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area, comprising the chief viscera; this area is governed by the sympathetic system, subordinate to and controlled by its connexions with the splanchnic or visceral branches of the spinal nerves.
The cerebral nerves are twelve in number (see note, p. 798), arranged in pairs; they present striking differences in origin, in distribution, and in functions.
The spinal nerves are usually thirty-one in number, also arranged in pairs. Each nerve arises by two roots from the spinal medulla, one posterior and gangliated, the other anterior and not gangliated. After each root has pierced separately the dura mater, the two roots become enclosed in a common sheath, and unite to form the spinal nerve in the intervertebral foramen; emerging from this, the nerve distributed to the trunk and limbs in a manner to be described later.
The nerves are designated cervical, thoracic, lumbar, sacral, and coccygeal, in relation to the vertebrae between which they emerge from the vertebral canal. Each nerve appears above the corresponding vertebra, in the cervical region, except the eighth, and below the corresponding vertebra in all other regions. There are thus eight cervical nerves (the last appearing between the seventh cervical and first thoracic vertebra); there are twelve thoracic, five lumbar, five sacral, and one coccygeal nerve, all appearing below the corresponding vertebræ.
The thirty-first nerve is occasionally absent; and there are sometimes one or two additional pairs of minute filaments below the thirty-first, which, however, do not emerge from the vertebral canal. These are rudimentary caudal nerves.
The size of the spinal nerves varies. The largest are those which take part in the formation of the great nerve-trunks of the limbs (lower cervical and first thoracic, and lower lumbar and upper sacral nerves); and of these the nerves destined for the lower limbs are the larger. The coccygeal nerve is the smallest of the spinal nerves; the thoracic nerves (except the first) are more slender than the limb nerves; and the cervical nerves diminish in size from below upwards.
Systema Sympathicum. The sympathetic system consists of a pair of gangliated trunks, connected, on the one hand, in certain regions to the spinal nervous system by series of white rami communicantes splanchnic or visceral branches of the spinal nerves : and, on the other hand, distributing branches (a) to the spinal nerves (gray rami com municantes), and (b) to the viscera and vessels occupying the splanchnic area.
splanchnic system serves to collect and transmit to the spinal medulla impulses from the viscera, and to distribute efferent fibres to vessels in the splanchnic area, and to glands and involuntary muscle-fibres.
DEVELOPMENT OF THE PERIPHERAL NERVES AND
DEVELOPMENT OF THE SPINAL NERVES.
I. Origin of the Spinal Nerve Roots. The process of development of the spinal nerves commences by means of the outgrowth of posterior and anterior roots from the medullary tube. The two roots take origin in pairs in quite different ways.
The posterior root is the first to appear,-before, during, or after the union of the medullary plates and the formation of the neural tube.
It takes origin as a ganglionic crest, forming a continuous lateral unsegmented band, on the dorsal surface of the medullary tube. It may arise in one of three ways: (1) from the junction of the medullary plate and surface epiblast, before the closure of the medullary groove; (2) from a neural crest, a median ridge on the dorsum of the completed tube; or (3) as a direct outgrowth from the dorsal surface of the medullary tube. The ganglionic crest becomes completely separated from the medullary tube, and secondarily its cells (neuroblasts) rapidly become spindle-shaped, and by the end of the fourth week give rise to two sets of processes: (1) a central series, which grow centrally
and are secondarily connected with the dorso-lateral aspect of the medullary tube as the fibres of the posterior root; and (2) a peripheral series, which constitute the posterior root-fibres of the spinal nerve and join the anterior root, to form the spinal nerve proper. It is only after the appearance of these nerve-fibres that the ganglionic crest becomes notched along its peripheral border, and it is gradually divided up to form the individual segmental spinal ganglia.
The anterior root of a spinal nerve arises in quite a different way, from cells (neuroblasts) in the substance of the medullary tube. In the account of the development of the spinal medulla it has been shown how the cellular constituents of the medullary tube are converted into two classes of cells: (1) spongioblasts, which produce the matrix (neuroglia) of the spinal medulla; and (2) neuroblasts, which produce the nerve-cells of the gray matter of the spinal medulla. The neuroblasts give rise to the axiscylinder processes or axons, which, penetrating the spongy tissue of the medullary tube and the outer limiting membrane, find their way into the mesodermic tissue on the ventro-lateral surface of the tube. Fibrillar from their earliest origin and derived from nerve-cells which remain within the medullary tube, the axons of the anterior root become surrounded by mesodermic cells immediately on their emergence, which give rise to the sheaths of the nerve. The anterior root is a little later in its date of appearance than the posterior root. It begins to be evident at the twenty-fourth day and is completely formed by the twenty-eighth day."
