In the central part of the inner end of the outer limb is seen a dark point when the structures are examined in transverse section (Ritter, Manz, Schiess, Schultze, and others). The cause of this is not clearly understood; some hold it to be a fibre (Ritter’s fibres), others hold it to be an artificial product (Hensen).

After treatment with certain reagents the outer limbs show a transverse striation, which is probably produced by the action of these reagents on the sheath of the outer limbs; that a sheath is present is proved by its possession of a different refractive index (Zenker, Schultze) to the rest of the outer limb, and this transverse striation is not seen until the whole organ has undergone considerable post-mortem changes (Hoffmann). Should this change be allowed to proceed a stage further, the outer limbs of the rods split transversely and form small discs from 0·0005–0·00055 mm. thick; this takes place in the outer limb only.

The inner segments of the rods (Figs. 258, 259) are short (0·020–0·022 mm.) and of the same thickness as the outer limbs. When perfectly fresh they appear homogeneous; very quickly changes commence, which are probably due to coagulation. A plano-convex figure (Fig. 259) is then seen at the outer portion of the segment (lens-shaped figure of Schultze); with staining reagents it gives the same reactions as the outer segment of the rods. The rest of this segment forms a short cylinder, which probably has no distinct sheath (Hoffmann, Merkel); some observers are inclined to think that a sheath exists (Landolt, Schwalbe).

The outer segments of the rods are of two chief sizes (Schwalbe). Those of the one kind are large; the second variety occurs less frequently, and the segments are shorter, measuring only 0·002 to 0·0025 mm. The inner segment is a long, thread-like process, except where it is swollen to enclose the lens-shaped body.

The rods are much more numerous than the cones, except at one small spot (macula lutea) on the posterior surface of the retina, where only cones are found (Krause).

The cones (coni) have each two segments like the rods (Figs. 258, 259). The outer segments are short (4–5 µ), they are slightly conical and terminate externally in a blunt point; they possess a longitudinal striation (Schultze), and very easily break up transversely into small discs, which, however, do not separate so completely as in the case of the rods, in consequence of the presence of a sheath continuous with a sheath on the inner segment.

The inner segments (Figs. 258, 259) have convex sides and measure 12–14 µ; like the corresponding parts of the rods they possess lens-shaped bodies at their junction with the outer segments, but the bodies differ in shape, being bi-convex or rather oval in form. The inner segments are enclosed in a delicate sheath continuous with that of the outer segments.

In some cases two cones are united to form a twin-cone; in such cases the one is always larger than the other (Fig. 259 9), and has several peculiarities which distinguish it from the smaller.

The smaller or secondary member of a twin-cone is longer, and possesses a lens-shaped body which is plano-convex. The larger or principal member of a twin-cone is shorter, has a plano-convex body, but also an oval, homogeneous, glistening body, which is directly attached to the plano-convex body. The shape of the two members is also different.

The outer nuclear layer (Fig. 258 f) is 14–16 µ thick; the nuclei lie in two layers. The nuclei belonging to rods and cones have the same characters, each nucleus being a large, oval, hyaline body, and enclosing a bright nucleolus. Each nucleus is surrounded by an extremely thin layer of finely granular matter. The inner processes of the nuclear bodies both of the rods and the cones extend to the outer molecular layer, are there dilated and serrated, where they become attached to the outer molecular layer (Schultze, Hoffmann). In some cases, however, the inner process of the nuclear bodies, belonging to the rods, forms only a short fine fibre.

In the case of twin-cones the corresponding parts in the outer nuclear layer possess two nuclei (Schultze).

(8) The pigment layer (Figs. 258 h, and 259 11, 12, 13, 14, 15) is not intimately attached to the rest of the retina. It consists of cylindrical cells in which two parts or segments are sharply differentiated; the external part, directed towards the choroid coat, is of pale, or colourless granular protoplasm, and occupies one-third of the length of the cell; this part encloses a large, round, nucleolated nucleus. This colourless segment of the cell also includes one or two bright yellow, fat globules (Morano). Seen from the surface the cells are hexagonal (Fig. 259 11). The remaining two-thirds of the cells consists of a brush formed of numerous fine pigmented processes; the ultimate terminations of the processes, which lie parallel to each other, are frequently unpigmented; each cell possesses thirty to forty such processes (Morano).

