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Coursing forwards through the anterior part of the ischio-rectal fossa, it passes between the two layers of fascia of the urogenital diaphragm towards the urethra. It supplies muscular branches to the anterior parts of the levator ani and external sphincter, to the transversus perinei superficialis and profundus, ischio-cavernosus, bulbo-cavernosus (or sphincter vagina), and sphincter urethræ membranacea. terminates as the nerve to the bulb, which, piercing the urogenital diaphragm, enters the bulb of the urethra and supplies the erectile tissue of the bulb and corpus cavernosum urethræ, as well as the mucous membrane of the urethra as far as the glans penis.
N. Dorsalis Penis vel Clitoridis.-The dorsal nerve of the penis or clitoris, the other terminal branch of the pudendal nerve, accompanies the internal pudendal artery above the fascia inferior of the urogenital diaphragm. It passes forward close to the pubic arch, lying under cover of the crus and ischio-cavernosus and fascia inferior of the urogenital diaphragm, and upon the sphincter urethræ membranaceæ muscle; piercing the fascia inferior of the urogenital diaphragm near
its apex, at the lateral side of the dorsal artery of the penis (or clitoris), it passes on to the dorsum of the penis or clitoris, to which it is distributed in its distal two-thirds, sending branches round the sides of the organ to reach its under surface. In the female the nerve is much smaller than in the male. The dorsal nerve of the penis supplies one branch, the nerve to the corpus cavernosum penis, as it lies between the fascia of the urogenital diaphragm. This is a slender nerve, which, piercing the fascia inferior of the urogenital diaphragm, supplies the erectile tissue of the crus and corpus cavernosum penis.
Morphology of the Pudendal Plexus.-The structures occupying the perineum are placed in the ventral axis of the body, and comprise, from before backwards, the penis and scrotum, or mons Veneris and vulva, the central point of the perineum, the anus and ischio-rectal fossa, and the coccyx. They are placed on the medial side of the attachment of the lower limbs-the penis or mons Veneris in relation to the preaxial border; the coccyx in relation to the postaxial border of the limb.
The nerves of the perineum, thus reaching the ventral axis of the trunk, are homologous with the anterior (ventral) terminations of other nerves. They are separable into two series. The perineum is supplied mainly through the pudendal plexus by the last four sacral and the coccygeal nerves, but it is also innervated to a minor extent by the first lumbar nerve through the ilioinguinal nerve, which reaches the root of the penis and the scrotum. The region is thus supplied by two series of widely separated nerves, which have their meeting-place on the dorsum and side of the penis and scrotum. This junction of the ilio-inguinal and pudendal nerves constitutes the beginning of the ventral axial line, which extends peripherally along
the medial side of the lower limb. Apart from this break in their distribution, a definite numerical order may be followed in the arrangement of the perineal nerves. The higher parts of the perineum are innervated by the higher spinal nerves; the lower parts, by the lower nerves. This is best exemplified in the distribution of the cutaneous nerves. The base of the penis and scrotum (or mons Veneris) is supplied by the first lumbar nerve (ilio-inguinal). The dorsal nerve of the penis (or clitoris), when traced back to the pudendal plexus, is found to come from the second, and to a less extent from the third sacral nerves; the scrotal nerves (perineal branches of the pudendal and posterior cutaneous nerve of the thigh) similarly arise from the third, and to a less extent from the second sacral nerves; the skin of the ischio-rectal fossa and anus is innervated by the inferior hæmorrhoidal (third and fourth sacral nerves), and the perineal branch of the fourth sacral nerve. The ano-coccygeal nerve (coccygeal plexus), lastly, supplies the skin round the coccyx (fourth and fifth sacral and coccygeal nerves). Judged from its nerve supply the perineum is to be regarded as occupying, for the most part, a position behind or more caudal than that of the lower limb in relation to the trunk. There is here
634.-SCHEME of the innervation of the hinder portion of the trunk and of the perineum, and the interruption of the segmental arrangement of the nerves associated with the formation of the limb.
a remarkable gap in the numerical sequence of the nerves supplying the ventral axis of the body. All the nerves between the first lumbar and the second sacral fail to reach the mid ventral line of the trunk and are wholly concerned in the innervation of the lower limb.
At the preaxial border of the limb (groin) the first lumbar nerve, the highest nerve supplying the perineum, is concerned also in innervating the skin of the limb. At the postaxial border of the limb (fold of the nates and back of the thigh), the nerves which are the highest of those constituting the pudendal plexus (the second and third sacral nerves) are also implicated in innervating that border of the limb. The fourth sacral nerve is concerned only to a very slight extent in the innervation of the limb by means of the perineal branch, which reaches the beginning of its postaxial border; the last two spinal nerves are wholly unrepresented in the limb proper and end entirely in the trunk behind the limb.
MORPHOLOGY OF THE LIMB-PLEXUSES.
