Yet another complication occurs in the formation of eyes, which is, perhaps, the most important of all: the retinal portion of the eye, instead of consisting of simple retinal cells, with their accompanying rhabdites, may include within itself a portion of the central nervous system. The rationale of such a formation is as follows: The external covering of the body is formed by a layer of external epithelial cells--the ectodermal cell-layer--and an underlying neural layer, of which the latter gives origin to the central nervous system. As development proceeds, this central nervous system sinks inwards, leaving as its connection with the ectoderm the sensory nerves of the skin. That part of the neural layer which underlies the optic plate forms the optic ganglion, and when the central nervous system leaves the surface to take up its deeper position, the strand of nerve-fibres known as the optic nerve, is left connecting it with the retinal cells as seen in Figs. 28, 29. It may, however, happen that part of the optic ganglion remains at the surface, in close connection with the retinal end-cells, when the rest of the central nervous system sinks inwards. The retina of such an eye is composed of the combined optic ganglion and retinal end-cells; the strand of nerve-fibres which is left as the connection between it and the rest of the brain, which is also called the optic nerve, is not a true peripheral nerve, as in the first case, but rather a tract of fibres connecting two parts of the brain, of which one has remained at the periphery. Such a retina, in contradistinction to the first kind, may be called a _compound retina_. The optic ganglion, as seen in eyes with a simple retina, consists of a cortical layer of small, round nerve-cells, and an internal medulla of fine nerve-fibres, which form a thick network known as 'Punctsubstanz,' or in modern terminology, 'Neuropil.' Fibres which pass into this 'neuropil' from other parts of the brain connect them with the optic ganglion. {72}At the present time, owing to the researches of Golgi, Ramón y Cajal, and others, the nervous system is considered to be composed of a number of separate nerve-units, called neurones, each neurone consisting of a nerve-cell with its various processes; one of these--the neuraxon--constitutes the nerve-fibre belonging to that nerve-cell, the other processes--the dendrites--establish communication with other neurones. The place where these processes come together is called a synapse, and the tangle of fine fibres formed at a number of synapses forms the 'neuropil.' [Illustration: FIG. 30.--DIAGRAM OF FORMATION OF AN UPRIGHT COMPOUND RETINA. _ABCD_, as in Fig. 28. _Op. g. I._ and _Op. g. II._, two optic ganglia which combine to form the retinal ganglion, _Rt. g._]