Appendix E · Major Vessels and Nerves

There are roughly a hundred named arteries, a hundred named veins, and a hundred named nerves in the human body, and no one memorizes them as a list. They are learned as a small number of organizing rules plus the exceptions to those rules. This appendix gives you the rules first, because everything after §E.1 is easier once you have them.

E.1 Four Organizing Principles

1 · Arteries, veins, and nerves travel together

A developing limb bud receives one arterial supply, one venous drainage, and one nerve trunk, and as it elongates all three follow the same path. The adult result is the neurovascular bundle: an artery, one or two accompanying veins, and a nerve, wrapped together in a common fascial sheath.

Three consequences follow immediately.

• One injury damages all three. A stab wound in the medial arm does not choose between the brachial artery and the median nerve; they are millimetres apart in the same sheath.
• Finding one finds the others. Regional anaesthesia works this way: locate the artery by palpation or ultrasound, and the nerve is beside it. The axillary block, the femoral block, and the interscalene block are all "find the artery" procedures.
• The order within the bundle is constant and clinically load-bearing. In the costal groove, from superior to inferior, run Vein, Artery, Nerve — which is precisely why a chest drain or a thoracentesis needle is passed over the top of the rib below the chosen space, hugging the upper border, away from the bundle tucked under the rib above. In the femoral triangle, from lateral to medial, run Nerve, Artery, Vein, Empty space, Lymphatics; the femoral vein's medial position is why it, not the artery, is cannulated for venous access. In the carotid sheath lie the common carotid artery medially, the internal jugular vein laterally, and the vagus nerve behind and between them.

2 · Vessels are named for the region they supply, and change name at landmarks

A single continuous tube of blood carries four or five different names on its way down a limb. The name changes are not arbitrary: each marks the point where the vessel crosses into a new anatomical compartment, which is exactly the information a clinician needs to localize a blockage.

Trace it once in each limb and the rule becomes concrete.

• Upper limb: the subclavian artery becomes the axillary at the lateral border of the first rib; the axillary becomes the brachial at the lower border of teres major; the brachial divides into the radial and ulnar arteries at the neck of the radius, about a finger's breadth below the elbow crease.
• Lower limb: the external iliac becomes the femoral as it passes deep to the inguinal ligament; the femoral becomes the popliteal as it passes through the adductor hiatus in adductor magnus; the popliteal divides into the anterior tibial artery and the tibiofibular trunk at the lower border of popliteus.

The same principle names veins, and it is why a "popliteal deep vein thrombosis" and a "femoral deep vein thrombosis" describe the same vessel above and below one muscular gap.

3 · Anastomoses decide whether an occlusion matters

Where two arterial territories meet and join, an occlusion of one may be silent because flow arrives from the other. Where they do not, an occlusion infarcts.

Watershed zones are the places where two territories meet at their weakest: the splenic flexure of the colon (between superior and inferior mesenteric supply) and the cortical border zones between the anterior, middle, and posterior cerebral arteries. In systemic hypotension, these are where infarction happens first, because they were furthest from both sources to begin with.

4 · Nerves carry a predictable mixture of fibre types

There is almost no such thing as a purely motor nerve. Every named peripheral nerve carries a mix, and knowing which mix predicts what a lesion will look like.

Two working rules come out of this table.

• A spinal nerve is always mixed; a named branch of a plexus may not be. The posterior interosseous nerve is purely motor, which is why a lesion of it causes finger drop with no sensory loss at all — a diagnostically decisive absence.
• A root lesion and a peripheral nerve lesion produce different maps. A root lesion produces a dermatomal sensory pattern and a myotomal motor pattern, both spanning several peripheral nerves. A peripheral nerve lesion produces the territory of that one nerve. Telling the two apart at the bedside — is the numbness in a stripe down the limb, or in one nerve's patch? — is what the dermatome map in §E.8 is actually for.

