The ependyma cells form the earliest support for the embryonic neurones of the brain and cord; in the adult they merely line the ventricles. They are columnar epithelial cells which reach from the internal to the external limiting membrane. The internal parts of these cells containing the nuclei remain fixed for a time in contact with the internal limiting membrane; the external parts, by the growth of the neural tube in thickness, become drawn out into tenuous fibers which radiate from the central canal to the exterior surface. Thus there is formed a radial "sustentacular apparatus" with a "nuclear zone" next the central canal. This radial framework is further elaborated by the development of spines and varicosities along the course of the tenuous fibers and by the arborization of those fibers in their peripheral portions. In the chick at the fourth day of incubation, there is no other sustentacular tissue in the neural tube (Villiger). Following the four-day stage, two processes occur simultaneously, viz., migration of cell-bodies from the nuclear zone and disappearance of the radial framework. The cell-bodies remaining in the nuclear zone develop cilia and become the permanent ependyma cells of the adult; the migrat ing cells form the neuroglia.
The neuroglia cells, as just stated, arise from the migrating ependyma cells. According to Villiger, these migrating cells appear in the chick on the tenth day of incubation. They lose their connection with the limiting membranes, to a large extent; and, branching, become typical spider cells with short, stocky branches or long filamentous branches, constituting the short rayed and long-rayed neuroglia cells. Those maintaining their connection with the limiting membranes give off long, parallel filamentous branches, which at the surface expand and fuse with one another. The latter are the arborescent neuroglia cells.
Connective-tissue Network.—That is of mesoblastic origin and is formed by branching processes from the inner surface of the pia mater. It transmits the blood-vessels into the nervous substance.
The neurones constitute 53 per cent. of the brain and cord (cell-bodies, 6 per cent.) and the sustentacular tissue 47 per cent. (Donaldson).
Grouping of bodies of neurones are massed in certain situations forming the cortex of the cerebrum and cerebellum, the basal complex nuclei (ganglia), the cranial and spinal nerve-nuclei, and the sensory and sympathetic ganglia.
A group of neurone-bodies located outside the cerebrospinal axis is called a ganglion; it is called a nucleus inside the axis. A ganglion is sensory if bipolar neurones compose it and its processes form a sensory nerve or the sensory part of a mixed nerve; such a ganglion is located on an afferent nerve. If a ganglion is made up chiefly of multiploar neurones whose axones supply involuntary muscle or glands, it is a sympathetic gang lion (autonomic ganglion). According to Dogiel sympathetic ganglia contain some afferent neurones and Robert B. Bean has demonstrated afferent and efferent neurones in the geniculate, petrosal and jugular ganglia, so there is a third class, called the mixed ganglia. The sympathetic and mixed ganglia are to a large extent self-regulating centers (autonomic centers).
Sensory Ganglia Sympathetic Mixed.
A group of neurone-bodies within the brain or cord consti tutes a nucleus. Nuclei may be very large, as the basal com plex nuclei of the cerebrum, the cerebral cortex, etc., or much smaller as is illustrated by the cranial and spinal nerve-nuclei. The nucleus of a peripheral nerve connected with the functions of the extremities or body wall is a somatic nucleus (soma-body); a nucleus that sends impulses to smooth muscle, glands or the great viscera, or receives impulses from them is a visceral nucleus. If the axones of a nucleus form an efferent nerve, that is a nucleus of origin, a genetic nucleus (genesis-origin); it is somatic if it supplies striated muscle and visceral if, through a sympa thetic ganglion, it innervates smooth muscle, heart muscle or a gland. Edward F. Malone and others have discovered that the cell-bodies of somatic nuclei of origin possess a much larger mass of cytoplasm than the visceral genetic nuclei and have more definite tigroid aggregations; also that the cell-bodies of the "nucleus cardiacus," which innervate striated heart muscle, are intermediate in position between those supplying voluntary and smooth muscle (Amer. Jour. Anat., Vol. 13, 1913). Other nuclei receive the axonic end-brushes of sensory nerves. These are terminal nuclei. They from only synapses with the axones of afferent nerves; there is no continuity between them. A terminal nucleus is somatic or visceral according to the per ipheral relations of the nerve which it receives. Furthermore, a visceral nucleus of termination is merely a part of a reflex arc; a somatic nucleus of termination is the same and in addition is a part of the path to the center of consciousness, where the impulses become sensations.
Somatic Genetic and Terminal Nuclei Visceral •i Genetic and Terminal Neurone strand of nerve fibers, medullated or non-medullated, constituting a distinct and separate bundle and located outside the cerebrospinal axis is a nerve. It may be afferent, efferent or mixed in function. A spinal nerve is connected with the central axis by a ventral and a dorsal root. The ventral root rises in the axis and is efferent. The dorsal root, rising in the ganglion located on the root, enters the cerebro spinal axis and terminates within it; the dorsal root is afferent. The afferent and efferent roots are joined together in the cord, either directly or by intercalated second-type neurones, and the simple reflex arc or neurone cycle is the result. Within the cerebrospinal axis, a bundle of fibers having the same general origin, destination and function is called a fasciculus or tract ; for example, the pyramidal tract or cerebrospinal fasciculus, the fasciculus gracilis, etc. Two or more fasciculi massed into a complex bundle form a funiculus or column; as the anterior, lateral and posterior funiculi or columns of the spinal cord. Fasciculi which join different parts or levels of the nervous system on the same side of the median plane are called associa tion fasciculi. Those are commissural bundles which extend transversely through the median plane and connect opposite sides; they form the commissures. When a pair of longitu dinal bundles, one on either side, cross each other obliquely through the median plane, they constitute a decussation.