THE RETINA 



417 



lachrymal gland is ordinarily carried away as fast as it 

 forms. 



But, under certain circumstances, as when the con- 

 junctiva is irritated by pungent vapours, or when painful 

 emotions arise in the mind, the secretion of the lachrymal 

 gland exceeds the drainage power of the lachrj-mal duct, 

 and the fluid, accumulating between the lids, at length 

 overflows in the form of tears. 



7. The Structure of the Retina. —If the globe of the 

 eye be cut in two, transversely, so as to divide it into an 

 anterior and a posterior half, the retina will be seen lining 



the whole of the concave 

 wall of the posterior half as 

 a membrane of great deli- 

 cacy, and, for the most part, 

 of even texture and smooth 

 surface. But almost exactly 

 opposite the middle of the 

 posterior wall, it presents a 

 slight circular depression of 

 a yellowish hue, the macula 

 lutea, or yellow spot (Fig. 

 135, m.l. ; Fig. 126, 8"),— 

 not easily seen, however, 

 unless the eye be perfectly 

 fresh, — and, at some distance from this, towards the inner, 

 or nasal, side of the ball, is a radiating appearance, pro- 

 duced by the entrance of the optic nerve and the spreading 

 out of its fibres into the retina. 



A very thin vertical slice of the retina, in any region 

 except the yellow spot and the entrance of the optic 

 nerve, may be resolved into the structures represented 

 separately in Fig. 136. The one of these (A) occupies 

 the whole thickness of the section, and comprises its 

 essential, or nervous, elements. The outer ^ fourth, or 



CD. 



Fig. 134. 

 A front view of the left eye, 

 with the eyelids partially dis- 

 sected to .show lachrymal gland, 

 L.G, and lachrymal duct, L.D. 



1 In the following account of the retina, the parts are described in re- 

 lation to the eyeball. Thus, that surface of the retina which touches the 



K E 



