434 ACTION OF THE IRIS. 



Inflammation of the eye, he asserts may produce the same 

 change, as seen especially on the uvea, in inflammation of 

 the iris around the pupillary margin, where is formed the brown 

 circle, so commonly attendant on severe iritis. In the early 

 state of development of the foetal eye, be found the sclerotica 

 and choroid protuberant inward, opposite the place where is 

 subsequently formed the yellow spot, at which point penetrate 

 some of the ciliary arteries that branch from thence into the 

 retina. The appearance of the black spot or foramen of Soem- 

 mering, he believes to be caused by a rupture of the delicate 

 retina at this point, bringing into view the black pigment on 

 the face of the choroid. In its first stage of formation the retina 

 appears to be a transparent layer, readily separated both from 

 the choroid and the optic nerve. 



The use of the yellow spot, which -he considers the most 

 sensitive part of the eye, is to give a common axis to the organ, 

 and stability during the exercise of vision. Hence the vacilla- 

 tion of the balls, and occasional squinting common to young 

 infants prior to the formation of the yellow spot, as well as the 

 rolling of the balls, in congenital cataract. " Centrum retina, 

 in quo macula flava occurrit, prae caeteris ejus partibus lucis 

 assimilation! optum esse videtur; necessario inde sequitior, 

 puncretinse locum imprimis lucis radiis offici et quasi detineri, 

 non potest igitur non uterque oculus uno eodumque dirigi 

 motu, et uno quasi stabiliri axe. Norrnalis autem haec oculi 

 condition! tune desideratur, cum macula lutea nondurn formata 

 est, sive cum suspicari licet, hujus organi genesin prohibatum, 

 aut ejus ceconomiam turbatum esse."* 



In the human eye, the external medullary pulpy layer of the 

 retina, when beaten up, presents all the appearance of cerebral 

 matter, a fact first observed by Galen. The nervous matter 

 is, however, arranged in the form of fibres, agreeably to the 

 recent microscopical observations of Langenbeck and Trevira- 



* Fred. Aug. ab Ammon, de Genesi et usu maculae lutese, in retina oculi 

 hutnani obvise. Accedit tabulae in aes incisa. Vinariae, 1830. 



