Human Anatomy - H. I. Koliadenko 2009

Nervous System
Central Nervous System
Spinal cord (medulla spinalis)

Embryogenesis. During the fifth week of embryonic development, the anterior median fissure forms on the anterior wall of the neural tube, while the posterior sulcus develops on the posterior wall, dividing the neural tube into right and left halves. Neuroblasts of the alar plate give rise to Nerve Cells that form the posterior horns of the Spinal Cord, whereas neuroblasts of the basal plate form motor Neurons of the anterior horns. Interneurons of the Autonomic (vegetative) Nervous system form between the anterior and posterior horns, constituting the lateral horns. Neurites of the anterior horns form the anterior, or ventral, spinal roots. The spinal ganglia give rise to the posterior sensory spinal roots. A distinctive feature of the posterior roots is the presence of a posterior sensory ganglion, known as the spinal ganglion. The anterior and posterior roots are joined by the ganglion and covered with a myelin sheath.

Structure OF THE spinal cord. The spinal cord is located within the vertebral canal and appears as a cylindrical cord 40–45 cm long, 1 to 1.5 cm in diameter, weighing up to 30 g, and slightly flattened in the anteroposterior direction (Fig. 143). In the embryo During the first 3 months of development, the spinal cord occupies the entire vertebral canal. After birth, however, the growth of the spine outpaces that of the spinal cord, which comes to occupy the vertebral canal from the first cervical vertebra (C1) to the first or second lumbar vertebrae (L1–L2), terminating in the conus medullaris. Extending downward from it is a delicate terminal filament (filum terminale) surrounded by the long roots of the lower spinal segments, which form the cauda equina.

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Fig. 143. Spinal cord: a — anterior view; b — posterior view:

1 — Pons; 2 — Medulla Oblongata; 3 — pyramidal decussation; 4 — cervical enlargement; 5 — anterior median fissure; 6 — anterolateral sulcus; 7 — lumbar enlargement; 8 — conus medullaris; 9 — filum terminale; 10 — Rhomboid fossa; 11 — posterior median sulcus; 12 — posterolateral sulcus

The spinal cord has four surfaces: an anterior flat surface, a posterior slightly convex surface, and two semi-rounded lateral surfaces. In addition, it features two enlargements: the cervical and lumbar enlargements. The first is located at the level of the V–VII cervical segments, and the second at the level of the III–IV lumbar segments. These enlargements are associated with clusters of nerve cells that innervate the upper and lower limbs, respectively.

In the center of the spinal cord lies a reduced canal filled with CEREBROSPINAL FLUID.

The anterior surface of the spinal cord features the anterior median fissure, while the posterior surface contains the posterior median sulcus. The fissure and sulcus divide the spinal cord into symmetrical right and left halves. On the anterior surface of the cord, lateral sulci run parallel to the median fissure. Similar sulci are situated on the posterior surface on both sides of the posterior sulcus.

The Gray matter of the spinal cord lies in its central region and resembles the letter H or butterfly wings in cross-section. This matter consists of neurocyte Cell bodies interconnected by processes and glial cells. The lateral PARTS OF THE gray matter are connected by a transverse bridge (gray commissure), through the center of which runs the central canal of the spinal cord. In adults, this canal may become obliterated in certain areas of the cord.

The gray matter of the spinal cord forms paired anterior and posterior horns. Lateral horns are present in the thoracic region and the upper segments of the lumbar region. Functionally, the anterior horns are motor, the posterior are sensory, and the lateral are autonomic. The right and left halves of the gray matter are connected by the intermediate gray matter, anterior to which lies the white commissure. In the region of the posterior median sulcus, a glial septum extends through the entire thickness of the White matter.

Within the gray matter, neuroocytes form specific nuclei: the posterior (dorsal) horn contains the proper Nucleus of the posterior horn and the thoracic nucleus (thoracic Column, Fig. 144). The ventral (anterior) horn contains 6 nuclei: anteromedial, anterolateral, posteromedial, posterolateral, and central. All these nuclei innervate the Muscles of the body.

Clusters of neuroocytes in the lateral horns of the thoracic and upper two lumbar segments constitute the central part of the Sympathetic division of the autonomic (vegetative) nervous system.

The posterior (afferent) roots of the Spinal Nerves enter the posterolateral sulci on the lateral surfaces of the spinal cord, whereas the anterior (efferent) roots emerge from the anterolateral sulci.

