436 



TUNICA JACOBI. 



retina goes, and adheres by a delicate filamentous structure to 

 both the retina and the choroid coat, but much more firmly to 

 the former. Mr. Jacobs asserts that this filamentous structure, 

 by which it adheres to the other two coats, is vascular: as this 

 assertion, however, is founded on a process of reasoning, and 

 not on absolute demonstration, our assent may very properly 

 be withheld from it till an injection is brought forward to prove 

 the fact. This membrane is well marked also in the sheep, ox, 

 horse, dog, and other domestic animals ; indeed, in all the 

 mammalia which he has had the opportunity of examining. It 

 exists, also, in birds and fishes, and is said to be particularly 

 strong in the latter. M. Cuvier has described it in fish as one 

 of the lamina of the retina, but its sensible properties are 

 asserted by Mr. Jacobs to be very dissimilar to those of the 

 retina. 



As this membrane may possibly be confounded by the stu- 

 dent with one of the lamina of the human retina, it should be 

 borne in mind that the latter consists of two parts, a pulpy or 

 nervous expansion and a reticulated vascular structure on 

 which the pulp is laid, the pulpy part being external. 



The reticulated structure is called by some the tunica. vascu- 

 losa retinae. Most anatomists have known and admitted these 

 two membranes as composing the retina, yet but few have had 

 the expertness to separate them from each other, and to leave at 

 the same time the membranous structure visible. Albinus had 

 the success to accomplish this separation ; and I am informed, 

 on the authority of Dr. Physic, that Mr. Hunter also succeeded 

 in it. 



When the common dissection of the retina is made and fixed 

 in clear water, this membrane will be seen floating around It 

 in delicate flocculi ; but if a more careful dissection be made, 

 by fixing the eye to the bottom of a basin of water, by its cor- 

 nea, through the aid of shoemaker's wax, and the sclerotica 

 and choroidea be removed from behind, this membrane may 

 be turned down entire from the retina, with the end of the 

 handle of a scalpel.] 