The processes extend between the rods and cones as far as the external limiting membrane (Figs. 258, 259), or sometimes a little further (Merkel, Morano, Hoffmann). The processes from one pigment-cell surround a number of rods and cones; according to Morano twelve to fifteen rods and cones may be encased or surrounded by the processes of a single cell.

The thickness of this layer varies from 60–70 µ; the nuclei of the cells have a diameter of 10–12 µ, the width of a single cell is from 20–25 µ (Hoffmann).

(10) The connective-tissue elements of the retina and the external and internal limiting membranes. The elements of the retina are supported by connective-tissue elements or sustentacular cells, which have a radial arrangement, and which form the two limiting membranes (Müller).

Each sustentacular cell (Fig. 259 10) has two segments, an inner and an outer, the boundary between these lying in the inner nuclear layer, and being marked by the presence of a large oval nucleus. The inner segment of each cell terminates internally in a wide ‘foot’ or base, or may form several such after having undergone division (Schultze): these bases together form a transparent, thin membrane, the internal limiting membrane (membrana limitans interna).

Within the ganglion-layer these cells possess peculiar appendages, which fit round the ganglion-cells and support them (Schwalbe).

The outer segments of the sustentacular cells extend into the outer molecular layer, and then break up into irregular processes which extend radially to the external limiting membrane, and which they probably form. The external limiting membrane (membrana limitans externa) is therefore a membrane corresponding to the internal limiting membrane, and formed by the flattened ends of the processes belonging to the sustentacular cells.

The sustentacular cells have a distinct, resistant cell-wall (Schwalbe); the cell-contents are a finely granular protoplasm, and a large oval, nucleated nucleus placed in the inner nuclear layer.

f. The ciliary processes have the same structure as the rest of the choroid coat: the vessels form more or less longitudinal meshes and are more irregular than in the rest of the choroid.

g. The posterior chamber and vitreous body (Fig. 260). The vitreous humour occupies the greater portion of the cavity of the eyeball, i.e. the posterior chamber. The humour consists of a mass of cells enclosed in a transparent hyaloid membrane (membrana hyaloidea), which is in contact with the internal limiting membrane of the retina.

The cells forming this structure are small, flattened, transparent, and nucleated (Iwanoff and Virchow); according to the former observer the cells have contractile powers.

The hyaloid membrane is described as structureless by Schwalbe, as fibrous by Pappenheim, Bowman, and Fuikbeiner.

The vessels of the vitreous body (Fig. 260) are as follows. The A. hyaloidea arises at the lowest point of the corpus ciliare; it almost immediately divides into two branches, which form a ring at a distance of about 0·5 mm. from the lens and lying on the surface of the vitreous body (Fig. 260 I, II). One, R. nasalis, passes to the nasal side and courses through one-fourth of the circle; the other, R. temporalis, courses through three-fourths of the circle. The branches are all given off proximally and at right angles to the circle (Fig. 260 I, II). From the R. nasalis only one branch arises, from the R. temporalis seven, the first of which corresponds in point of origin with the branch from the R. nasalis. The branches on the nasal and temporal surfaces of the vitreous body are the shortest.

These branches form a capillary network (Fig. 260 I) with elongated meshes, formed by the capillaries anastomosing at acute angles. The capillary network is more dense towards the middle of the proximal surface than in other parts.

The veins arising from this network are three in number (Fig. 260 III); two of these accompany the arteries from their origin, and form a somewhat similar circle around the lens, while the third passes backwards along the ventral surface of the vitreous body to the papilla nervi optici. The nasal vein, however, takes a more proximal course than the corresponding artery, the branches of which it crosses; consequently the venous ring is not so perfect as the arterial. The nasal vein is larger and the temporal vein smaller than the corresponding arteries.