A section of the spinal cord with a pair of sensory and a pair of motor roots is called a segment. The spinal cord comprises 31 segments: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal.

In addition to the anterior and posterior spinal roots, a spinal segment includes: the spinal ganglion, where The Cell bodies of sensory fibers of afferent nerves are localized, and the spinal nerves, which are formed by the union of sensory and motor nerve fibers.

Fig. 144. Spinal cord (horizontal section):

1 — posterior median sulcus; 2, 14 — posterior ROOT; 3, 15 — posterior horn of gray matter; 4, 12 — lateral horn; 5, 10 — anterior horn; 6 — central canal; 7 — gray commissure; 8 — anterior funiculus; 9 — anterior median fissure; 11 — anterior root; 13 — lateral funiculus; 16 — posterior funiculus (fasciculus cuneatus / Burdach's fasciculus); 17 — posterior funiculus (fasciculus gracilis / Goll's fasciculus)

A spinal nerve has four branches: anterior, posterior, meningeal, and communicating. The anterior (ventral) branch contains sensory and motor nerve fibers. It participates in The formation of nerve plexuses in all Regions of the spinal cord except the thoracic. Twelve pairs of anterior branches segmentally innervate the intercostal spaces and lie inferior to the intercostal Arteries. All intercostal nerves give off a lateral cutaneous branch.

The six lower nerves continue into the anterior abdominal wall, where they pierce the rectus abdominis Muscle and emerge subcutaneously as the anterior cutaneous branch. Thus, the intercostal nerves innervate the Skin of the chest and abdomen, as well as ventral trunk muscles: the external and internal intercostal muscles, levatores costarum, transversus thoracis, posterior serratus muscles, external and internal oblique Abdominal muscles, transversus abdominis, rectus abdominis, and pyramidalis muscles.

The posterior (dorsal) branch is also mixed; it segmentally innervates the muscles and skin of the back. The meningeal branch arises from the anterior branch, loops back toward the spinal cord, and innervates its dura mater. The communicating branch connects the segment with the ganglion of the Sympathetic trunk.

The WHITE MATTER OF the spinal cord is divided into three paired funiculi (cords) surrounding the gray matter. The anterior funiculus lies between the anterior median fissure and the anterolateral sulcus (the exit site of the anterior spinal roots); the lateral funiculus lies between the anterolateral and posterolateral sulci; and the posterior funiculus lies between the posterolateral and posterior median sulci.

In the upper thoracic and cervical regions of the spinal cord, between the posterolateral and posterior median sulci, there is an intermediate sulcus that divides the posterior funiculi into two fasciculi (Fig. 145): the fasciculus gracilis (Goll's tract) and the fasciculus cuneatus (Burdach's tract). Between the posterior and anterior horns lies a network of intersecting bands of gray matter known as the reticular formation.

Fig. 145. Distribution of nerve tracts in the white matter of the spinal cord:

1 — dorsal (posterior) median sulcus; 2 — dorsal (posterior) root of spinal nerve; 3 — dorsal (posterior) horn of gray matter; 4 — central canal; 5 — lateral horn of gray matter; 6 — ventral (anterior) horn of gray matter; 7 — ventral root of spinal nerve; 8 — ventral (anterior) median fissure; 9 — direct pyramidal tract; 10 — ventral spinocerebellar tract (Gowers' tract); 11 — posterior spinocerebellar tract (Flechsig's tract); 12 — rubrospinal tract (Monakow's bundle); 13 — lateral pyramidal tract; 14 — cuneate fasciculus (fasciculus cuneatus / Burdach's column); 15 — gracile fasciculus (fasciculus gracilis / Goll's column)

The white matter located close to the central gray matter contains short intersegmental fibers of the principal tracts. Originating from the reticular formation, these fibers extend upward and downward across 2–3 segments to terminate in the motor neurons of the anterior horns. These fibers provide communication between individual spinal segments. Together, the principal tracts form the intrinsic apparatus of the spinal cord.

The spinal cord also houses reflex centers. Nerve fibers from the spinal ganglia pass through the posterior horns and terminate: some on the intercalated (interneuron) Cells of the posterior horns or contralaterally, and others on neurons of the autonomic (vegetative) nervous system, as well as on motor neurons of the anterior horns on the same and opposite sides, and on fibers of the reticular formation. Through these connections, the spinal cord mediates reflex responses originating from both the body's outer integument and muscles, as well as from Internal Organs.