E.2 The Aorta and Its Branches

E.3 Arteries by Region

Head and neck

Upper limb

Thorax

Abdomen and pelvis

Lower limb

E.4 The Cerebral Arterial Circle and Its Territories

        THE CEREBRAL ARTERIAL CIRCLE (CIRCLE OF WILLIS) — from below
                              ANTERIOR
                                 ▲
                    ┌────── ANTERIOR COMMUNICATING a. ──────┐
                    │           (one, midline)              │
              ╭─────┴─────╮                           ╭─────┴─────╮
              │  ANTERIOR │                           │ ANTERIOR  │
              │ CEREBRAL a│                           │CEREBRAL a │
              │   (ACA)   │                           │   (ACA)   │
              ╰─────┬─────╯                           ╰─────┬─────╯
      LEFT           │                                       │        RIGHT
                ╔════╧════╗                             ╔════╧════╗
                ║ INTERNAL║                             ║ INTERNAL║
                ║ CAROTID ║                             ║ CAROTID ║
                ╚════╤════╝                             ╚════╤════╝
              ╭──────┴──────╮                       ╭────────┴────╮
              │   MIDDLE    │                       │   MIDDLE    │
              │ CEREBRAL a. │◄── lenticulostriate   │ CEREBRAL a. │
              │    (MCA)    │    perforators ──►    │    (MCA)    │
              ╰─────────────╯                       ╰─────────────╯
                    │                                       │
         POSTERIOR COMMUNICATING a.            POSTERIOR COMMUNICATING a.
                    │        (CN III runs beneath)          │
              ╭─────┴─────╮                           ╭─────┴─────╮
              │ POSTERIOR │                           │ POSTERIOR │
              │CEREBRAL a.│                           │CEREBRAL a.│
              ╰─────┬─────╯                           ╰─────┬─────╯
                    └───────────┬───────────────────────────┘
                          ╔═════╧═════╗
                          ║  BASILAR  ║ ← superior cerebellar aa.
                          ║  ARTERY   ║ ← pontine branches
                          ║           ║ ← AICA
                          ╚═════╤═════╝
                    ╭───────────┴───────────╮
                    │  L VERTEBRAL   R VERT │ ← PICA from each
                    ╰───────────────────────╯
                                 ▼
                             POSTERIOR

   ═══════════════════════════════════════════════════════════════════
   CORTICAL TERRITORIES ON THE HOMUNCULUS

        ACA ──► medial surface = LEG and foot area
                                          ┌──────────┐
        MCA ──► lateral surface =          │  L E G   │ ACA
                FACE, ARM, hand,       ┌───┴──────────┴───┐
                and language           │ trunk  arm  face │ MCA
                                       └──────────────────┘
        PCA ──► occipital lobe = VISION

   → an ACA stroke weakens the LEG more than the arm
   → an MCA stroke weakens the FACE and ARM more than the leg
   The circle is anatomically COMPLETE in only about a third of people.

Wallenberg's crossed findings, explained. A PICA occlusion infarcts the lateral medulla, producing ipsilateral face pain and temperature loss (the spinal trigeminal nucleus, which has not yet crossed), contralateral body pain and temperature loss (the spinothalamic tract, which crossed in the spinal cord), plus ipsilateral Horner syndrome, ataxia, and dysphagia with hoarseness from the nucleus ambiguus. Motor power is spared, because the corticospinal tract is in the medial medulla. The crossed pattern is not a curiosity; it is the fastest way to localize a lesion to the brainstem rather than the hemisphere (Chapter 12).

E.5 Venous Drainage

Superior vena cava and its tributaries

Inferior vena cava and its tributaries

Dural venous sinuses

Endothelium-lined channels between the two layers of dura. They have no valves and no muscular walls, which makes them both efficient low-resistance drainage channels and a route by which infection spreads.

Why CN VI fails first in cavernous sinus disease. The abducens nerve is the only cranial nerve that runs free within the sinus lumen, surrounded by blood, while III, IV, V1, and V2 sit protected in the lateral wall. Anything raising pressure in the sinus reaches CN VI first, and an isolated lateral rectus palsy is therefore an early sign (Chapters 13 and 15).

The hepatic portal system

             THE HEPATIC PORTAL SYSTEM AND ITS FOUR ESCAPE ROUTES

    ┌──────────────────────────────────────────────────────────────┐
    │ CAPILLARY BEDS OF THE GUT, SPLEEN, PANCREAS, GALLBLADDER      │
    │  (everything from lower esophagus to upper rectum)            │
    └───────┬──────────────────┬──────────────────┬─────────────────┘
            │                  │                  │
     SUPERIOR MESENTERIC   SPLENIC v.      INFERIOR MESENTERIC v.
          v.                   ▲                  │
            │                  └──────────────────┘
            └───────┬──────────┘   (IMV joins the splenic)
                    ▼
           ╔══════════════════╗  + left & right gastric vv.
           ║  HEPATIC PORTAL  ║  + cystic v.
           ║      VEIN        ║  + paraumbilical vv.
           ╚════════┬═════════╝
                    ▼
           ┌──────────────────┐   ← FIRST PASS: everything absorbed
           │   LIVER SINUSOIDS│      from the gut is processed here
           │  (2nd capillary  │      before reaching the rest of the
           │      bed)        │      body.  Nutrients stored, toxins
           └────────┬─────────┘      detoxified, drugs metabolised.
                    ▼
              HEPATIC VEINS ──► INFERIOR VENA CAVA ──► HEART

