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The only exception to this rule is on the ventral (anterior) surface of the arm, where a sup pression of the muscle elements leads to an absence of the regular series of segmental nerves (C. 8, T. 1) on its postaxial border. These nerves reappear in the forearm, and the occasional
DIAGRAM of the segmental distribution of the muscular nerves of the upper and lower limbs. "axillary arches" may be regarded as the muscular elements usually suppressed, and, when present, supplied by these nerves.
Muscles with a Double Nerve-supply. The existence of more than one nerve to a muscle indicates usually that the muscle is composite and is the representative of originally separate elements, belonging to more than one segment or to both surfaces of the limb. In the case of the pectoralis major, subscapularis and flexor digitorum profundus, adductor magnus, and soleus. parts of the same (ventral or dorsal) stratum have fused, to form muscles innervated from the corresponding ventral or dorsal nerves. The other muscles having a double nerve-supplybrachialis, biceps femoris, and (sometimes) pectineus are examples of fusion at the preaxial or postaxial border of muscular elements derived from the dorsal and ventral surfaces of the limb, which are correspondingly innervated by branches from both dorsal and ventral series: eg. the brachialis is innervated by the musculo-cutaneous and radial nerves; the biceps femoris by the peroneal (short head) and tibial nerves (long head); and the pectineus, by the femoral and (sometimes) obturator nerves.
B. Innervation of the Skin of the Limbs. While the scheme of cutaneous innervation of the limbs is fundamentally segmental, yet the arrangement is confused and complicated by various causes. The growth of the limb from the trunk has caused the skin to be drawn out over it like a stretched sheet of india-rubber (Herringham), and at the same time the extent of the dorsal area of the limb is increased at the expense of the ventral area. The central nerves of the plexus remain buried deeply in the substance of the limb, only coming to the surface towards the periphery. The proximal parts of both surfaces of the limb thus become innervated by cutaneous nerves otherwise not necessarily concerned in the innervation of the limbs. Herring ham has shown that-(A) Of two spots on the skin, that nearer the preaxial border tends to be supplied by the higher nerve. (B) Of two spots in the preaxial area, the lower tends to be supplied by
the lower nerve; and of two spots in the postaxial area, the lower tends to be supplied by the higher nerve. In other words, from the root of the limb along the preaxial border to its distal extremity, and along the postaxial border to the root of the limb again, there is a definite numerical sequence of spinal nerves supplying skin areas through nerves of the limb-plexuses. A similar numerical sequence in the arrangement of the nerves is also found extending over the dorsal and ventral surfaces of the limbs from preaxial to postaxial border, except in certain situations.
On the dorsal and ventral surfaces of both upper and lower limbs there is a hiatus, for a certain distance, in the numerical sequence of the spinal nerves in their cutaneous distribution, explicable on the ground that the central nerves of the plexus, which fail to reach the surface in these situations, are replaced by cutaneous branches from neighbouring nerves. This hiatus has been named the axial area or line.
In the upper limb, the dorsal axial area or line extends from the median line of the back, opposite the vertebra prominens, to the insertion of the deltoid.
The ventral axial area or line extends anteriorly from the median plane of the trunk, at the sternal synchondrosis, across the chest, distally along the front of the arm and forearm to the wrist.
In the lower limb, the dorsal axial area or line may be traced from the median plane of the back over the posterior superior iliac spine, across the buttock and thigh, to the head of the
A ventral axial area or line can also be traced from the root of the penis along the medial side of the thigh and knee, and along the back of the leg to the heel.
These areas or lines represent the meeting-place and overlapping of nerves, which are not in numerical sequence; and it is only at the peripheral parts of the limbs, on the dorsal and ventral surfaces, that the nerves appear in numerical sequence from the preaxial to the postaxial border. In the case of the upper limb the hiatus is caused, in both surfaces of the limb, by the absence of cutaneous branches of the seventh cervical nerve; in the case of the lower limb the hiatus is due to the absence of branches from the fifth lumbar nerve on both surfaces of the limb, and the absence of branches from the fourth lumbar nerve, in addition, on the dorsal surface.
Understanding the significance of these dorsal and ventral axial areas or lines, and at the same time bearing in mind the overlapping which occurs in the cutaneous distribution of each spinal nerve, the areas of skin supplied through the limb-plexuses can be mapped out with considerable precision, as indicated in the following tables :
Two different kinds of variations occur in relation to the limb-nerves.