External to the principal tracts of the spinal cord lie the conduction pathways (tracts), which are subdivided into ascending and descending. The ascending tracts include the fasciculus gracilis, fasciculus cuneatus, spinothalamic tract, anterior spinocerebellar tract, and posterior spinocerebellar tract; the descending tracts include the anterior corticospinal tract, lateral corticospinal tract, rubrospinal tract, tectospinal tract, and vestibulospinal tract (see "Conduction Pathways of the Brain AND SPINAL cord", p. 330).

Meninges of the Spinal Cord. The spinal cord is enclosed by three membranes (pia mater, arachnoid mater, and dura mater), which are continuous with the corresponding meninges of the brain (Fig. 146).

The innermost layer—the pia mater—closely invests the spinal cord, extending into and lining all of its fissures and sulci.

The arachnoid mater is a delicate, transparent membrane that covers all elevations of the cord. Unlike the pia mater, it does not extend into the sulci and fissures, thereby creating subarachnoid spaces filled with cerebrospinal fluid.

The dura mater is the outermost layer, forming a dense fibrous sac around the spinal cord that extends onto the nerve roots and ganglia. Inferiorly, the dura mater surrounds the cauda equina and, together with the filum terminale, attaches to the periosteum of the coccyx. Connective Tissue fibers (22–23 pairs) extend from the dura mater to form denticulate ligaments between the pia and dura mater, thereby stabilizing the spinal cord. A subdural space filled with cerebrospinal fluid is formed between the arachnoid and dura mater. The space between the dura mater and the periosteum of THE Vertebral Column is termed the epidural space; it contains adipose tissue and venous plexuses.

Fig. 146. Meninges of the spinal cord:

1 — vertebral body; 2, 8 — dura mater; 3, 16 — arachnoid mater; 4 — ventral (anterior) root of spinal nerve; 5 — dorsal (posterior) root of spinal nerve; 6 — spinal nerve; 7 — section through the vertebral arch root; 9 — superior articular process; 10 — vertebral venous plexus; 11 — vertebral arch; 12 — spinous process; 13 — transverse process; 14 — ligamentum flavum; 15 — spinal ganglion (root ganglion); 17 — pia mater; 18 — posterior spinal arteries; 19 — spinal cord

Spinal nerves (nn. spinales).

Thirty-one pairs of spinal nerves arise from the spinal cord, comprising 8 cervical pairs, 12 thoracic pairs, 5 lumbar pairs, 5 sacral pairs, and 1 coccygeal pair. Each nerve is formed by a sensory dorsal root and a motor ventral root, making them mixed nerves both morphologically and functionally. The dorsal (posterior) rami of the nerves are significantly thinner than the ventral (anterior) rami; they innervate the skin and muscles of the occipital region and back, as well as parts of the gluteal skin. Each dorsal ramus runs independently to its corresponding body segment. The ventral rami are massive trunks, particularly those supplying the limbs. At the points where the ventral rami emerge from the spinal ganglion, they form loops that interconnect to constitute plexuses: cervical, brachial, lumbar, and sacral. The ventral rami of the II–XI Thoracic nerves do not form plexuses, but instead run symmetrically within the intercostal spaces.

The Cervical plexus (plexus cervicalis) is formed by the ventral rami of the upper four cervical nerves (Fig. 147). It lies upon the Deep Muscles of the Neck and is covered superiorly by the sternocleidomastoid muscle. The nerves of this plexus primarily supply the skin, though it also gives off branches to striated muscles.

The cutaneous NERVES OF THE cervical plexus include: the greater auricular nerve, which innervates the skin around the auricle; the lesser occipital nerve, which supplies the skin of the occiput and neck; the transverse cervical nerve, supplying the lateral aspect of the neck; and the supraclavicular nerves, supplying the lower part of the neck and the upper chest wall.

Fig. 147. Nerves of the cervical plexus:

1 — submandibular gland; 2 — parotid gland; 3 — greater occipital nerve; 4 — Accessory nerve; 5 — greater auricular nerve; 6 — transverse cervical nerve; 7 — supraclavicular nerves; 8 — Brachial Plexus; 9 — trapezius muscle; 10 — sternocleidomastoid muscle

The muscular branches innervate the muscles of the neck of ventral origin: the deep vertebral muscles, scalene muscles, and the Infrahyoid muscles.