The arrangement of the limb nerves is rendered complex and the significance of the plexuses is obscured by the changes through which, coincidently, the nerves, on the one hand, and the parts supplied by them, on the other hand, have passed in the course of development.
Nature of the Limbs.-As already described, the mammalian limbs arise as flattened buds from the extremities of the Wolffian ridge. Each bud possesses a preaxial and a postaxial border, and a dorsal and a ventral surface, continuous with the dorsal and ventral aspects of the trunk and homologous with its lateral and ventral surfaces. Each bud consists at first of a mass of undifferentiated, unsegmented mesoderm, covered with epithelium. Around the central core of mesoderm which produces the skeletal axis, the vessels and muscles of the limb are formed in situ, the muscles as double dorsal and ventral strata, beneath the corresponding surfaces of the bud.
Each limb bud is connected to the lateral and ventral aspects of the trunk, and is associated with a number of body segments, varying in the two extremities and in different animals. Although the mesodermal material of which the limb bud is composed exhibits in itself no segmental divisions at any period of its development, a clear indication of the segmental relations of the limbs is obtained from the arrangement of the limb nerves. Taking the nerves which supply the limbs in man as a guide, the segments engaged in the formation of the upper extremity are the last five cervical and first two thoracic. The lower extremity is related by its nerves to all the lumbar and the first three sacral segments. In each limb, the segments at the preaxial and postaxial borders are only partially concerned in limb formation.
It has been already shown that the somatic branches of the nerves enter the substance of the embryonic limb and divide in their course into dorsal and ventral trunks, which supply the dorsal and ventral surfaces of the limb bud. The higher nerves supply the preaxial border, the lower nerves supply the postaxial border, while the nerves most centrally situated extend furthest towards the periphery of the limb.
In order to understand properly the constitution of the limb-plexuses, it is necessary, further, to make a comparison of the surfaces and borders of the embryonic and adult limbs.
Upper Limb. (4) Borders.-The preaxial border of the upper extremity extends from the
middle of the clavicle, in the line of the cephalic vein, distally along the front of the shoulder, the lateral border of the arm, forearm and hand, to the lateral border of the thumb. The postaxial border extends from the middle of the axilla along the medial side of the arm (in the line of the basilic vein), the medial side of the forearm and hand, to the medial border of the little finger.
(B) Surfaces.-The areas of the limb between these lines, anteriorly and posteriorly, correspond to the ventral and dorsal surfaces of the embryonic limb bud. The ventral surface is represented by the front of the chest, arm, and forearm, and the palm of the hand. The dorsal surface is represented by the scapular and deltoid regions, the back of the arm, forearm, and hand.
Lower Limb.—(A) Borders.—The preaxial border of the lower limb extends from the middle of the inguinal ligament distally along the medial side of the thigh and leg in the line of the great saphenous vein, to the medial side of the great toe. The postaxial border, beginning at the coccyx, extends along the fold of the nates and the lateral border and back of the thigh and leg (in the line of the small saphenous vein) to the lateral border of the foot and little toe.
(B) Surfaces.-The areas between these lines correspond to the primitive dorsal and ventral surfaces of the embryonic limb bud. The unequal amount of rotation in the parts of the lower limb obscures the relation of foetal and adult surfaces, which are most easily made out in the infantile position of the limbs, with the thighs and knees flexed and the soles of the feet inverted. The ventral surface of the embryonic limb is represented by the medial side and posterior part of the thigh, the back of the leg, and the sole of the foot. The dorsal surface is represented by the front of the thigh and buttock, the front of the leg, and the dorsum of the foot.
Composition of the Limb-plexuses. In all mammals the same definite plan underlies the constitution of the limb-plexuses. The nerves concerned are the anterior rami of certain segmental spinal nerves, which (with certain exceptions at the preaxial and postaxial borders are destined wholly and solely for the innervation of the limb. Each of the anterior rami engaged divides into a pair of secondary trunks, named dorsal or posterior, ventral or anterior. The dorsal and ventral trunks again subdivide into tertiary trunks, which combine with the corresponding subdivisions of neighbouring dorsal and ventral trunks to form the nerves of distribution. The combinations of dorsal trunks provide a series of nerves for the supply of that part of the limb which is derived from the dorsal surface of the embryonic limb bud; the combinations of ventral trunks give rise to nerves of distribution to the regions corresponding to its ventral surface. The relation of the nerves derived from the limb-plexuses to the areas of the limbs is given in the accompanying tables :—
In the regions of the limbs no anterior cutaneous branches, derived from the limb nerves, supply the trunk. The whole of the nerve is carried into the limb and is absorbed in its innervation, and the dorsal and ventral trunks forming the limb-plexuses are to be looked upon as homologous with the lateral and anterior trunks of an intercostal nerve. Two series of anomalies in relation to the formation and distribution of the nerves to the, limbs must, however, be considered, because it has been suggested (Goodsir) that the nerves of the limbs are serially homologous not with the whole, but only with the lateral branches of the anterior rami of the intercostal nerves.
