On the inside of the choroid, surrounding the aperture into which the iris is fitted, and corresponding in position within to the ciliary ligament without, exists another peculiar pro vision destined to establish a connexion between this part and the hyaloid membrane of the vitreous humour, as the ciliary ligament esta blishes a similar connexion between the sclerotic and choroid. This is the corpus ciliare or ciliary processes, called sometimes incorrectly ciliary ligament, and by Sommerring corona ciliaris. It is composed of a number of dis tinct folds or productions of the choroid, having their anterior extremities extended to the back of the iris, while the posterior gradually dimi nish until lost in the membrane from which they originate. Each fold or ciliary process is a production or continuation of the choroid, and cannot be separated from it unless clipped off by the scissors. They appear to be com posed altogether of a remarkable interlacement of arteries and veins derived from those of the choroid, and exhibit no appearance whatsoever of muscular organization, although considered by Porterfield and others as endowed with that function. These are sixty or seventy in num ber, fifty-seven being enumerated by SUmmer ring, and seventy by Zinn. They are about two lines in length, but are not equally so, every alternate one being shorter than the next to it. The free internal margin of each ciliary process is buried in the hyaloid membrane of the vitreous humour at its anterior part, round the circumference of the crystalline lens, and a corresponding production of the hyaloid mem brane projects into the space between these processes so as to establish a most perfect bond of union between the two structures. The ciliary processes appear to be attached to the circumference of the lens, and are often de scribed as having such connexion. This, how ever, is not the case. The anterior extremities do not touch the circumference of the lens ; they project into the posterior chamber of the aqueous humour up to the back of the iris, and consequently constitute the circumferen tial boundary of that cavity. When the eye becomes flaccid from evaporation after death, the ciliary processes fall down to the margin of the lens and appear to adhere ; but if the cornea and iris be removed from the eye of a subject recently dead, a circle of hyaloid membrane may distinctly be seen occupying the space between the ciliary processes and lens, through which the observer can see to the bottom of the eye. This space is represented and pointed out in Sommerring's plates. The annexed figure from Zinn's work represents the corpus ciliare or circle of ciliary processes on a large scale.
The choroid, in common with several other parts of the eye and its appendages, is stained by a black colouring matter secreted in and upon different textures. In man it is of a dark brown colour, but in other animals is generally black, and so loosely connected with the struc ture in which it is deposited, that in dissecting the eyes of our common gmininivorous animals under water it becomes diffused, and colours the fluid as the ink of the cuttlefish obscures the water into which it is shed. It is not confined to any one particular structure, but is deposited in every situation where it is necessary for the purpose for which it is destined. It is found in considerable quantity on the inner surface of the choroid, where it appears as if laid on in the form of a paint, and is frequently so described ; but it is much more probable that it is deposited in the interstices of the exqui sitely fine cellular membrane which connects the choroid with the delicate covering of the retina. In this situation it often, especially in
infants, presents the appearance of a perfectly distinct black membrane, which may be peeled off in flakes or allowed to remain on the retina in patches, as noticed by !latter. It also per vades the structure of the choroid, at least in the adult, and even stains the inner surface of the sclerotic and the cellular layer which con nects these two membranes. It is deposited in larger quantity in the ciliary processes and upon the back and in the texture of the iris. In many animals it is found forming a black ring round the margin of the cornea and in the edge of the third eye-lid, as well as in the peewit or marsupium nigrum in birds. It is even sometimes found scattered, as if acci dentally, as in the texture of the sclerotic in hogs, and within the sheaths of the optic nerve in oxen ; it is obvious that it does not require any special form of organization for its produc tion, but is merely secreted into the cellular membrane, where necessary, as the colouring matter is secreted with cuticle on the skin.
It is darker in the earlier periods of life, and in the infant is more confined to the inner sur face of the choroid and to the posterior surface of the iris, tban pervading the texture of either of these membranes. In old age it evidently fades, and even appears as if absorbed in patches. It is sometimes altogether absent, as in those animals called albinos, where all the parts usually coloured are unstained. Its use is obviously toprevent the rays of light from being reflected from surfaces where they should be absorbed, a provision as essential to the perfection of the animal eye as to the artificial optical instrument. It is also applied to give complete opacity to prevent the transmission of light, and hence is deposited in large quantity in and on the iris, as well as in the ciliary pro cesses which correspond in situation to the exposed part of the sclerotic, through which the light might otherwise pass to the bottom of the eye, and disturb correct vision. The layer of black pigment on the inner surface of the choroid has undergone a careful microscopic investigation, especially by Mr. T. W. Jones, the results of which arc stated in a short account of the anatomy of the eye prefixed to the second edition of Mr. M'Kenzie's work on Diseases of the Eye. lie says that it possesses organization and constitutes a real membrane, and when examined with the miscroseope " is seen to consist of very minute flat bodies of a hexagonal form, joined together at their edges. These bodies, which are about of an inch in diameter, consist of a central transparent nucleus, surrounded by an envelope of colour ing matter, which is most accumulated at their edges. The centre, indeed, of each hexa gonal plate is a transparent point, and appears somewhat elevated, the elevations on the inner surface corresponding to depressions to be described in the membrane of Jacob. That part of the membrane of the pigment situated on the pars nun plicate: of the ciliary body around the ciliary processes, and on the poste rior surface of the Iris, is composed of irregu larly rounded bodies, analogous to the hexa gonal plates. In albinos the same membrane exists, but contains no pigment. The bodies composing it are but little deve loped, being nothing but the central nuclei separated from each other by large intervals, and not hexagonal, but circular, or even globular." The annexed figure represents this mem brane of the pigment as described.