Central Termination and Terminal Nuclei (Fig. 146).—The ascending divisions of the outer set of fibers run a short distance within the apex of the posterior columna, and end in ramifica tions about the cell-bodies of the substantia gelatinosa. They form the marginal tract (of Lissauer). Probably their de scending branches have the same ending. The T-branches and collaterals of the inner set of fibers from the posterior root run: (I) To the gray matter of the cord, viz. : To all parts of the posterior columna, to the center of the crescent, and to the anterior columna on the same side. These fibers end in relation with the dendrites or cell-bodies situated in those several regions of the crescent at various levels: (a) At the same level as the nerve, (b) at a lower level than the nerve, through the descend ing radicular tracts, and (c) at a higher level, through the col laterals given off by the ascending tracts. (2) The large ascending T-branches of the inner set of fibers run to the medulla oblongata. They form the fasciculus gracilis and fasciculus cuneatus and ascend to the nuclei of those columns in the medulla oblongata. They terminate in the nuclei funiculi gracilis and cuneati. Hence the terminal nuclei (nn. terminales) of any spinal nerve are composed of the gray crescent of the cord, chiefly at the same level, and of the nucleus funiculi gracilis or the nucleus funiculi cuneati in the medulla. All these are somatic terminal nuclei, except the nucleus dorsalis of Clark; that is a visceral terminal nucleus. Like other nuclei of the kind, it receives non-sensory impulses which excite reflexes, and it belongs to the afferent side of the sympathetic system. Such im pulses do not reach consciousness normally; but under powerful stimuli, they may overflow their proper bounds and pass through the cerebellum to the centers of the cerebrum.
Spinal Ganglia (ganglia spinalia).—On the posterior root of every spinal nerve, with the possible exception of the first cervical, there is a fusiform swelling, 4-6 mm. long, called a spinal ganglion. The spinal ganglia are often bifurcated at the proximal end, where they are continuous with the two fasciculi of the posterior root. They lie upon the neural arches of the first two vertebr w; but, below that, are located in the interverte bral foramina, down to the last lumbar nerve. The ganglia of the sacral and coccygeal nerves are within the vertebral canal; but, excepting the last, are outside the dural sheath. The ganglia in man are composed of pear-shaped, bipolar cell-bodies with their processes and nucleated capsules. Dogiel also de scribes typical multipolar cells (sympathetic neurones) in spinal ganglia, and they are undoubtedly present in the mixed cranial ganglia (geniculate of the seventh, petrosal of the ninth, and jugular of the tenth); but recent studies have not shown them in purely sensory ganglia. The cell-bodies vary in size from 25 i70µ, the greater number measure from 6o-8ogin their long axis.
Each cell contains a large spherical nucleus, with one or more nucleoli, and possesses all the cytoplasmic constituents common to neurones (p. 169). The axone hillock is well developed and from it originates the single composite process, formed by the approximation of axone and dendrite. This composite process,
constituting the stem of the pear-shaped cell, pursues a tortuous course of variable length, sometimes equaling three or four times the diameter of the cell-body; it then separates into its compo nent axone and dendrite which continue in opposite directions, the dendrite toward the periphery and the axone into the cord as a posterior root-fiber. The dendrites are the larger processes, though many of them are non-medullated. The caliber of the axones is proportionate to the size of the cell-bodies from which they arise: about one-third of them are large and are medullated; two-thirds are small or of medium size and most of them are non medullated (Ranson). As the axones enter the cord, through the posterior lateral sulcus, they fall into two strands, a medial and a lateral. The lateral bundle, made up of fine non-medullated fibers, with a few medullated fibers of medium size, enters the apex of the posterior columna, where its fibers divide T-like and form the marginal tract of Lissauer. The medial bundle is composed of the large medullated fibers and of a con siderable number of small and medium sized fibers, some of which possess a myelin sheath. The small and medium fibers end quickly in the gray crescent above and below the point of entrance; the large fibers terminate chiefly in the dorsal nucleus of the cord and the nucleus gracilis and nucleus cuneatus of the medulla, their descending rami terminate in segments of the cord below the points of entrance.
Besides the single composite process given off by the spinal ganglion neurones, many investigators have observed accessory processes which end in the form of points or bulbs within the capsule of the cell; and protoplasmic loops and plexuses continu ous with the cytoplasm of the cell which, likewise, are contained within the capsule. These are not permanent formations; they change quickly and may disappear entirely. They are found during development and under conditions of stimulation, par titularly, and are probably the result of amceboid movement. Dogiel may have mistaken the accessory processes for the dendrites of a multipolar cell.
The nucleated capsule of each cell in a spinal ganglion is con tinuous with the neurolemma of its processes; it invests the whole neurone from the surface of the spinal cord to a point near the end-tufts of its dendrites; and it incloses a lymph space in which, according to Orr and Rows, there is a centripetal current flowing into the spaces of the cord and its membranes.
Spinal ganglion neurones carry all kinds of common sensory impulses, in harmony with the law that first order common sensory neurones conduct impulses in combinations; second order common sensory neurones form specific paths each of which conveys only one variety a impulse. Hence, lesion of the posterior column of the cord, not involving the nerve roots, causes more or less loss of a whole group of sensations—as muscle-sense, tactile discrimination, sense of size, shape and form, of weight, of vibration, etc.; while localized lesions of the lateral column of the cord may produce the loss of a single variety of common sensation—as pain, heat, cold, or tactile localiza tion—without affecting any others conducted by the lateral column.