II. Formation of the Spinal Nerve. The fibres of the posterior root ganglion and the anterior root grow by extension from the cells with which they are respectively connected, and meet in the space between the myotome and the side of the medullary tube to form the spinal nerve. In the adult there is a fundamental division of the spinal nerve into posterior and anterior rami. In the process of development this separation is even more obvious. As the fibres of the posterior and anterior roots approximate, they separate at the same time each into two unequal portions: the smaller parts of the two roots unite together to form the posterior ramus, and the larger parts unite to form the anterior ramus of the spinal nerve.
The posterior ramus, curving laterally and dorsally, passes through the myotome and is connected with it. In the substance of the myotome it separates into branches as it proceeds towards the dorsal wall of the embryo. At a later stage, the branches are definitely arranged into a lateral and a medial series.
The anterior ramus grows gradually in a ventral direction to reach the somatosplanchnopleuric angle, under cover of the growing myotome. It spreads out at its distal end and eventually separates into two portions: a smaller, splanchnic, or visceral and a larger, somatic, or parietal portion. (1) The smaller, splanchnic, or visceral portion grows inwards, dorsal to the Wolffian ridge, to be connected through the sympathetic trunk with the innervation of organs in the splanchnic area. This branch of the spinal nerve becomes the white ramus communicans of the sympathetic. It is not present in the case of all the spinal nerves, but only in relation to the thoracic and upper lumbar and the third and second or fourth sacral nerves. It will be referred to again in connexion with the sympathetic system. (2) The larger, somatic, or parietal portion becomes the main part of the anterior ramus of the nerve. It continues the original ventral course of the nerve, and, reaching the body wall, subdivides into two terminal branchesa lateral branch, which grows laterally and downwards and reaches the lateral aspect of the trunk, after piercing the myotome; and a ventral or anterior branch, which grows onwards in the body wall to reach the ventral axis. This arrangement is met with in the trunk between the limbs and in the neck.
III. Formation of Limb-plexuses. The method of growth of the spinal nerves, just described, is modified in the regions where the limbs are developed. In relation to the limbs, which exist in the form of buds of undifferentiated cellular mesoblast before the spinal nerves have any connexion with them, the development of the anterior ramus of the nerve proceeds exactly in the way described, up to the point of formation of somatic and splanchnic branches. The somatic branches then stream out into the limb bud, passing into it below the ends of the myotomes and spreading out into a bundle of fibres at the basal attachment of the limb. Later, the nerves separate, each into a pair of definite trunks, which are named posterior or dorsal and anterior or ventral. and which, dividing round a central core of mesoderm, proceed to the dorsal and ventral surfaces respectively of the limb bud. While this process is going on, a secondary union takes place between parts of adjacent dorsal and ventral trunks. Dorsal trunks unite with dorsal trunks, ventral trunks unite with ventral trunks, to form the nerves distri buted ultimately to the surfaces and periphery of the limb. These dorsal and ventral
trunks are homologous with the lateral and ventral branches of the somatic nerves in other regions.
DEVELOPMENT OF THE SYMPATHETIC SYSTEM.
There are two conflicting views of the mode of development of the sympathetic E system.
In birds and mammals the first rudiment of the sympathetic trunk occurs in the formation of a longitudinal unsegmented column of mesodermic cells (which stain more deeply than the mesoderm in which they lie) on each side of the aorta, and coterminous with it. This column of cells becomes joined at an early stage by the visceral branches of the spinal nerves which grow inwards from the main nerve trunks into the splanchnic area, and result from the division of the nerve into somatic and visceral parts. These visceral branches constitute the white rami communicantes. They receive contributions usually from both posterior and anterior roots, and gradually approaching the above-mentioned column of mesodermic cells, they become intimately associated with the cells. In some cases fibres of the visceral nerves pass over the cellular column into the splanchnic area without connexion with it (Fig. 603). By the junction of these visceral nerves with the cells of the column, certain cells persist and produce the ganglia. The intervening portions of the column, by changes in the cells, and by the addition of fibres belonging to the visceral nerves, give rise to the connecting cords. The cellular column, besides producing the gangliated trunk, by the further growth of its cells and their extension centrally and peripherally, produces the gray rami communicantes, parts of the peripheral branches, and the peripheral (collateral and terminal) ganglia, as well as the medullary portion of the suprarenal gland. The cervical, lower lumbar, and sacral portions of the sympathetic gangliated trunk are secondary extensions from the primitive trunk, gradually growing upwards and downwards along the main vessels. These portions of the system are not provided with white rami communicantes. The ganglia of the sympathetic assume their segmented appearance (1) from the persistence of the primitive cells and their connexion with the spinal nerves by means of the white and gray rami communicantes, and (2) from the way in which the primitive column is moulded by the surrounding structures (bones, segmental arteries, etc.).
