THE SENSES 



423 



bony socket? A stalklike structure, the optic nerve, connects the eye with 

 the bram. Free movement is obtained by means of six little muscles which 

 are attached to the outer coat, the eyeball and to the bony S(.cket around 

 the eye.i Notice the living frog. Compare the movements of your own 

 eye with those of the frog. Note any differences in position of the eye of 

 the frog and of that of man, and try to account for these difTercu. Ixiok 



for adaptations for the protection of your eye. Among such adaptaticina 

 are the position, structure of the lids, and the lashes. The latter are useful 



in protecting the eye from foreign substances 

 outer lids are not always found in 

 lower vertebrate animals. In some 

 vertebrates, however, as in the bird, 

 lizard, frog, and some lower mam- 

 mals, we find a third or winking eye- 

 lid. In man this eyelid is reduced to 

 a small fleshy fold seen in the inner 

 angle of the eye. Glands which se- 

 crete a salty, watery fluid are present. 

 This fluid keeps the eye moist, and 

 prevents friction between the eyeball 

 and its coverings. A small duct, which 

 can be found in the inner corner of 

 the eye, carries off all waste secretion 

 into the nose. During a cold, when 

 this passage is stopped up, the tears 

 overflow. Other glands which secrete 

 an oil prevent tears, under normal 

 circumstances, from flowing out of 

 the eyes. 



As we have soon, tlio two 



o / l 



0J (_ ^*- 



Section of the retina; .1, rHafrr.im of the 

 structure of the retina as .s<'t'n with the 

 compound microscope; B, the oji.M*ntial 

 nervous elements of tlie ret ina as <leinon- 

 strated by the Golgi method ; /, internal 

 limiting membrane; 2, nerve-fiber layw; 



3, nerve-ceil or HJinclion-crli layer; 



4, inner molecular layer; o, iriner granu- 

 lar layer; 6, outer molecular layer; 

 7, outer jjranular layer: .•?.•' ' ' ■'• 

 iting memlirane; .'^i nxl-anu ; 

 lU, pigment-cell layer. 



Internal Structure.^ — The hu- 

 man eyeball, if cut in a longi- 

 tudinal median section, from the 

 front backwards, will show the 

 following structures: — 



The wall of the eveball is made u]:> of three coats. An outer 

 tough white coat, of connective tissue, is called the sclerotic coat: 

 this coat is lacking in the exposed part of the eyeball. ])Ut may he 

 seen by lifting the eyelid. Under the sclerotic coat, in front, the 

 eye bulges outward a little. Here the outer coat is rrplace<i by 



1 Use for the following work a living frog 



2 For laboratory work on the eye of the sheep or the Imnuin eye. sec Hunter nntl 

 Valentine, Maniuil, pages 189-192. 



