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The thorax is a mobile osteocartilaginous enclosure with two pleural compartments and a central mediastinum; its visceral boundaries change normally with breathing and posture. The sketch is an anterior projection, with the subject's right on the viewer's left. It distinguishes the lower margin of a lung from the lower reflection of its pleura: those are different boundaries.

The posterior wall is formed by the twelve thoracic vertebrae and adjacent parts of the ribs; laterally the ribs and intercostal muscles form a flexible wall. Anteriorly lie the sternum, costal cartilages and anterior intercostal structures. The superior aperture communicates with the neck, while the respiratory diaphragm closes the inferior aperture. The lower thoracic wall therefore overlaps upper abdominal organs: not every organ under a lower rib is inside the pleural compartment.

Each lung is covered by visceral pleura. Parietal pleura lines the chest wall, upper surface of the respiratory diaphragm and side of the mediastinum, becoming continuous with visceral pleura at the root. The potential pleural space contains a lubricating fluid film; it is not normally an air-filled gap. The right lung has three lobes, the left two, with the cardiac notch and lingula accommodating the leftward projection of the heart. Each apex projects above the first rib into the root of the neck. The roots carry the bronchi, pulmonary vessels, nerves and lymphatic connections.^[ref]

The heart and pericardium occupy chiefly the middle part of the inferior mediastinum, behind the sternum and between the lungs, with most of the cardiac bulk to the left of the median plane. The great vessels approach superiorly. The trachea divides into main bronchi behind the upper central region; the oesophagus descends behind the airway and heart. The descending thoracic aorta lies posteriorly, chiefly left of the vertebral bodies; the azygos venous route lies chiefly on the right. The thoracic duct and sympathetic chains belong to the posterior relations. A frontal projection superposes these structures and must not be mistaken for their order from front to back.

For a conventional quiet-breathing surface description, the lower lung border reaches approximately the sixth rib in the midclavicular line, eighth in the midaxillary line and tenth posteriorly near the scapular line. The pleural reflection descends approximately two ribs farther at corresponding landmarks: eighth, tenth and twelfth. These are orientation points, not immutable lines in every healthy individual. Inferiorly, the extra pleural extent forms the costodiaphragmatic recess. The right diaphragmatic dome usually stands higher, related to the underlying liver; the left is related to the stomach and other abdominal viscera.

In inspiration the respiratory diaphragm descends, the rib cage expands and lower lung margins move into the recesses; in expiration they retreat. The heart also shifts with the diaphragm and changes its surface projection as neighbouring lung expands. Deep inspiration, quiet expiration and forced expiration therefore cannot share a single normal map. The heart's own beat adds smaller cyclic movements.

Posture shifts the diaphragm and mediastinal contents: a supine position generally raises the domes compared with standing, while lateral positioning displaces mobile contents towards the dependent side. Body build, age, abdominal fullness and pregnancy change the available thoracic shape and diaphragmatic level. A distended stomach can raise the left dome and alter the cardiac projection without implying primary cardiac disease. The clinically useful comparison records posture and respiratory phase before judging an apparent difference in organ position.

This combination warrants a grave prognosis and closely supervised treatment of both the pneumonia and the delirium tremens; recovery cannot be promised or ruled out from the diagnosis alone. The immediate conflict is that the agitation and sleeplessness of alcohol withdrawal call for effective control, while pneumonia reduces respiratory reserve. Excessive sedation can then worsen hypoventilation and aspiration, whereas uncontrolled delirium tremens can itself be life-threatening.

An historically reasoned 1864 approach would emphasise continuous nursing, a quiet environment, nourishment and fluids that the person could safely swallow, and repeated examination of the thorax and circulation. Suppressing a cough or inducing sleep would not demonstrate resolution of the pneumonia. The traditional strongly depleting approach to inflammation, including bloodletting and large doses of tartar emetic, would be especially hazardous in an exhausted person. Opium and alcoholic stimulation were debated contemporary remedies; escalating opium simply because sleep had not occurred could deepen respiratory depression. Period discussion of delirium tremens explicitly recognised that treatment response varied and that intercurrent disease altered the outlook.^[ref] These are historical treatment controversies, not present instructions to administer alcohol or opium.