The ventral vein is formed near the papilla nervi optici by the union of two smaller branches. The capillary system of these vessels has the usual structure of capillaries, the cells being united by cement-substance (Zimmermann).

The blood-vessels of the vitreous body are accompanied by lymphatics; according to Iwanoff they completely enclose the capillaries: Zimmerman contradicts this view, as he has been unable to find lymphatics on that side of the capillaries directed towards the vitreous body.

B. Appendages of the eye.

The appendages of the eye are the eye-muscles (see pp. 55–59), the eyelids, the Harderian gland, and the lachrymal duct.

a. The eyelids are two in number, an upper and a lower. The upper eyelid is intimately attached to the eyeball and follows the movements of that organ.

The lower eyelid (membrana nictitans) is much larger than the upper and has the same functions as the lower eyelid of higher vertebrates. It forms a transparent covering for the eyeball, and is raised by a special muscle (see p. 58); functionally it takes the place of both eyelids of higher vertebrates.

The lower eyelid is a prolongation of the skin, but has only a few pigment-cells, except at its free margin, and no serous glands. Mucous glands are found in two or three rows, closely applied to one another, on the superficial surface of the lid; on the deeper surface they are wanting. The stroma of the lid, like the cutis, is of connective-tissue.

Nerve-fibres can be traced in all directions through the substance of the lid, forming a wide-meshed plexus. Around each gland the plexus becomes finer and by numerous branchings much closer; from the plexus twigs are given off, which divide to form a number of fibrils traceable into the epithelial cells of the glands (Openchowski).

The vessels of the lower eyelid have been investigated by Stricker, (l. c.); according to him they possess some interesting peculiarities. Many of these capillaries course within a lymphatic vessel, in some places the capillary being contracted by a projection from its inner wall; where this is found the accompanying lymphatic is correspondingly dilated. Such points are especially met with where the capillaries branch; in many cases the capillary was contracted to such an extent that the blood-corpuscles were unable to pass the obstruction. Stricker further observed in the living tissue that such constrictions could take place in a part which a short time previously had been comparatively wide and dilated; further, that many of the nerves were enclosed in similar lymphatics. Langer, however (l. c.), describes the vessels as being accompanied by an irregular network of small lymphatic vessels.

The distribution of the nerves in the lower eyelid has been described by Klein. In the epithelium they form a network resembling that found in the cornea (Fig. 261 II); along the blood-vessels the fine fibrils form a perivascular network, which supplies fine twigs to the walls of the vessels (Fig. 261 I). He distinguishes three kinds of pigmented cells.

b. The Harderian gland is situated at the inner angle of the eye, and is pear-shaped in form. It consists of a number of racemose glands held together by connective-tissue, the whole being enclosed in a relatively thick and strong capsule of connective-tissue.

The alveoli have a diameter of 0·040–0·060 mm.: they possess a lining of epithelium and a lumen which varies considerably in size. The epithelial layer is bounded externally by a membrana propria. The cells are placed eccentrically, they are cylindrical, and composed of finely granular protoplasm; each cell contains a pale, rounded nucleus. The ducts of the alveoli are lined with a single layer of cylindrical epithelium, the cells of which are usually shorter and narrower than those of the epithelium of the alveoli; the ducts open into a single main tube, lined with similar epithelium but strengthened externally by a layer of connective-tissue. The glands secrete a fluid which moistens the free surface of the eye.

The Harderian glands are surrounded by a rich capillary anastomosis which completely invests the alveoli.

c. The lachrymal duct opens behind and below into the nasal cavity (see p. 389), anteriorly it can be traced forwards, as a small tube imbedded in connective-tissue and lying immediately beneath the skin, to the outer angle of the eye, where it opens by numerous tubules.

The lachrymal duct is lined with ciliated columnar epithelium.