An important branch of the cervical plexus is the phrenic nerve (n. phrenicus), which runs across The surface of the anterior scalene muscle, descends, passes through the superior thoracic aperture, and is situated between the subclavian vein and Subclavian Artery. Within the thoracic cavity, the phrenic nerve courses between the mediastinal Pleura and the Pericardium, anterior to the root of the lung. Upon reaching the Diaphragm, the nerve ramifies within it. Its sensory branches transmit impulses from the diaphragm, mediastinal pleura, and pericardium to the phrenic nerve, while its motor fibers innervate the muscular portion of the diaphragm. Additionally, the phrenic nerve extends into the Abdominal cavity, where it partly supplies the Peritoneum and the ligaments of the Liver.

The brachial plexus (plexus brachialis) is formed by the ventral rami of the lower four cervical nerves and the greater part of the ventral ramus of the first thoracic nerve. This plexus lies between the anterior and middle scalene muscles and consists of supraclavicular and infraclavicular parts.

The supraclavicular part gives rise to short branches that innervate the levator scapulae, rhomboids, teres major, latissimus dorsi, supraspinatus, infraspinatus, subscapularis, pectoralis Major and minor, and serratus anterior muscles (Fig. 148).

The infraclavicular part is composed of three cords—lateral, medial, and posterior—from which the long nerves of the upper limb originate (Fig. 149).

The lateral cord gives rise to a branch contributing to the median nerve and to the musculocutaneous nerve. The medial cord gives off a branch to form the median nerve, along with the ulnar nerve and the medial cutaneous nerves of the arm and forearm.

The posterior cord gives rise to the axillary and radial nerves.

The musculocutaneous nerve (n. musculocutaneus) originates from the lateral cord, pierces the coracobrachialis muscle, runs downward between the biceps brachii and brachialis muscles, and innervates them. The nerve emerges on the anterior and lateral aspects of the forearm and continues as the lateral cutaneous nerve of the forearm.

The median nerve (n. medianus) is formed by the lateral and medial cords and runs alongside the brachial artery. It gives off no branches in the arm. In the forearm, it occupies a median position and innervates all muscles of the palmar surface. Passing through the carpal tunnel, the nerve enters the hand, supplying the skin and muscles of the lateral side of digits I, II, III, and IV, while the Medial surface of digits IV and V is innervated by the ulnar nerve.

Fig. 148. Supraclavicular part of the brachial plexus:

1 — Hypoglossal nerve; 2 — Internal jugular vein; 3 — phrenic nerve; 4 — thoracic nerves; 5 — subclavian artery and vein; 6 — long thoracic nerve; 7 — ulnar nerve; 8 — median nerve; 9 — axillary artery; 10 — musculocutaneous nerve; 11 — suprascapular nerve

The ulnar nerve (n. ulnaris) branches off from the medial cord of the brachial plexus and descends along the medial aspect of the arm. In the forearm, it innervates the muscles located along the ulna, as well as the skin of the palmar surface and digits IV and V of the PALMAR ASPECT OF the hand.

The radial nerve (n. radialis) is a mixed, most massive nerve of the upper limb, innervating all muscles on its posterior aspect.

Fig. 149. Infraclavicular part of the brachial plexus:

1 — axillary artery; 2 — axillary vein; 3 — brachial plexus; 4 — thoracic nerves; 5 — axillary nerve; 6 — subscapular nerve; 7 — long thoracic nerve; 8 — thoracodorsal nerve; 9 — medial cutaneous nerve of the arm; 10 — radial nerve; 11 — ulnar nerve; 12 — medial cutaneous nerve of the forearm; 13 — median nerve; 14 — musculocutaneous nerve

Emerging from the posterior cord of the brachial plexus, the radial nerve winds around the humerus and enters the thickness of the triceps brachii muscle, innervating it and the skin of the posterior arm. Reaching the lateral epicondyle of the humerus, it then divides into deep and superficial branches. The deep branch supplies all muscles of the dorsal forearm and the brachioradialis muscle, whereas the superficial branch supplies the skin of the dorsal hand and fingers, as well as the supinator and anconeus muscles.

