The choroid is supplied with blood from the ophthalmic artery by the short ciliary arteries, which penetrate the sclerotic at a short distance from the entrance of the optic nerve, and are distributed to it in nearly twenty small branches. These branches ramify and inosculate freely on the outside of the membrane, and are visible as distinct vessels, especially on the posterior part of the sphere. They finally terminate on the inner surface, forming a beautiful vascular expansion. The long ciliary arteries give scarcely any twig to the choroid, being distri buted to the iris, and the anterior branches furnished to the sclerotic, as described in speaking of that membrane, do not penetrate to the choroid. The veins of the choroid pre sent a peculiar appearance. The ramifications are arranged in the form of arches or portions of a circle, bending round to a common trunk like those of certain trees with pendulous branches. They discharge their blood into four or five larger branches which penetrate the sclerotic at nearly equal distances from each other behind the middle of the eyeball. On account of this peculiar arrangement they have received the name of vasa vorticosa. They lie external to the ciliary arteries, but the ultimate ramifications pervade the inner surface in the same manner as the arteries ; and if the venous system of the eye be minutely injected, the same beautiful uniform villous vascularity is displayed as in the arterial injections.
The annexed figure is a copy of Zinn's re presentation of the vasa vorticosa.
The numerous nerves which pierce the scle rotic and run forward between that membrane and the choroid, called ciliary nerves, being distributed almost exclusively to the iris, are to be noticed when that organ is described ; small branches of them are, however, probably distributed to the choroid and its appendages, and possibly even to the retina and hyaloid membrane.
The inner villous surface of the choroid, which in man is stained with the black pig ment, in several other animals presents a bril liant colour and metallic lustre. This is called the tapetum. It is not a superadded material nor dependent on any imposed or separable colour ing matter, but is merely a different condition of the surface of the choroid or tunica Ruys chiana, by means of which rays of light of a certain colour only are reflected. It exists in the form of a large irregular patch, occupying the bottom of the eye toward the outside of the entrance of the optic nerve. It is of a beautiful blue, green, or yellow colour, with splendid metallic lustre, and sometimes white as silver. It is not obscured by the black pigment which covers the rest of the surface and even encroaches a little on its margin, and consequently it acts most perfectly as a concave reflector, causing the rays of light previously concentrated on the bottom of the eye by the lens to be returned, and to produce that re markable luminous appearance observed in the eyes of cats and other animals when seen in obscure situations. This provision is absent in man, the quadrumanous animals, bats, the insectivorous order, perhaps all the rodentia, the sloths and many other of the class mammalia ; while it is present in the majority if not all of the ruminants, as well as in the horse, the cetacea, and most of the carnivorous tribe. It
does not appear to exist in birds or reptiles, and is absent in the osseous, although present in the cartilaginous fishes. I must here, how ever, state that I am obliged to speak loosely respecting this matter, as the subject has not yet been thoroughly investigated. The use of this tapetum has not been ascertained, or the reason why it exists in some and is ahsent in other animals explained. It is obvious that where it is present the rays of light are trans mitted through the retina, and again when reflected by the tapetum are returned through the same retina, thus twice pervading that structure.
On the outside and anterior part of the choroid, where the margin of that membrane corresponds to the place of union between the sclerotic and cornea, a peculiar and distinct formation exists apparently for the purpose of securing a firm union between the two mem branes. It is commonly called the ciliary liga ment, also orbiculus ciliaris, circulus ciliaris, by Lieutaud plexus ciliaris, by Zinn annulus cellalosus, and by Sommerring gangliform ring. It is a gray circle of soft cellular membrane about two lines broad, applied like a band round the margin of the aperture into which the iris is fitted. It adheres closely to the choroid, and almost equally closely to the scle rotic, especially in the groove where the cornea joins that membrane. It contains few red vessels, and is not stained by the black pig ment; consequently it is of a whitish colour. The ciliary nerves penetrate it and subdivide in its structure. Hence it has been considered by Sommerring as a ganglion, and had been previously described by Lieutaud as a nervous plexus. The ciliary nerves, however, merely pass through, and may easily be traced on to the iris. It is evidently a mere band of cellular membrane serving to bind the choroid and sclerotic together at this point, and is obviously a provision essentially necessary for the perfec tion of the anatomical mechanism of the eye, as without it the aqueous humour must, from pressure on the eyeball, be forced back be tween the two membranes. In man it is broader in proportion than in the larger quadrupeds, and in birds it is particularly large and dense, adhering more closely to the circle of osseous plates than to the choroid, and consequently presents a very remarkable appearance when the latter membrane is pulled off with the ciliary processes and iris, an appearance to which the attention of anatomists was first drawn by Mr. Crampton. From .its position and appearance the ciliary ligament has often been suspected to be a muscular organ, destined by its contraction to alter the form of the cornea, and thus adapt the eye to distance. There is not, however, sufficient evidence to sustain such an opinion. The plate introduced to represent the ciliary nerves, as well as that which represents the iris, exhibit this part of the organization of the eyeball in connexion with the choroid.