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Introduction; Structure and Function: parent facial nerve, middle-ear course, taste and secretomotor pathways.

The sensory long buccal branch of the mandibular division, distinct from the motor buccal branches of the facial nerve.

The four names identify distinct pathways: chorda tympani, the sensory buccal nerve, the radial nerve formerly called musculospiral, and the obturator nerve. Origin must include the parent nerve or nerve plexus, while the function must follow the actual fibre supply rather than the name of a structure crossed.

Chorda tympani. This branch leaves the facial nerve in its descending facial canal before the stylomastoid foramen. It enters the tympanic cavity from behind, passes forwards between the malleus and incus close to the inner surface of the tympanic membrane, and leaves through the petrotympanic fissure. In the infratemporal fossa it joins the lingual nerve, a branch of the mandibular division of the trigeminal nerve. It then travels with that carrier towards the tongue and submandibular ganglion.

The chorda tympani carries taste from the anterior two thirds of the tongue, with sensory cell bodies in the geniculate ganglion and central connections to the nucleus of the solitary tract. It also carries preganglionic parasympathetic fibres from the superior salivatory nucleus towards the submandibular ganglion; postganglionic fibres supply the submandibular gland and sublingual gland. It does not provide the general touch supply of that part of the tongue; that travels through the lingual nerve's trigeminal nerve pathway. Taste and tactile testing can consequently distinguish two functions that briefly share one peripheral trunk.^[ref]

Buccal nerve. In the usual anatomical meaning intended by this list, this is the long buccal nerve from the anterior division of the mandibular trigeminal nerve. It commonly emerges between the heads of the lateral pterygoid muscle, descends near the anterior border of the mandibular ramus and the temporalis muscle insertion, and passes towards the cheek. Its branches reach cheek skin, the buccal mucosa, and the buccal gingiva beside the lower molars. It is principally sensory. The motor supply of the buccinator muscle comes from the facial nerve, whose buccal branches are a different set of nerves. Thus the ability to hold the cheek against the teeth and sensation in the cheek are not tests of the same efferent pathway.^[ref]

Radial nerve, historically musculospiral. This is the major continuation of the posterior cord of the brachial plexus, usually containing fibres from C5–T1. It passes from the axilla into the posterior arm, accompanying the profunda brachii artery in the radial groove of the humerus. It pierces the lateral intermuscular septum and reaches the anterior compartment between brachialis and brachioradialis. Near the lateral elbow it divides into a superficial sensory branch and a deep motor branch. The latter passes through the supinator muscle and continues as the posterior interosseous nerve.

The radial nerve supplies triceps brachii and anconeus, then brachioradialis, the radial wrist extensors and, through its deep continuation, the principal finger and thumb extensors and associated posterior forearm muscles. Its sensory branches supply posterior arm and forearm regions and part of the dorsolateral hand. A lesion's level matters: an arm lesion can impair wrist and finger extension, while an isolated superficial branch lesion is sensory and does not itself cause a wrist drop. Conversely, a posterior interosseous nerve lesion chiefly affects motor function. The fingertip pads are not the characteristic autonomous sensory territory of the superficial radial nerve.^[ref]

Obturator nerve. It arises from the anterior divisions of the anterior rami of L2–L4 in the lumbar plexus. It emerges from the medial border of psoas major, crosses the pelvic side wall and leaves through the obturator canal with the obturator vessels. Its anterior and posterior divisions characteristically run on opposite sides of adductor brevis, although the level of division varies. The anterior division supplies adductor longus, gracilis and commonly adductor brevis; the posterior division supplies obturator externus and the adductor portion of adductor magnus, with variable additional contributions. The hamstring portion of adductor magnus has a different supply. Cutaneous supply to a variable medial-thigh patch and articular branches to the hip and knee supplement its principal motor role in thigh adduction. Obturator internus is supplied by its own sacral-plexus nerve, not by the obturator nerve merely because the names resemble one another.^[ref][ref]

The fourth ventricle is the cavity of the hindbrain between the dorsal surfaces of the pons and upper medulla oblongata anteriorly and the cerebellum posteriorly. It is continuous above with the cerebral aqueduct and below, at the obex, with the central canal of the spinal cord. Its floor is diamond-shaped when viewed from behind, while its roof rises dorsally towards the fastigium.

The floor, the rhomboid fossa, consists of an upper pontine triangle and a lower medullary triangle. Its upper apex points towards the cerebral aqueduct; its lower apex is the obex; the two lateral angles continue into the lateral recesses of the fourth ventricle. A median groove divides it into right and left halves. On each side, the medial eminence lies medial to the sulcus limitans, which separates predominantly motor territories medially from sensory territories laterally. The striae medullares of the fourth ventricle cross the floor near the junction of its pontine and medullary portions; their extent varies.

Several surface features identify deeper relations. In the pontine part, the facial colliculus overlies the abducens nucleus and the internal looping course of fibres of the facial nerve; it is not the exposed facial motor nucleus. The upper part of the sulcus limitans has the superior fovea, and near its rostral lateral region lies the locus coeruleus. Laterally, the vestibular area overlies the vestibular nuclei. In the lower triangle, the hypoglossal trigone overlies the hypoglossal nucleus, and the vagal trigone, lateral to it, overlies the dorsal motor nucleus of the vagus. The area postrema lies still farther caudolaterally, separated from the vagal trigone by the funiculus separans. These are landmarks and underlying relations, not claims that the cranial nerve nuclei lie loose within the cavity.^[ref]

The upper lateral boundaries are formed by the superior cerebellar peduncles. The lower lateral boundaries relate to the inferior cerebellar peduncles and adjacent dorsal medullary margins. The lateral recess on each side curves around the inferior cerebellar peduncle towards its external opening near the flocculus.

The roof has superior and inferior slopes. Superiorly the thin superior medullary velum spans the superior cerebellar peduncles, with the lingula of the cerebellum on its dorsal surface. Inferiorly, the roof relates to the inferior medullary velum, nodule of the cerebellum and thin tela choroidea. Vascular folds of the tela choroidea carry the choroid plexus of the fourth ventricle, with median and lateral extensions. The cavity has an ependymal lining; the choroid plexus has its specialized epithelial covering and produces cerebrospinal fluid.

There are three important exits to the subarachnoid space: one median aperture of the fourth ventricle through the inferior roof, opening towards the cisterna magna, and two lateral apertures of the fourth ventricle at the ends of the lateral recesses of the fourth ventricle. These must be distinguished from the inlet through the cerebral aqueduct and the inferior continuity with the central canal of the spinal cord. Most cerebrospinal fluid leaving the fourth ventricle reaches the subarachnoid space, not the narrow adult central canal of the spinal cord.

The three-dimensional arrangement explains the method of examination: reflecting the cerebellum and opening the delicate roof exposes the rhomboid fossa, but destroys part of the structure one is trying to describe. Studying an intact roof, then the exposed floor, and using a median section to locate the fastigium avoids interpreting a dissected cavity as naturally open behind. The modern nomenclature is useful here; not every fine landmark or physiological interpretation in this account should be read as an assertion about what an 1867 examiner required.^[ref]