888 ANATOMY [ORGANS OF SENSE Etellate cells: in dark-coloured eyes, both are filled with dark pigment granules ; whilst in light-coloured eyes the stellate cells of the stroma are either devoid of pigment or only faintly coloured. The iris contains numerous fasciculi of involuntary or non-striped muscular fibre arranged in two directions. Circularly arranged fibres surround the aperture of the pupil, and form the sphincter muscle, by the contraction of which the size of the pupil is diminished. Smooth muscular fibres also radiate from the pupillary to the ciliary border of the iris and form the dilatator muscle. The muscular nature of these fibres in the human iris was long disputed, but was satisfactorily demonstrated in 1852 by Lister. Jeropheef has also described circular fasciculi surrounding the ciliary border. In birds and reptiles the muscular tissue of the iris consists of transversely striped fibres. The arteries of the iris arise from the circulus arteriosus, and run radially forwards towards the pupil, where they anastomose and form the circulus iridis minor. They possess relatively thick external and muscular coats. The capillaries form a plexus not so compact as that of the choroid coat. The veins of the iris end in the venae vorti- cosae. In the foetus the pupil is closed in by a delicate membrane, membrana pupillaris, into which the blood vessels of the iris are prolonged. This membrane disap pears by absorption during the later months of embryo life. The nerves of the middle coat of the eyeball are the long ciliary branches of the ophthalmic division of the 5th and the short ciliary branches of the ciliary ganglion (PI. XIX. fig. 7, j). They pierce the sclerotic near the optic nerve, and run forward in the lamina fusca of the choroid. They give off branches to the choroid which form in it a plexus in which H. Miiller found nerve cells. From this plexus delicate branches pass to the muscular coat of the choroidal arteries. The ciliary nerves then enter the ciliary muscle, and form plexuses with interspersed nerve cells, from which branches pass to the muscular fibres. Other branches of the ciliary nerves enter the iris, and form plexuses, from which branches proceed to the muscular tissue. The Retina is the delicate nervous coat of the eyeball which lies immediately internal to the choroid, and extends Fig. 78. Diagrammatic section through the retina to show the several layers which are numbered as in the text. Ct, the radial fibres of the supporting con nective tissue. from the place of entrance of the optic nerve as far forward as the ciliary processes, where it forms a jagged border, the ora serrata. In the living eye it is translucent and colour less, but shortly after death it becomes grey : it is soft and so easily torn that it is difficult to display it in a dissection without injury. Its inner or anterior surface, concave forwards, is moulded on the vitreous body, and presents the following appearances : Almost exactly in the antero- posterior axis of the eyeball is a transversely oval yellow spot, about -^th inch in its long diameter, which amongst mammals is found only in man and apes, though, as Knox and Hulke have shown, it exists in reptiles ; in the centre of this spot is a depression, the fovea centralis ; about -|th inch to the inner side of the yellow spot is a slight elevation, the papilla optica, which marks the disc-like entrance of the optic nerve into the retina ; here the fibres of the nerve radiate outwards and forwards to the ora serrata, and branches of the arteria centralis retinae accompany then. The retina is highly complex in structure, and consists of nerve fibres and cells, of peripheral end-organs, of con nective tissue, and of blood-vessels, arranged in several layers. Max Schultze, who is the chief authority on the subject, recognises ten layers, but includes among these the layer of hexagonal pigment cells just described as the inner pigmentary layer of the choroid. If this layer be omitted, nine layers may then be recognised, and, following Schultze, be named from before backwards as follows: 1. Mem brana limitans interna ; 2. Layer of optic nerve fibres ; 3. Layer of ganglion cells ; 4. Internal granulated (molecular) layer; 5. Internal granule layer; 6. External granulated layer ; 7. External granule layer ; 8. Membrana limitans externa ; 9. Bacillary layer (Fig. 78). The nervous elements of the retina will first be con sidered. The optic nerve fibres (2), where they pierce the sclerotic, as a rule lose the medullary sheath, and radiate outwards as non-medullated fibres from the optic disc to the ora serrata immediately behind and parallel to the mem brana limitans iuterna. These fibres vary greatly in size, and are frequently varicose. When any of the optic nerve fibres retain the medullary sheath the retina is there ren dered opaque. Immediately behind the nerve fibres is the layer of ganglionic nerve cells (3). These cells are either bi polar or multipolar. In the living eye the cell substance is hyaline and the nucleus transparent, but after death the substance both of the body of the cell and the processes assumes a fibrillated appearance, like the axial cylinder of an optic nerve fibre. One process, the central process, extends into the layer of optic nerve fibres ; and another, the peripheral, into the internal granulated layer. The internal granulated layer (4) contains the branching pro cesses of the nerve cells, some of which apparently become continuous with an arrangement of excessively fine fibrils, probably nervous in their nature. These fibrils are inter mingled with a delicate plexus of connective tissue. The internal granule layer (5) contains numerous fusiform nucleated enlargements, the so-called internal granules, arranged in superimposed strata; from each fusiform enlargement a fibre proceeds in two directions, one centrally into the internal granulated layer, and one peripherally into the external granulated layer. These fibres possess vari- cosities, and resemble the optic nerve fibres. The external granulated layer (6) is very thin, and consists of an ex panded network of minute fibres, with nuclei situated at the points of intersection of the fibres. Krause has called it the membrana fenestrata. The external granule layer (7) uun tains numerous fusiform nucleated enlargements, the so-called external granules, arranged in superimposed strata : from each enlargement a fibre proceeds in two directions, one centrally into the external granulated layer, and one peripherally through the membrana limitans externa to the bacillary layer, where it becomes continuous with the anterior end of either a rod or a cone, as the case may be. Hence these fibres of the external granule layer are called by Schultze rod and cone fibres, and the external granules are nucleated enlargements of these fibres. These fibres possess varicosities like those of the internal granule layer. The bacillary layer (9) or membrane of Jacob consists of multitudes of elongated bodies arranged side by side like rows of palisades, and vertically to the surfaces of the retina. Some of these bodies are cylindrical, and are named the rods of the retina ; others flask-shaped, and named the cones of the retina : the rods equal in length the entire thickness of the bacillary layer ; the cones are shorter than the rods, and are interspersed at regular intervals between them ; the apex of each cone is directed towards, but does not reach, the plane of the posterior or choroidal surface of the retina. The posterior or outer end
of each rod rests against the pigmentary layer of the choroid.
