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cup, the narrow tube connecting the vesicle to the brain becoming the optic stalk. This stalk becomes solid, and forms the basis of the optic tract, optic chiasma, and optic nerve. The optic cup, bilaminar in form, and by its edge clasping the lens, is imbedded in mesodermic tissue, which gives rise to the envelopes of the eyeball, etc. The outer layer of the optic cup produces the layer of hexagonal pigment cells of the retina. The cells of the inner layer produce the tissues of the retina proper. They form neuroblasts with peripheral and central processes. The peripheral processes are converted into retinal nerve tissues; the central processes extend backwards along the optic stalk, and give rise to the optic nerve, optic chiasma, and optic tract. Spongioblasts in the inner lamina of the optic cup produce the sustentacular tissue of the retina (Müller's fibres). The mesodermic tissue surrounding the optic cup and lens gives rise to the rest of the structure of the eyeball, the formation of which is described in the section which deals with the organs of sense.
III. The oculomotor nerve arises, like the ventral root of a spinal nerve, from a group of neuroblasts in the medial part of the basal lamina of the mid-brain. The peripheral fibres extend forwards, to end around the optic cup in the mesodermic tissue, from which the eye muscles are derived. Numerous cells are carried along with the cell processes in their course, and these have been described as being concerned in the formation of the ciliary ganglion.
IV. The trochlear nerve also arises from a group of neuroblasts occupying the medial part of the basal lamina of the mid-brain, close to its junction with the hind-brain. The peripheral processes do not emerge directly from the brain, but extend dorsally from their origin along the side of the brain to its dorsal aspect, where they appear, after decussating with the fibres of the opposite nerve, just behind the quadrigeminal lamina. V. The trigeminal nerve is developed by means of a large posterior and a small anterior root. Their origin to a large extent resembles the mode of formation of the roots of a spinal nerve.
The large posterior (afferent) root is formed by means of a cellular bud from the alar lamina of the hind-brain. This bud separates from the brain, and forms the semilunar ganglion. Its cells becoming bipolar, like the cells of a spinal ganglion, are secondarily connected with the brain by means of their central processes; while the peripheral processes, separating into three groups, proceed along the fronto-nasal and maxillary processes, and along the mandibular arch, to form the three main divisions of Numerous cells accompany each main division in its course from the ganglion, and form eventually the subordinate ganglia-the ciliary on the ophthalmic nerve, the spheno-palatine on the maxillary nerve, and the otic ganglion on the mandibular nerve.
The small anterior (efferent) root of the trigeminal nerve, like the motor anterior root of a spinal nerve, is later in its appearance than the sensory root. It arises as the peripheral fibres of a group of neuroblasts occupying the lateral part of the basal lamina of the hind-brain, which proceed directly to the surface to join the mandibular division of
VI. The abducens nerve resembles in its mode of development the oculomotor and trochlear nerves with which in its origin it is in series. It is formed by the peripheral processes of a group of neuroblasts in the medial part of the basal lamina in the upper part of the hind-brain. These processes pierce the part of the brain in which, at a later stage, the fibres of the pyramid are developed. They then proceed to the mesodermic tissue round the optic cup, which is destined to form the eye muscles.
VII. The facial nerve has developmentally a double origin. (1) In connexion with the formation of the acoustic nerve a group of cells becomes separated from the alar lamina of the hind-brain opposite the auditory vesicle. This group becomes separated into three parts, of which the middle portion is the rudiment of the genicular ganglion which becomes incorporated with the efferent part of the facial nerve, and is connected to the brain by a slender root, known as the nervus intermedius (O.T. pars intermedia). (2) The efferent root of the nerve arises from a group of neuroblasts in the lateral part of the basal lamina of the hind-brain, in series with efferent fibres of the vago-glossopharyngeal nerves; after a tortuous course within the brain its fibres emerge beneath the above-mentioned cellular mass, opposite the auditory vesicle. They are joined by the ganglionic root, and in their course round the auditory vesicle become imbedded in the auditory capsule (canalis facialis). The chorda tympani nerve appears early as a branch of the facial nerve. It is probable that the nervus intermedius, the genicular ganglion, and the chorda tympani nerve together represent the posterior afferent element in the constitution of this nerve.
