The following layers compose the retina (Fig. 7), named from without inward; (I) layer of rods and cones; (2) membrana limi tans externa; (3) outer nuclear layer; (4) outer plexiform layer; (5) inner nuclear layer; (6) inner plexiform layer; (7) ganglion cell layer; (8) nerve fibre layer; (9) membrana limitans interna. The layers are connected partly by extensions from the individual ele ments and partly by a supporting framework — the fibres of Mueller.
The rods are cylindrical structures ; the cones flask-shape. Both possess a thinner outer half and a thicker inner part. The rods con tain the visual purple, which bleaches out readily in the cadaver but regenerates in the dark when the relationship to the pigment epithelium is undisturbed. The cones are found in all parts of the retina; in the fovea there are no rods. The raembrana limitans externa is a very delicate sieve-lilce membrane, having a perforation through which each rod and cone sends a prolongation to its own nucleus in the outer nuclear layer. The outer plexiform layer is essentially a supporting layer, composed of proximal portions of rods and cones on the one side and of fine extensions from the inner nuclear layer of the other side. The inner nuclear layer contains the nuclei of Muellees fibres and nerve cells. There are also larger cells in this layer provided with Nissl's granules. The sixth layer, inner plexiform layer, resembles the outer plexiform layer and belongs to the supporting tissue. The ganglion cells of the next layer are multipolar and send off numerous dendntic processes into the under lying inner plexiform layer. The nerve-fibre layer, with its neuroglia cells, is composed of afferent and efferent fibres passing to and from the optic nerve. In this layer the larger vessels of the retina are lodged. Finally, the very thin glass membrane — membrana hmitans intema — separates the nerve-fibre layer from the vitreous.
The retina is composed of three neurons, or nervous units, named as follows in the sense of their conduction. The rods and cones, with the outer nuclei, belong to the first neuron. This unit serves for the reception of the indi vidual light impressions. The nerve cells among the inner nuclei form the second unit. These
cells come in contact vrith ele.ments from the first and third neurons. The latter is repre sented by the ganglion cells, the axis cylinders of which reach through the optic nerve, the chiasm, and optic tract to the brain.
The rods and cones, outer nuclear layer and part of the outer plexiform layer are nourished by the chorio-capillaris. All the other layers have a vessel system of their own in branches of the arteria centralis retinae and veins of the same name. The. blood vessels are the only elements in the retina of mesodermic origin; all else is derived from the ectoderm. In the peripheral portions of many eyes one encounters peculiar cavities due to cystoid de generation. These are the spaces of Lessing or Ivanoff's retinal oedema.
The optic nerve (second cranial) unites the retina to the brain. (Fig. 2). That portion of the nerve lying within the wall of the eYe is termed bulbar or intraocular. The orbital or retrobulbar portion lies between the eyeball and the canalis opticus. Another division is into a medullated and non-medullated portion. At the lamina cribrosa the medullary sheaths cease; in consequence the nerve-fibre layer of the retina has no sheaths of Schwann, and is trans parent. The cord is surrounded by three sheaths; outer, dura mater; middle arachnoid; and inner, pia mater — all continuous with the membranes of the brain. Between the dura and the pia is the intervaginal space. This is divided by the arachnoid into two cavities, an outer, subdural, and an inner, subamchnoid. Both communicate with spaces of the same name in the cranium. The pia mater closely invests the nerve sub4tance sending numerous trabeculae among the nerve-fibres and separat ing them into incomplete bundles. About 20 mm. behind the eyeball the central artery and vein of the retina enter the nerve at right angles and continue in its axis to the retuia.
They, are surrounded by connective tissue car ried in with them from the pia mater. The nervous element is composed of extremely nu merous fibres estimated at half a million or more, with its own supporting framework the filial cells. The lamina cribrosa is the per forated space in the sclera for the entrance of the nerve fibres.