The axillary nerve (n. axillaris) innervates the deltoid muscle, teres minor muscle, skin of the arm, and the shoulder joint.

The medial cutaneous nerve of the arm innervates the skin of the medial surface of the arm.

The medial cutaneous nerve of the forearm innervates the skin of the anterior and medial surfaces of the forearm (Figs. 150, 151).

The Lumbar plexus (plexus lumbalis) is formed by the ventral rami of the upper three lumbar nerves and communicating branches from the IV lumbar and XII thoracic nerves (Fig. 152). This plexus lies posterior to the psoas major muscle and is covered anteriorly by it. The Short Branches of the lumbar plexus include rami that innervate the iliopsoas muscle, quadratus lumborum muscle, and partially the muscles of the lateral and posterior abdominal walls.

Fig. 150. NERVES AND ARTERIES of the shoulder girdle and arm:

1 — brachial plexus; 2 — axillary artery; 3 — subscapularis muscle; 4 — latissimus dorsi muscle; 5 — triceps brachii muscle; 6 — profunda brachii artery; 7 — radial nerve; 8 — brachial artery; 9 — biceps brachii muscle; 10 — musculocutaneous nerve; 11 — median nerve; 12 — axillary nerve; 13 — pectoralis major muscle (cut); 14 — deltoid muscle; 15 — pectoralis minor muscle (cut); 16 — clavicle

The primary Long branches of the lumbar plexus include the femoral nerve (n. femoralis), which is the largest nerve in the lumbar plexus. This nerve emerges onto the thigh beneath the inguinal ligament alongside the iliopsoas muscle. In the thigh, the nerve divides into numerous branches (cutaneous and muscular) that supply the quadriceps femoris muscle, sartorius muscle, and the skin of the anterior thigh. The longest branch of the femoral nerve is the saphenous nerve, which runs along the medial surface of the leg and FOOT.

Fig. 151. Arteries and nerves of the forearm and hand:

1 — biceps brachii muscle; 2 — ulnar nerve; 3 — brachial artery; 4 — median nerve; 5, 8 — ulnar artery; 6 — pronator teres muscle (cut); 7 — common interosseous artery; 9 — flexor digitorum profundus muscle; 10 — flexor carpi ulnaris muscle; 11 — tendon of flexor digitorum superficialis muscle; 12 — superficial palmar arch; 13 — common palmar digital arteries; 14 — proper palmar digital arteries; 15 — superficial palmar branch of radial artery; 16 — tendon of flexor carpi radialis muscle; 17 — flexor pollicis longus muscle; 18 — radial artery; 19 — radial nerve (superficial branch); 20 — extensor carpi radialis longus muscle; 21 — brachioradialis muscle; 22 — brachial artery; 23 — radial nerve (deep branch); 24 — tendon of biceps brachii muscle

Fig. 152. Lumbar plexus and its branches (diagram):

1 — XII thoracic vertebra; 2 — I lumbar vertebra; 3 — sympathetic trunk; 4 — V lumbar vertebra; 5 — branch of lumbar nerve to Sacral Plexus; 6 — psoas major muscle; 7 — rectus abdominis muscle; 8 — genitofemoral nerve; 9 — obturator nerve; 10 — Inguinal Canal; 11 — obturator foramen; 12 — fascia lata of thigh; 13 — external iliac vein; 14 — femoral nerve; 15 — anterior superior iliac spine; 16 — transversus abdominis muscle; 17 — internal oblique muscle; 18 — external oblique muscle; 19 — iliacus muscle; 20 — lateral cutaneous nerve of thigh; 21 — branch of iliohypogastric nerve to skin of gluteal muscles; 22 — iliohypogastric nerve; 23 — ilioinguinal nerve; 24 — quadratus lumborum muscle; 25 — 12th intercostal nerve; 26 — XII rib

On the lateral aspect of the thigh lies the lateral cutaneous nerve of the thigh, which supplies the skin of this region. The obturator nerve (n. obturatorius) runs along the lateral wall of the lesser pelvis and emerges onto the thigh through the obturator foramen, innervating the obturator muscles and the muscles of the medial thigh. The iliohypogastric nerve is also among the long nerves; it lies between the transverse and internal oblique abdominal muscles, supplying them as well as the skin around the external opening of the inguinal canal. In addition, the lumbar plexus includes the ilioinguinal nerve, which passes through the inguinal canal, supplying it and the external genitalia.