VIII. The acoustic nerve arises as a cellular bud from the alar lamina of the hind
brain, dorsal to the efferent portion of the facial nerve, opposite to the auditory vesicle, and in close association with it.
Becoming separated from the brain, the cellular mass separates into three portions, of which the intermediate part is associated with the facial nerve and intermediate nerve (as the genicular ganglion), while the medial and lateral parts are converted into the medial (vestibular) and lateral (cochlear) ganglia and the roots of the acoustic nerve. The cells becoming bipolar, their central processes are secondarily connected with the brain on the dorsal (lateral) aspect of the facial nerve; the peripheral processes proceed to the auditory vesicle, to which they are distributed as the vestibular and cochlear nerves. Numerous cells are carried along with the nerve trunks into relation with the auditory capsule, and constitute the vestibular and cochlear ganglia.
IX. and X. The glossopharyngeal and vagus nerves are developed from the side of the hind-brain, both in the same way, and each by two roots. A collection of cells separates itself from the alar lamina of the hind-brain behind the auditory vesicle to form the ganglionic afferent root. The ganglion of the vagus is much larger than that of the glossopharyngeal. Each ganglion becomes divided into two parts, a proximal and a distal portion, connected together by a commissural band of fibres. The proximal ganglion (superior ganglion of the glossopharyngeal; jugular ganglion of the vagus) is secondarily connected by centripetal fibres to the hind-brain. From the distal ganglion (petrous
FIG. 604. COMPARISON OF ORIGINS OF NERVE ROOTS FROM SPINAL MEDULLA AND HIND-BRAIN (after His). A. Spinal medulla; B. Hind-brain.
ganglion of the glossopharyngeal; ganglion nodosum of the vagus) peripheral fibres grow outwards to form the nerve trunk.
Each nerve is also provided with a small efferent root, consisting of nerve fibres, arising from a collection of neuroblasts in the lateral part of the basal lamina of the hindbrain, and emerging beneath the ganglionic root at the junction of the alar and basal laminæ (in series with the fibres of the efferent root of the facial nerve above and of the accessory nerve below).
XI. The accessory nerve arises in two parts-one medullary, the other spinal. The medullary (accessory) portion develops as the processes of a series of neuroblasts in the lateral portion of the basal lamina of the hind-brain, which emerge in series with the efferent roots of the glossopharyngeal and vagus nerves. The spinal portion arises as the processes of a group of neuroblasts in the anterior part of the medullary tube (anterior column), which, turning outwards, emerge as a series of roots on the lateral aspect of the spinal medulla.
XII. The hypoglossal nerve is developed, not in series with the nerves above mentioned, but like the third, fourth, and sixth nerves, from the medial part of the basal lamina of the hind-brain, in the space between the glossopharyngeal and other nerves above, and the first cervical nerve below. It is formed as a series of peripheral processes from a collection of neuroblasts occupying the hind-brain. Froriep's ganglion is a transitory collection of nerve cells developed from the alar lamina of the hind-brain on the dorsal aspect of the nerve, and represents in a rudimentary condition its posterior ganglionic root. The ganglion gives off no branches and soon disappears.
DESCRIPTION OF THE PERIPHERAL NERVES AND
I. THE SPINAL NERVES.
Origin of the Spinal Nerves. Each spinal nerve is attached to the spinal medulla by two roots, called respectively posterior (dorsal, afferent) and anterior (ventral, efferent).