FIG. 603. THE DEVELOPMENT OF THE
Sympathetic trunk; Spl, Splanchnic branches
In another account of the development of the sympathetic system (Onodi), the gangliated trunk is described as an outgrowth of the thoracic spinal ganglia of the spinal nerves. It is said that each ganglion gives off a bud at its inferior end, which, growing inwards into the splanchnic area, becomes attached to the trunk of the spinal nerve just beyond the union of the posterior and anterior roots. The bud still extending inwards into the splanchnic area, remains associated with the nerve by an attenuated stalk. These buds, it is said, become the ganglia, which, after reaching their permanent place in the splanchnic area, are supposed to grow upwards and downwards so as to coalesce and form a beaded chain of ganglia. The stalks connecting the ganglia with the spinal nerves become the white rami communicantes. This mode of development does not satisfactorily account for several important features of the sympathetic system-the development of those parts of the gangliated trunk which possess no white rami, the absence of a truly segmental character in the trunk, and the constancy of its continuity. No instance is recorded
of a hiatus between two ganglia. It is, on the other hand, an attractive view, as it ascribes to one germinal layer (ectoderm) the formation of all the elements of the nervous system, and it brings the sympathetic ganglia into serial homology with the isolated ganglia-ciliary, spheno-palatine, and otic-associated with the trunks of the trigeminal cerebral nerve.
THE DEVELOPMENT OF THE CEREBRAL NERVES.
The cerebral nerves are divisible morphologically into three series: (1) those associated with sense organs-the first or olfactory, second or optic, and eighth or acoustic; (2) those connected with the embryonic branchial arches-the fifth or trigeminal, seventh or facial, ninth, tenth, and eleventh, glossopharyngeal, vagus, and accessory; and (3) motor nerves distributed to muscles derived from cephalic myotomes the third or oculomotor, fourth or trochlear, sixth or abducent, and twelfth or hypoglossal.
Omitting the olfactory and optic nerves, which are special vesicular outgrowths of the brain itself, it is possible to trace a distinct homology in the process of development of the other cerebral and the spinal nerves. In the primitive brain the gray matter is arranged into Alar and Basal Lamina (His), comparable to the postero-lateral and anterior areas of gray matter (columns) of the spinal medulla. Further, the basal lamina may be split up into lateral and medial areas.
The origin of the third, fourth, sixth, and twelfth cerebral nerves-all motor efferent nerves-is from the medial part of the basal lamina of the primitive brain, in serial homology with the anterior efferent roots of the spinal nerves.
The efferent motor roots of the fifth, seventh, ninth, tenth, and eleventh nerves arise from the lateral part of the basal lamina, and so may be differentiated from the preceding series.
The afferent sensory roots of the fifth, seventh (nervus intermedius), eighth, ninth, and tenth nerves are homologous with the posterior roots of the spinal nerves. They are all gangliated, and are connected with the alar lamina of the brain.
I. The olfactory nerves are associated in their development with the formation of the olfactory pit and the olfactory bulb.
The olfactory pits appear on each side of the front of the head at a little later period than the formation of the lens and the auditory vesicle. They become converted into the nasal cavities by the formation of the pre-oral visceral clefts and arches,-fronto-nasal and ethmo-vomerine in the median plane, and lateral ethmoid and maxillary processes at the sides (p. 49).
The Rhinencephalon or olfactory bulb is a hollow outgrowth from each telencephalon or cerebral hemisphere, and appears in the first month. It grows forwards into relation with the deep surface of the nasal pit. In many animals (as in the horse) the olfactory bulb remains hollow; but in the human subject it loses its lumen and becomes a solid bulb (olfactory bulb) connected to the brain by a narrow stalk, the olfactory tract.
The epithelium of the olfactory pit is responsible for the formation of the olfactory nerves. There are two views as to the mode of their development from the epithelial cells. Both views admit the proliferation of the epithelium of the nasal pit so as to produce neuroblasts. According to the one view these neuroblasts detach themselves from the epithelial surface, and constitute an olfactory ganglion which becomes applied to and incorporated with the olfactory bulb. The cells of the ganglion become bi-polar, and the peripheral axons constitute the olfactory nerves, while the central axons (in the second month) proceed backwards to the brain along the olfactory tract. According to the other view (based on Disse's investigations), the proliferating cells of the nasal epithelium remain in the wall of the nasal pit, and become the olfactory cells of the nasal cavity, with peripheral processes projecting to the surface of the epithelium. Their central axons become the olfactory nerve fibres which end in the olfactory bulb, forming dendrites associated with the dendritic processes of the nerve-cells of the bulb. The central axons of these latter cells develop into the fibres of the olfactory tract (see p. 622).
II. The optic nerve is developed wholly from the brain. Its formation begins with the outgrowth of the optic vesicle, a paired hollow outgrowth from the ventral surface of the diencephalon. The ectodermic invagination of the lens, growing inwards from the surface of the head, causes the collapse of the vesicle and its conversion into the optic