The sacral plexus (plexus sacralis) is formed by the anterior rami of the IV and V lumbar nerves and three sacral nerves (Fig. 153). This largest plexus lies within the lesser pelvis on the piriformis muscle. Its short branches (superior and inferior gluteal nerves) supply the gluteal muscles, the Posterior Thigh Muscles, and partially the adductor magnus muscle.

The long nerves include the sciatic and posterior cutaneous nerves.

Fig. 153. Nerves of the thigh (anterior view):

1 — lumbar plexus; 2 — sacral plexus; 3 — femoral nerve; 4 — obturator nerve; 5 — intermediate cutaneous nerve of the thigh; 6 — muscular Branches of the femoral nerve

The sciatic nerve (n. ischiadicus) is the largest and thickest nerve in The Human Body, lying deep to the gluteus maximus muscle. It emerges from the pelvic cavity through the greater sciatic foramen. On the posterior aspect of the thigh, the sciatic nerve courses between the bellies of the biceps femoris muscle, giving off branches to the posterior thigh muscles, and before reaching the popliteal fossa, it divides into the tibial and common fibular nerves.

The tibial nerve (n. tibialis) runs in the leg between the triceps surae and the deep muscles of the calf, where it innervates the triceps surae, tibialis posterior, flexor digitorum longus, and flexor hallucis longus muscles. The tibial nerve then gradually passes to the medial side of the leg and medial malleolus, emerging onto the sole as the plantar nerve, where it divides into lateral and medial branches that supply the muscles and skin of the sole.

Fig. 154. Nerves and Vessels of the thigh (posterior view):

1 — gluteus medius muscle; 2 — superior gluteal artery and Veins; 3 — superior gluteal nerve; 4 — gluteus minimus muscle; 5 — piriformis muscle; 6 — quadratus femoris muscle; 7 — sciatic nerve; 8 — biceps femoris muscle; 9 — popliteal vein; 10 — common fibular nerve; 11 — small saphenous vein; 12 — gastrocnemius muscle; 13 — tibial nerve; 14 — popliteal artery; 15 — semimembranosus muscle; 16 — semitendinosus muscle; 17 — inferior gluteal artery and veins; 18 — inferior gluteal nerve; 19 — gluteus maximus muscle

Fig. 155. Nerves, arteries, and VEINS OF THE posterior leg region:

1 — tibial nerve; 2 — popliteal vein; 3 — popliteal artery; 4 — common fibular nerve; 5, 7 — posterior tibial vein; 6 — small saphenous vein; 8 — posterior tibial artery; 9 — soleus muscle; 10 — fibular artery and vein; 11 — flexor hallucis longus muscle; 12 — triceps surae muscle; 13 — posterior tibial veins; 14, 16 — posterior tibial artery; 15 — flexor digitorum longus muscle

Fig. 156. General Overview of the Peripheral Nervous System:

1 — cutaneous branches of the ophthalmic nerve; 2 — lesser pes anserinus (branches of the maxillary nerve); 3 — brachial plexus; 4 — axillary nerve; 5 — musculocutaneous nerve; 6 — radial nerve; 7 — median nerve; 8 — ulnar nerve; 9 — sciatic nerve; 10 — common fibular nerve; 11 — tibial nerve; 12 — saphenous nerve; 13 — obturator nerve; 14 — femoral nerve; 15 — lateral cutaneous nerve of the thigh; 16 — sacral plexus; 17 — lumbar plexus

The common fibular nerve (n. peroneus communis) reaches the lateral aspect of the leg at the level of the fibular HEAD, where it divides into the superficial and deep fibular nerves.

The superficial fibular nerve supplies the fibular (peroneal) muscles, then continues onto the dorsum of the foot and branches out in its skin.

The deep fibular nerve innervates the anterior compartment Muscles of the Leg, then extends onto the dorsal aspect of the foot, where it supplies the dorsal foot muscles. The tibial and common fibular nerves anastomose with each other to form the sural nerve, which runs down the posterior leg and supplies its skin. This nerve then continues onto the dorsum of the foot, where it is known as the dorsolateral cutaneous nerve of the foot. The pudendal nerve also arises from the sacral plexus, innervating the skin and muscles of the Perineum as well as the external genitalia (see Figs. 154, 155, 156).