The posterior root is larger than the anterior root; it contains a larger number of radicular fibres, and the individual fibres are of larger size than in the anterior root. It has a vertical linear attachment to the postero-lateral sulcus of the spinal medulla. The fibres of contiguous posterior roots are in close relation, and, in some instances, overlap. The posterior root separates, as it passes away from the spinal medulla, into two bundles, both of which become connected with the proximal end of a spinal ganglion. From the distal end of this ganglion the posterior root proceeds to its junction with the anterior root in the intervertebral foramen.
The spinal ganglia are found on the posterior roots of all the spinal nerves. (In the case of the first cervical or sub-occipital nerve, the spinal ganglion may be rudimentary or absent; and the posterior root itself may be wanting, or derived from the accessory nerve.) They occupy the intervertebral foramina, except in the case of the sacral and coccygeal nerves, the ganglia of which lie within the vertebral canal; and the first and second cervical nerves, the ganglia of which lie upon the vertebral arches of the atlas and epistropheus respectively. With the exception of the coccygeal ganglia they are outside the cavity of the dura mater, but are invested by the membrane. The ganglia are of ovoid form, bifurcated in some cases at their proximal ends. They consist of unipolar nerve-cells, whose axons, after a very short course, divide into central (root) and peripheral (trunk) fibres. The central fibres form the portion of the root entering the spinal medulla; the peripheral fibres are continued in a lateral direction from the ganglion into the spinal nerve.
Ganglia Aberrantia (aberrant spinal ganglia).-Between the spinal ganglion and the spinal medulla small collections of cells are occasionally found on the posterior roots, either as scattered cells or distinct ganglia. They are most frequently met with on the posterior roots of the lumbar and sacral
The anterior root is smaller than the posterior root. It arises from the anterior surface of the spinal medulla (anterior root zone) by means of scattered bundles of nerve-fibres, which occupy a greater horizontal area and are more irregular in their arrangement than the radicular fibres of the posterior root. It possesses no ganglion in its course. The rootlets sometimes overlap, and are not infrequently connected with neighbouring radicular fibres above and below.
FIG. 605.-DIAGRAMMATIC REPRESENTATION OF THE ORIGIN OF THE SPINAL NERVES, showing the position of their roots and ganglia respectively in relation to the vertebral column. The nerves are shown as thick black lines on the left side.
The dorsal and ventral roots, from their attachment to the spinal medulla, proceed
laterally in the vertebral canal towards the intervertebral foramina, where they unite to form the spinal nerve. The direction of the roots of the first two nerves is upwards and laterally; the roots of the remaining nerves course obliquely downwards and laterally, the obliquity gradually increasing until, in the case of the lower lumbar, the sacral and coccygeal nerve-roots, their course is vertically downwards in the vertebral canal. The collection of nerve-roots which occupies the lower part of the canal, below the first lumbar vertebra, and comprises all the nerve-roots below those of the first lumbar nerve, is designated the cauda equina. They arise from the lumbar enlargement and conus medullaris, and surround the filum terminale of the spinal medulla.
Within the vertebral canal the nerve-roots are in relation with the meninges of the spinal medulla, and are separated from one another by the ligamentum denticulatum, and, in the neck, by the spinal part of the accessory nerve. Each receives a covering of pia mater, continuous with the neurilemma; the arachnoid invests
FIG. 606. THE ORIGIN AND DISTRIBUTION OF A TYPICAL SPINAL NERVE.
each root as far as the point where it meets with the dura mater; and each root pierces the dura mater separately. The two roots are thereafter enclosed in a single tubular sheath of dura mater, in which is included the spinal ganglion of the posterior root. The spinal nerve thus ensheathed occupies the intervertebral foramen (except the first two cervical and the sacral and coccygeal nerves).
Divisions of a Spinal Nerve.-After emerging from the intervertebral foramen the nerve immediately divides into two primary divisions, named respectively the posterior and anterior rami (O.T. posterior and anterior primary divisions). Just before its division each nerve gives off a minute meningeal (recurrent) branch, which re-enters the vertebral canal after effecting a junction with a branch from the sympathetic trunk, and is distributed to the spinal medulla and its membranes. The posterior and anterior rami of the spinal nerves are mainly somatic in their distribution, and are responsible for the innervation of the skeletal muscles and of the skin covering the trunk and limbs.
