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rones, the cell-bodies and dendrites of which are grouped to form the sympathetic ganglia, form terminal links in nerve or neurone chains; the second link of these chains is formed by neurones the cell-bodies of which are situated in the spinal cord or medulla, the

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Fig. 322. From section of sympathetic ganglion of turtle, showing white rami fibers wound spirally about a large process of a unipolar cell, and ending in pericellular plexus (Huber, Journal of Morphology, 1899).

neuraxes leaving the cerebrospinal axis through the white rami as small medullated nerve-fibers, which terminate in pericellular plexuses inclosing the cell-bodies of the sympathetic neurones.

Large medullated nerve-fibers, the dendrites of spinal ganglion neurones, reach the sympathetic ganglia through the white rami.

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Fig. 323. From section of sympathetic ganglion of frog, showing spiral fiber (white ramus fiber) and pericellular plexus (Huber, Journal of Morphology, 1899).

They make, however, no connection with the sympathetic neurones, but pass through the ganglia to reach the viscera, where they terminate in special sensory nerve-endings or in free sensory nerveendings.

G. GENERAL SURVEY OF THE RELATIONS OF THE NEURONES TO ONE ANOTHER IN THE

CENTRAL NERVOUS SYSTEM.

The following figures illustrate the modern theories with regard to the relationship of the neurones in a sensorimotor reflex cycle. The pathway along which the impulse from the stimulated. area of the body is transmitted to the motor nerve end-organ traverses two neurones (primary neurones) which are in contact by means of their telodendria situated within the gray matter of the spinal cord. The cell-body of the sensory neurone lies within the spinal ganglion; that of the motor neurone, in the anterior horn of the spinal cord. The dendrite of the sensory neurone commences

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Fig. 324.-Schematic diagram of a sensorimotor reflex arc according to the modern neurone theory; transverse section of spinal cord: mV, Motor neurone; s, sensory neurone; C1, nerve-cell of the motor neurone; C2, nerve-cell of the sensory neurone; d, dendrite; n, neuraxis of both neurones; 7, telodendria; M, muscle-fiber; h, skin with peripheral telodendrion of sensory neurone.

as a telodendrion in the skin and transmits a cellulipetal impulse, while its cellulifugal neuraxis and telodendrion (the latter in the gray matter of the cord) transfer the impulse to the cellulipetal telodendrion of the motor neurone. The cellulifugal neuraxis of the latter finally ends as a telodendrion in the muscle. (Figs. 324 and 325.)

In the case of longer tracts the conditions are somewhat more complicated, as, for instance, in tracing the impulse along the sensory fibers to the cortex of the brain, and from there along the motor fibers to the responding muscle. In such cases secondary neurones are called into play by means of their telodendria, which are necessarily in contact with the primary neurones just described.

When we take into consideration the simplest possible case, that of the motor segment of such a neurone-chain, we find, for instance. (Fig. 326), that the neuraxis of a pyramidal cell in the brain cortex. (psychic cell) enters the white substance and traverses it as a nervefiber through the peduncle and the pyramid into the crossed pyramidal tract of the opposite side. Here its telodendria come in contact with those of the motor neurone of the anterior horn.

In the foregoing instance the motor nerve tract is composed of two neurones-of a motor neurone of the first order, extending from the cortex of the brain to the anterior cornua of the spinal cord, and of a motor neurone of the second order, the elements of which extend from the anterior cornua to the telodendria in the muscle.

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Fig. 325.-Schematic diagram of a sensorimotor reflex cycle; sagittal section of the spinal cord: C, Motor cells of the anterior cornua; n, n, neuraxes; s, sensory neurone; C2, spinal ganglion cell; C, collaterals of the sensory neuraxes; d, dendrite of sensory neurone; the broken lines at the cells on the left indicate their dendrites.

The sensory tract may likewise be composed of neurones of the first and second orders. The cellulifugal neuraxis arising from a cell of the spinal ganglion passes to the posterior column of the cord, gives off collaterals to the latter, and then passes upward by means of its ascending branch through the posterior column to the medulla. Although here the relationship is not so clearly defined as in the motor tract, it may nevertheless be assumed that the cellulifugal (but centripetally conducting) neuraxis at some point or other terminates in telodendria (sensory neurone of the first order), which enter into contact with the corresponding structures of a cell of the spinal cord or medulla oblongata. These cells would then

constitute the sensory neurones of the second order. Exactly how their cellulifugal neuraxes end has not as yet been fully determined, but it is very probable that in this case the telodendria are represented by the coarse end-fibers which penetrate into the brain cortex, and here seem to come in contact with the dendrites of the pyramidal cells.

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Fig. 326.-Schematic diagram of the reflex tracts between a peripheral organ and the brain cortex: H, Cerebral cortex; mN1, motor neurone of the first, sN2, sensory neurone of the second, degree; C1, motor cell of the spinal cord; C2, sensory cell of a spinal ganglion; C3, pyramidal cell of the brain cortex (pyschic cell); C4, nerve-cell of a sensory neurone of the second degree; n, n, n, n, neuraxes; d, d, dendrites; c, c, c, c, collaterals; t, t, telodendria; sÑ1, sensory neurone first degree; mN2, motor neurone second degree.

H. THE NEUROGLIA.

We may now consider the neuroglia, a tissue distributed throughout the central nervous system and looked upon as a supporting tissue. Its relation to the other tissues has long been a matter of controversy, but modern observers have shown conclusively that the neuroglia is of ectodermic origin, at least so far as its cellular elements are concerned.

At an early stage of embryonic development there are seen in the spinal cord, and also in the brain, elements radially disposed around the neural canal, which upon closer observation appear to be processes emanating from the epithelial cells lining the neural These processes may undergo repeated dichotomous divi

canal.

Fig. 327.-Neurogliar cells: a, From spinal cord of embryo cat; b, from brain of adult cat; stained in chrome-silver.

b

sion, ending finally in a swelling near the periphery of the cord. These cells are known as ependymal cells, and are differentiated from ectodermal cells, called spongioblasts. In later stages the radial arrangement is still preserved, but the cell-bodies no longer all border upon the central canal, many being found at varying distances from the latter. At this stage in the development of the spinal cord, the elements retaining their original characteristics are situated only in the region of the ventral and dorsal fissures of the spinal cord, and during further development increase in number.

These observations observations would seem to indicate that at least a portion of the neurogliar cells, which develop from the ependymal cells previously mentioned, originate from the epithelium of the central canal, and that from here they are gradually pushed toward the periphery of the cord. This assumption is still further strengthened by the fact that later the epithelial cells of the central canal still continue to divide. Later observations (Schaper, 97) show, however, that neurogliar cells develop also from certain undifferentiated germinal cells of the neural canal, of ectodermal origin, which wander from their position near the neural canal toward the periphery of the medullary tube, where they develop into neurogliar cells.

In the adult the epithelium of the central canal and that of the brain cavities (the ependyma) is of the pseudostratified variety with

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