sheep or an Ox, leaving untouched the 

 retina. If the eye, thus prepared, be presented to any object, a mi- 

 nute, inverted, and most beautiful view of the picture will appear 

 upon the retina, arrayed in all its natural colors. If the picture so 

 formed be magnified with a lens of moderate amplifying power (one 

 inch, or 1^ inch focus), a very charming sight will be offered to the 

 spectator. 



1288. A well formed visual organ appears to require the follow- 

 ing parts : a transparent cornea, or outer covering ; an aqueous (wa- 

 tery) humor; an iris, or curtain of the eye, to limit and control 

 the quantity of light to be admitted ; a crystalline lens ; a vitreous, 

 or glassy humor; a pigment or paint, frequently but not always 

 black, to absorb the excess of light which has entered the eye, and 

 thereby give increased sharpness and intensity to the picture ; a 

 retina, or thin membraneous expansion of the optic nerve, forming 

 the white curtain (similar to the ground-glass of a camera) upon 

 which the images are depicted ; and finally, an optic nerve, whose 

 function it is to transmit faithfully to the brain, the full particulars 

 of color, and general appearance of the picture formed upon the 

 retina. 