(1) Individual variations, both in the extent of origin and in the area of distribution of given nerve, are not uncommon; these variations are usually concomitant with compensatory variations in adjacent nerves, and are due to the fibres of a given spinal nerve taking a abnormal course in the trunk of another nerve of distribution and effecting a communicatio with the proper nerve peripherally. In this way the variations in the origin and distribution of the intercosto-brachial nerve may be explained; and, similarly, the ulnar nerve may hav some of its fibres carried as far as the forearm, incorporated with the median and transferred to it by a communication between the two nerves in that region.
(2) Variations in the limb-plexus, in relation to the vertebral column, are the chief cause of variations in the constitution of the limb-nerves. These variations affect more or less the whole series of nerves in the plexus.
The brachial plexus is subject only to very slight variation in position and arrangement It may be reinforced at the upper end by a slender trunk from the fourth cervical nerve, and, more frequently, by an intra-thoracic communication between the second and first thoraci The presence of one or other of these nerves is an indication of a slight tendency towards a cephalic or caudal shifting of the whole plexus in relation to the spinal medulla. It however, never sufficient to cause the exclusion to any extent of the nerves normally implicated. The presence of a cervical rib may coincide with little or no change in the relation of the nerves. Indeed, the inclusion of the second thoracic nerve in the plexus may be, as already statel merely an individual variation, a change in the path to the limb of the intercosto-brachial nerve. Concomitant variations occur among groups of nerves, however, which indicate a certain tendency to variation in the position of the whole plexus. At one end, the suprascapular ani musculo-cutaneous nerves may arise from the fourth and fifth, fifth alone, or fifth and sixth cervical nerves. At the other end of the plexus, the radial may or may not receive a root from the first thoracic nerve, and this addition is rather more likely to occur when the second thoracic nerve is implicated in the plexus.
The lumbo-sacral plexus shows a very considerable variability in position and constitution. Eisler records concomitant variations in the plexus in 18 per cent. of the cases examined by him. The variations occur within wide limits. The plexus may begin at the eleventh or twelfth
thoracic or first lumbar nerve. The last nerve in the sciatic cord may be the second, third, or fourth sacral nerve. The position of the n. furcalis is a guide to the arrangement of the plexus. It may be formed by the third, third and fourth, fourth, fourth and fifth, or fifth lumbar nerves. The resulting variations are illustrated by the following extreme cases :
Those variations in the constitution of the lumbo-sacral plexus are most numerous which are due to the inclusion of nerves more caudally placed. Thus, out of twenty-two variations in the position of the n. furcalis, in nineteen Eisler found it formed by the fifth lumbar nerve; in two cases only, by the third lumbar nerve. There is further evidence that variations in the position of the plexus are accompanied by variations in the vertebral column itself. Out of the twenty-two abnormal plexuses examined by Eisler, sixteen were coincident with abnormal arrangement of the associated vertebræ.
SIGNIFICANCE OF THE LIMB-PLEXUSES.
From the above considerations, it is obvious that something more than convenience of transit for the spinal nerves to skin and muscles is secured by the formation of the limb-plexuses. It has been shown that by their combinations in the plexuses, every spot or area of skin in the limbs is innervated by more than one spinal nerve; and generally, also, each limb-muscle is supplied by more than one spinal nerve. Each cutaneous area and each limb-muscle is thus brought into relationship with a wider area of the spinal medulla than would occur if the plexuses were non-existent. A simultaneous record of sensation is thus transmitted from any given point on the surface of the limb through more than one posterior root; and a more ready co-ordination of muscular movement is brought about by the transmission of motor impulses from the anterior root of a given spinal nerve to more than one muscle at the same time. In a word, a plexus exists to supply the whole limb and the limb as a whole, as an organ which has its different active parts connected with the central nervous system by means of the limb-plexus.
SYSTEMA NERVORUM SYMPATHICUM.
The sympathetic nervous system comprises a pair of elongated gangliated trunks, extending through the whole length of the body from the base of the skull to the coccyx, connected to the peripheral spinal nerves by one series of nerves, and to the viscera by another series. At its cephalic end each sympathetic trunk passes into the cranial cavity along with the internal carotid artery, on which it forms plexuses, and thereby forms complex relations with certain cerebral nerves. At their caudal ends the two sympathetic trunks are joined together by fine filaments and unite with the coccygeal ganglion (g. impar).