The posterior and anterior rami of the nerves contain fibres from both posterior and anterior roots. Indeed, each root can be seen, on removal of its sheath, to divide into two portions, of which one portion enters into the formation of the posterior ramus, the other into the formation of the anterior ramus. The posterior rami, with the exception of the first two, are smaller than the anterior rami. They are responsible for the innervation of the skin and axial muscles of the back. They do not supply the muscles of the limbs, although in their cutaneous distribution they are prolonged on to the back of the head, the shoulder, and the buttock. They form two small plexuses-the posterior cervical and the posterior sacral plexuses. The anterior rami are, with the exception of the first two cervical nerves, much larger than the posterior rami. They supply the sides and anterior parts of the body, the limbs, and the perineum. For the most part they have a complicated arrangement. The thoracic or intercostal nerves alone have a simple mode of distribution; the other nerves give rise to the three great plexuses— cervical, brachial, and lumbo-sacral.
White Rami Communicantes. From the anterior rami of certain nerves (second thoracic to second lumbar inclusive) a series of fine nerves arises, which serves to connect the spinal with the sympathetic system. These visceral or splanchnic branches, or white rami communicantes, through the medium of the gaugliated trunk of the sympathetic, serve to innervate the vessels and viscera in the splanchnic area. A second stream of pelvic splanchnic nerves, associated with the second and third, or third and fourth sacral nerves, connects these spinal nerves with the pelvic sympathetic plexuses (p. 766).
Distribution of the Spinal Nerves. Although the distribution, like the origin of the spinal nerves, presents primarily and essentially a segmental arrangement, this is masked, and in some instances obliterated, by developmental changes in the parts supplied. In no region can an isolated nerve be traced to a complete segment. The nearest approach to a complete girdle of innervation is found in the thoracic region, in such a nerve as the sixth thoracic nerve. Yet even such a nerve is not distributed to any part entirely alone. In its cutaneous distribution it supplies a complete belt of skin, a distinctly segmental area from the median plane posteriorly to the median plane anteriorly; yet at the same time the adjacent nerves overstep, so to speak, the boundaries of the area and assist in the cutaneous nerve supply. Its muscular distribution, also, is segmental; the anterior ramus supplies the intercostal muscles of the space in which it lies; but in this it forms communications with adjacent nerves. The posterior ramus supplies axial muscles of the back, not, however, in an obviously segmental manner, on account of the fusion of the segmental myotomes in the formation of complex longitudinal muscles, which are together supplied by the series of muscular branches derived from the posterior rami of contiguous nerves. In other regions still greater changes of structure are accompanied by deviations from a segmental type of distribution, causing the foundation of the nerve-plexuses by which the trunk and limbs are innervated.
POSTERIOR RAMI OF THE SPINAL NERVES.
The posterior rami (O.T. posterior primary divisions) of the spinal nerves innervate both skin and muscles; the skin of the trunk posteriorly, the back of the head, the shoulder and the buttock, and the longitudinal muscles of the back, but not the muscles of the limbs.
Each posterior ramus divides as a rule into two parts, a medial and a lateral trunk (Fig. 606, p. 686). In the upper half of the body the medial trunks generally supply the cutaneous branches, while the lateral trunks are purely muscular nerves. In the lower part of the body the opposite is the case: the lateral trunks provide the cutaneous nerves and the medial trunks are distributed entirely to muscles. The cutaneous branches have a different course in the two cases. In the upper half of the back they course backwards beneath and among the muscles to within a short distance of the spinous processes of the vertebræ, close to which they become superficial. They then extend laterally in the superficial fascia. In the lower half of the back the cutaneous nerves are directed downwards