(1) Nerves in connexion with the primitive borders of the Limbs.-At the preaxial border of the upper limb, at its root, the fourth cervical nerve, which supplies the anterior and lateral surfaces of the neck, is also distributed through the supraclavicular nerves to the skin of both ventral and dorsal surfaces of the limb. The nerves and surfaces are here not merely homologous, but in actual continuity.
At the preaxial border of the lower limb, similarly, the first lumbar nerve, by means of the ilio-hypogastric and ilio-inguinal branches, supplies on the one hand the buttock, in series with the lateral branches of the lower thoracic nerves, and, on the other hand, the lower part of the abdominal wall and the adjacent medial side of the thigh, in series with the anterior terminal branches of the lower thoracic nerves.
At the postaxial border of the upper limb the first and second thoracic nerves are concerned in supplying trunk segments as well as parts of the limb. The first thoracic nerve, besides supplying the limb through the medial cord of the plexus, also innervates at least the muscles of the first intercostal space; the second thoracic nerve is concerned in the innervation of the limb, principally by means of its lateral branch only, which, as the intercosto-brachial nerve, supplies the skin along the postaxial border of the limb and on its dorsal side. At the postaxial border of the lower limb, in the same way, the third and fourth sacral nerves, partially implicated in the innervation of the limb (through the tibial, posterior cutaneous nerve of the thigh, perforating cutaneous nerve, and perineal branch of the fourth sacral nerve), are also engaged in supplying the trunk (perineum) through the pudendal nerve. These peculiarities of arrangement of the nerves at the borders of the limbs may be explained on the supposition that the segment corresponding to the nerve named is only partially concerned in limb formation, and is, at the same time, implicated to a greater or less extent in the formation of structures belonging to the trunk. (2) The origin and distribution of the nerves at the postaxial border of the limbs present
a special difficulty. In the composition of the brachial plexus the first thoracic nerve is almost wholly engaged in forming the ventral series of nerves. It only gives a minute nerve to join the posterior cord, and this is not always present. In the case of the lumbo-sacral plexus the third sacral nerve does not as a rule divide into ventral and dorsal trunks, but contributes only to the formation of the ventral series of nerves. A solution of this difficulty may be found in the examination of the areas of distribution of the nerves derived from the first thoracic and third sacral nerves respectively. In the case of the brachial plexus (the medial cord of which receives normally the whole contribution of the first thoracic nerve) the medial cutaneous nerve of the arm, the ulnar branch of the medial cutaneous nerve of the forearm, and the dorsal branch of the ulnar nerve supply the dorsal aspect of the limb on its postaxial border. These nerves are in serial homology with the intercosto-brachial and lateral trunks of intercostal nerves. In the case of the lumbo-sacral plexus similarly, in which the third sacral nerve does not divide into ventral and dorsal trunks, the posterior cutaneous nerve of the thigh and tibial nerves containing the contribution from the third sacral nerves innervate, by means of the gluteal and lateral femoral branches of the former and the medial sural nerve (medial cutaneous nerve of the calf) of the latter, the dorsal surface of the limb along the postaxial border, in series with the perforating cutaneous nerve and the perineal branch of the fourth sacral.
These apparent anomalies appear to indicate that, instead of dividing into its proper dorsal and ventral trunks, the entire contribution of the spinal nerve concerned, is in these instances carried undivided along the postaxial border of the limb in association with the ventral trunks, and that the dorsal subdivisions are thrown off successively as the plexus cords approach the periphery. Indeed, in the case of the posterior cutaneous nerve of the thigh, Eisler has shown that, when the common peroneal and tibial nerves are separated at their origin, its gluteal and lateral femoral branches arise from and are connected with the dorsal trunk, and the perineal and medial femoral branches with the ventral trunk.
By dissection, experiment, and clinical observation, it is conclusively proved that, as a rule, each nerve of distribution in the limb, whether to muscle or skin, is made up of fibres derived from more than one spinal nerve; and, further, that in cutaneous distribution a considerable overlapping occurs in the course of the several peripheral nerves. Moreover, the arrangement of the distribution of the nerves to skin and to muscles is not identical. In the case of the skin of the limbs, by the covering of the limb being drawn on to it from adjacent parts in the process of growth, cutaneous nerves are engaged which are derived from sources not represented in the muscular innervation of the limbs. Again, among the muscles, some have undergone fusion, others have become rudimentary, and others again have altered their position in the limb. Bearing these qualifications in mind, it is possible to formulate a definite plan for the innervation of the skin and muscles of the upper and lower limb. The accompanying tables give an analysis of the distribution of the spinal nerves to the skin and muscles of the upper and lower limb respectively :
THE DISTRIBUTION OF THE SPINAL NERVES TO THE MUSCLES AND SKIN OF THE LIMBS.
I. Upper Limb. A. Cutaneous Nerves.