The sympathetic system is essentially dependent on and subservient to the spinal nervous system. It distributes efferent fibres from the peripheral spinal nerves to (a) the viscera and vessels of the splanchnic area, and (b) through recurrent (gray) rami to vessels, glands, and involuntary muscles in the course of the somatic divisions of the spinal nerves. It further collects and transmits to the cerebro
spinal system afferent fibres from the viscera (Fig. 635). General Structure of the Sympathetic System.-The sympathetic system is composed of two elements-ganglia and nerve fibres.
Ganglia Trunci Sympathici. The ganglia are variable in number, form, size, and position. They are not definitely segmental in position, but they are always connected together by a system of narrow cords of nerve fibres. A ganglion (Fig. 635) consists of a larger or smaller number of multipolar nerve cells, enclosed in a capsule of connective tissue. Each cell is provided with one axon and a number of dendrites. The axon may enter into the composition of (a) the connecting cord; (b) a central branch (gray ramus communicans); or (c) a peripheral branch from the sympathetic trunk. These axons are commonly medullated at their origin, but become non-medullated in their course from the parent cell. Besides these ganglia, two other series of ganglia are present in connexion with the peripheral branches of the sympathetic ganglia plexuum sympathicorum,
intermediate or collateral ganglia, on the branches or in the sympathetic plexuses: and terminal ganglia, in close relation to the endings of the nerves in the viscera.
The nerve-fibres in the sympathetic system are of two classes, medullated and non-medullated. The distinction is not absolute. The medullated fibres may lose their medullary sheaths before reaching their terminations; and the non-medullated fibres may at their origin possess a medullary sheath. The medullated fibres form the series of white rami communicantes (the visceral branches of the spinal nerves). They take origin from the anterior rami of certain spinal nerves in two streams: thoracico-lumbar from the second thoracic to the second lumbar nerve inclusive, and pelvic, or sacral, from the second and third, or third and fourth sacral nerves. The roots of these rami arise from both posterior and anterior roots of the spinal nerves,
but in largest numbers from the anterior root. The fibres from the anterior root are of very small size. They are the axons of nerve cells within the spinal medulla, which enter the sympathetic trunk through the white ramus, and end by forming arborisations around the cells of a sympathetic ganglion. There are three known courses for such a fibre to take in relation to the sympathetic system—(a) It may end in the ganglion with which the ramus is immediately related; (b) it may course upwards or downwards in the connecting cord to reach a neighbouring ganglion; (c) it may pass beyond the sympathetic trunk to end in relation to cells of the peripheral (collateral) ganglia along with fibres of distribution from the sympathetic ganglia. These fibres are splanchnic efferent fibres; motor fibres for the unstriped muscular tissue of the vessels and viscera, and secretory fibres for the glands in the splanchnic area. The fibres from the posterior root of the spinal nerve entering into the composition of the white ramus communicans are the axons of spinal ganglion cells. They constitute the splanchnic afferent fibres, and probably traverse the sympathetic gangliated trunk, passing upThe roots and trunks of a spinal nerve are wards, downwards, and peripherally, without shown, with the white ramus passing between the spinal nerve and a sympathetic being connected with its cells. They are the ganglion (Sy). The splanchnic efferent sensory fibres from the viscera, with which fibres (in red) are shown, partly ending in they are associated along with the peripheral the ganglion, and partly passing beyond the ganglion into a peripheral branch. The branches arising from the sympathetic trunk splanchnic afferent fibres (in blue) are itself. It is not certain that fibres from the shown, partly entering the ganglion, and spinal ganglia are only found in connexion passing upwards or downwards in the gangliated cord; partly passing over the with nerves provided with distinct white rami. Similar medullated fibres are found also in the gray rami communicantes.
FIG. 635.-SCHEME OF THE CONSTITUTION OF
cord into peripheral branches.
The non-medullated fibres in the sympathetic system are derived from the axons of the sympathetic ganglion cells. They have different destinations. (a) Some fibres appear to contribute to the formation of the cord connecting the ganglia together, and to end in arborisations round the cells of a neighbouring ganglion. (b) Non-medullated fibres form a large part of the system of peripheral splanchnic efferent) branches, streaming into the splanchnic area in an irregular manner, both from the ganglia and the connecting cords. (c) The gray rami communicantes form a series of non-medullated fibres (with a small number of medullated fibres intermingled) proceeding centrally from the ganglia to the spinal nerves. These gray rami are found in connexion with each and all of the spinal nerves. Their origin from the gangliated trunk is quite irregular: they may come from the ganglia or the commissure; they may divide after their origin, so