    ══ WHEN PORTAL PRESSURE RISES, BLOOD FINDS FOUR WAYS AROUND ══

    site            portal side              systemic side        result
    ─────────────────────────────────────────────────────────────────────
    ① lower         left gastric v.    ↔    esophageal vv.    ESOPHAGEAL
      esophagus                              → azygos          VARICES
                                                              (the lethal one)
    ② umbilicus     paraumbilical vv.  ↔    superficial       CAPUT
                    (in falciform lig.)      epigastric vv.    MEDUSAE
    ③ rectum        superior rectal v. ↔    middle + inferior ANORECTAL
                                             rectal vv.        VARICES
    ④ retro-        colic vv.          ↔    retroperitoneal   (usually
      peritoneum                             vv. of Retzius    silent)

Why the portal system exists. Everything absorbed from the gut — glucose, amino acids, drugs, bacterial products — is delivered first to the liver rather than to the systemic circulation. The liver stores or converts nutrients, detoxifies, and metabolises drugs on this first pass, which is why many oral medications have far lower bioavailability than the same dose given intravenously, and why hepatic failure produces encephalopathy: gut-derived nitrogenous compounds reach the brain unprocessed. It is the only place in the body, apart from the kidney's glomerulus-to-peritubular arrangement and the hypothalamic-hypophyseal portal system, where blood passes through two capillary beds in series before returning to the heart.

Superficial and deep veins of the limbs

The calf muscle pump. Deep veins are squeezed by contracting muscle inside an inextensible fascial compartment; valves ensure the blood can only move toward the heart. Every step therefore pumps venous blood upward. That is why immobility — a long flight, a hospital bed, a plaster cast — is the dominant risk factor for deep vein thrombosis, and why early mobilization is the single most effective prophylaxis. Superficial thrombophlebitis is painful but rarely embolizes; deep vein thrombosis is often silent and does (Chapter 19).

E.6 The Twelve Cranial Nerves

The five highest-yield cranial nerve lesions

CN III — pupil-sparing versus pupil-involving. The parasympathetic fibres that constrict the pupil travel on the outside of the oculomotor nerve; the somatic motor fibres to the extraocular muscles run in the core. Compression from outside — a posterior communicating artery aneurysm, uncal herniation — therefore hits the pupil fibres first, giving a painful, dilated, unreactive pupil, and this is a neurosurgical emergency. Ischemia from small-vessel disease, as in diabetes, infarcts the poorly perfused core and spares the surface fibres, giving a painless palsy with a normal pupil. Same nerve, two blood supplies, two urgencies.

CN VII — upper versus lower motor neuron. The upper face receives corticobulbar input from both hemispheres; the lower face from the contralateral hemisphere only. A hemispheric stroke therefore spares the forehead; a lesion of the facial nerve itself does not (see Appendix D §D.2).

CN X — the recurrent laryngeal nerves and the aortic arch. Both recurrent laryngeal nerves supply all the intrinsic laryngeal muscles except cricothyroid. The right hooks under the right subclavian artery; the left descends into the thorax to hook under the aortic arch before ascending again. Consequently, a left recurrent laryngeal palsy causing hoarseness is a recognized presenting sign of a left hilar lung tumour, an aortic arch aneurysm, or an enlarged left atrium — chest pathology presenting as a voice change.

CN XI — trapezius weakness. The accessory nerve crosses the posterior triangle of the neck superficially, covered only by skin and fascia. It is at risk in lymph node biopsy, and injury produces shoulder droop, weak shrug, and difficulty raising the arm above the horizontal because trapezius can no longer rotate the scapula upward.

CN XII — tongue deviation toward the lesion. Genioglossus protrudes the tongue. With one side paralyzed, the intact side pushes unopposed and the tip swings toward the weak side.

E.7 The Four Plexuses

   THE BRACHIAL PLEXUS — Roots · Trunks · Divisions · Cords · Branches
        "Real Texans Drink Cold Beer"

   ROOTS        TRUNKS        DIVISIONS      CORDS          BRANCHES
   (ant. rami)                (ant./post.)   (named for their relation
                                              to the AXILLARY ARTERY)

   C5 ──┐
        ├──► UPPER ────┬─ant──────────┐
   C6 ──┘              └─post──┐      ├──► LATERAL ──┬──► MUSCULO-
                               │      │     CORD     │    CUTANEOUS
   C7 ─────► MIDDLE ───┬─ant───┼──────┘              │    (C5–C7)
                       └─post──┤                     └──┐
   C8 ──┐                      │                        ├─► MEDIAN
        ├──► LOWER ────┬─ant───┼──► MEDIAL ──┬──────────┘  (C6–T1)
   T1 ──┘              └─post──┤     CORD    │
                               │             └──► ULNAR (C8–T1)
                               └──► POSTERIOR ─┬─► AXILLARY (C5–C6)
                                      CORD     └─► RADIAL  (C5–C8)

   ═══════════════════════════════════════════════════════════════════
   RULE 1  All POSTERIOR divisions form the POSTERIOR cord, which gives
           the two nerves of the EXTENSOR compartments (axillary, radial).
   RULE 2  ANTERIOR divisions form the lateral and medial cords, which
           give the nerves of the FLEXOR compartments (musculocutaneous,
           median, ulnar).
   RULE 3  The MEDIAN nerve has TWO roots, one from each of the lateral
           and medial cords — which is why it carries C6 through T1.

