426 NERVOUS 



was thought, for some time, that the yellow spot and the fora- 

 men were peculiar attributes of the human being: more ex- 

 tended and successful observation has corrected this mistake, 

 by dt tecting them in several classes of animals,* 



In the centre of the optic nerve, where it enters the eye, ig 

 a foramen for the passage of the artery and vein belonging to 

 the retina.t These vessels ramify, by a few branches, on the 

 internal surface of the membrane, and form a sort of circle sur- 

 rounding the yellow spot. -Neither the branches of the ar- 

 tery nor of the vein communicate with those of the cho- 

 roides; and, as observed, never go beyond what we just con- 

 sidered as the anterior margin of the retina, but rather run 

 along it. 



Interposed between the retina and the choroides, is a most 

 delicate serous membrane, which was discovered by Mr. Jacobs, 

 Demonstrator of Anatomy in Trinity College, Dublin. By 

 preparing the retina in the usual way, and then floating the 

 eye in a saucer of water, this membrane may be turned down 

 with the handle of a scalpel from the optic nerve to the termi- 

 nation of the retina. It is supposed to be the seat of the ossi- 

 fications which are sometimes met with in the eye. 



Humours of the Eyeball. 



The Vitreous Humour (Humor Vitreus, Corpus Vitreum,) 

 occupies, with the exception of a very small part just behind 



* In a careful examination of the eye of Williams the murderer, executed in 

 the Moyamensing Prison Aug. 9, 1839, I found (in three hours after the drop 

 fell, the eye consequently being- perfectly fresh,) the retina, in both eyes, of the 

 colour of oiled white paper or 0round glass: it was seen distinctly to terminate 

 at the beginning of the ciliary plaits of the choroid. The spot of Sojmmering was 

 seen, but it was of a sea green, oval, a line in length, the centre marked by an 

 olive spot: no foramen was seen satisfactorily. The fold of the retina, running 

 to the entrance of the optic nerve, was very distinct, as well as the button-like 

 appearance of the nerve at this point. 



t The point where the central artery of the Retina enters, is called the Macula 

 Lutea. Michaelis asserts, that around it the filaments of the retina are arranged 

 in arches, of which one part meets in the n>acula lutca, the next in succession 

 converge regularly towards a line stretching from the macula lutea. Muller ut 

 Supra. 