   BRANCHES OFF THE ROOTS/TRUNKS (before the cords):
     dorsal scapular (C5) ····· rhomboids, levator scapulae
     long thoracic (C5–C7) ···· serratus anterior  → WINGED SCAPULA
     suprascapular (C5–C6) ···· supraspinatus, infraspinatus
     nerve to subclavius ······ subclavius

   CLASSIC LESIONS
     UPPER trunk (C5–C6)  = ERB–DUCHENNE  → arm adducted, medially
                            rotated, forearm pronated = "WAITER'S TIP"
     LOWER trunk (C8–T1)  = KLUMPKE       → clawed hand, intrinsic
                            muscle loss, ± HORNER'S SYNDROME

Cervical plexus — anterior rami C1–C4

Brachial plexus — anterior rami C5–T1

Lumbar plexus — anterior rami L1–L4

Sacral plexus — anterior rami L4–S4

Reflexes and their roots — the fastest bedside test of a segment:

E.8 Dermatomes

A dermatome is the strip of skin supplied by a single spinal nerve root. The map below gives the landmark that is easiest to find and remember at each level; the bolded ones are the ones worth committing to memory, because they let you localize a root lesion in seconds.

Four caveats that matter more than the map itself.

1. Dermatomes overlap substantially. Each patch of skin receives fibres from at least two adjacent roots, so cutting a single dorsal root produces reduced sensation, not anesthesia. To produce true anesthesia you must interrupt three consecutive roots. This is why radiculopathy causes tingling and altered sensation far more often than numbness.
2. Published maps disagree with one another, particularly in the limbs, because they were derived by different methods — clinical case series, residual sensation after root section, and the distribution of herpes zoster.
3. Myotomes are a separate map, and the two do not overlie one another neatly. A given root supplies skin in one place and muscle in another.
4. The pattern is the diagnosis. A stripe of altered sensation running down a limb is a root problem — a radiculopathy. A discrete patch matching one named nerve's territory is a mononeuropathy. A symmetrical stocking-and-glove distribution beginning distally is a polyneuropathy, because the longest axons fail first. Three different patterns, three different differential diagnoses, one bedside examination (Chapter 13).

E.9 Pulse Points

What the pulse examination actually tells you. Rate and rhythm are only the beginning.

• Volume reflects stroke volume and pulse pressure. A thready pulse suggests a low stroke volume; a bounding one suggests a wide pulse pressure, as in aortic regurgitation, sepsis, or thyrotoxicosis.
• Character carries diagnostic information. A collapsing (water-hammer) pulse — a rapid rise and rapid fall, best felt with the arm raised — points to aortic regurgitation. A slow-rising pulse points to aortic stenosis. Pulsus paradoxus, an inspiratory fall in systolic pressure greater than 10 mm Hg, points to cardiac tamponade, constrictive pericarditis, or severe asthma. Pulsus alternans, alternating strong and weak beats, points to severe left ventricular failure.
• Symmetry localizes disease. Radio-radial delay suggests a subclavian stenosis or an aortic arch problem, including dissection. Radio-femoral delay suggests coarctation.
• Capillary refill, pressed for five seconds at the fingertip or sternum and released, should return colour within two seconds. It is a crude but fast measure of peripheral perfusion.

E.10 Localization Self-Test

For each, name the vessel or nerve and the level of the lesion.

1. Weakness of the right leg with relative sparing of the right arm, plus urinary incontinence and apathy.
2. Dense left face and arm weakness, a left visual field defect, and fluent but meaningless speech.
3. Sudden hoarseness in a 62-year-old smoker with a left hilar mass on a chest radiograph.
4. A painful, dilated, unreactive right pupil with the right eye deviated down and out.
5. Loss of pain and temperature on the left side of the face and the right side of the body, with hoarseness and a left Horner syndrome.
6. Inability to extend the fingers, with a normal wrist extension and completely normal sensation.
7. Numbness of the thumb, index, and middle fingers that wakes the patient at night, with normal sensation over the thenar eminence itself.
8. A patient standing on the left leg whose pelvis drops on the right.
9. Vomiting of large volumes of blood in a patient with a large spleen, ascites, and dilated veins radiating from the umbilicus.
10. An adolescent with hypertension in both arms, weak femoral pulses, and a delay between the radial and femoral pulses.

Use this appendix with Appendix D, which names the muscle supplied by every nerve listed here, and with Appendix C §C.8 for the surface landmarks under which these vessels and nerves run.