Modern management is in hospital, with an appropriate level of observation and early critical-care assessment if breathing or consciousness deteriorates. Assess the airway, respiratory rate, oxygen saturation, blood pressure, temperature, blood glucose and electrolytes. Establish the severity and likely cause of the pneumonia, including possible aspiration; a chest radiograph and microbiological samples are used as indicated. Give timely antibiotics selected for the setting, severity, likely organisms and local resistance patterns. Provide oxygen therapy or more advanced respiratory support when indicated, while reassessing the response.^[ref]

Treat the delirium tremens using an appropriate benzodiazepine regimen with frequent reassessment; the respiratory illness makes skilled monitoring particularly important, rather than making untreated withdrawal acceptable. Provide thiamine, with the parenteral route when indicated by malnutrition or suspected Wernicke encephalopathy, and correct relevant deficiencies. Hypoglycaemia requires prompt treatment. Antipsychotics are not a replacement for treatment of the underlying alcohol withdrawal.^[ref]

Historically the absence of effective antibiotics made the additional pneumonia particularly ominous. Today the outlook depends on respiratory failure, sepsis, coexisting organ disease and treatment response. No severity findings, age or organ-function results are supplied, so neither a mortality percentage nor an inevitable fatal outcome follows. A poor result would also not, by itself, prove that a particular historical remedy caused or prevented death.

Arterial recoil, the muscle pump, respiratory movements, venous tone and the mechanics of cardiac filling assist blood circulation. The heart remains its principal continuing source of mechanical energy. These agencies differ in whether they store cardiac work, contribute new muscular work, or direct flow without supplying power.

During ventricular systole, the aorta and other large elastic arteries expand and store elastic potential energy. During diastole, their recoil continues to drive blood towards the peripheral vessels, while closure of the aortic valve limits return into the heart. This elastic reservoir smooths an intermittent cardiac input into a more continuous downstream flow. It does not manufacture extra energy: it releases part of the work performed during the preceding ejection.

Contraction of surrounding skeletal muscles compresses veins. Venous valves favour displacement towards the heart and limit backward flow when the compression is released. Alternating contraction and relaxation therefore produces a muscle pump, especially useful for return from dependent limbs. The added work comes from the contracting muscles; valves guide the displacement but supply no power themselves. Persistent compression without refilling is not equivalent to a series of pumping strokes.

Spontaneous inspiration supplies another assistance. Descent of the respiratory diaphragm raises abdominal pressure while expansion of the thorax lowers intrathoracic pressure. The resulting changes favour a pressure gradient towards the thoracic veins and right side of the heart. The effect depends on posture, the respiratory phase and whether vessels remain open. Positive-pressure inflation does not reproduce the same pressure changes as spontaneous inspiration.^[ref]

Venous smooth muscle can reduce the storage capacity of peripheral veins, transferring blood towards the central circulation. This changes the volume available for cardiac filling; it does not require a travelling wave of venous peristalsis. Relaxation and recoil of the heart, together with movement of the atrioventricular plane, also influence filling pressures. The older expression cardiac suction is therefore best interpreted through specific changes of chamber volume and pressure, rather than a separate force pulling uniformly through every vessel.

Gravity assists a downward column and opposes an upward one. For a complete circuit returning to its initial height it supplies no net gravitational work. Its important effects include hydrostatic pressure differences and dependent pooling. Likewise, the residual pressure gradient across the capillaries is chiefly the downstream consequence of the cardiac pump. Ordinary capillary action at a liquid meniscus is not a universal extra pump inside a closed, fluid-filled vascular network. Local vasodilation can improve perfusion by reducing vascular resistance, but it does not create the necessary pressure source.

With $Q$ the mean volumetric flow rate, $R$ an effective vascular resistance, and $\Delta P$ the driving pressure difference, the useful bookkeeping relation isIt separates an increase of driving pressure from a reduction of resistance. An instructive apparatus compares the outflow of a pulsatile pump through rigid tubing with its outflow through an elastic reservoir feeding the same resistance. Continued outflow between strokes demonstrates stored elastic potential energy. A separately compressed valved flexible tube demonstrates the additional muscle pump principle. Neither observation requires a mysterious attraction of blood by the tissues.

Quiet breathing chiefly uses active inspiration and passive expiration; deeper or more forceful breathing recruits additional muscles and changes their coordination. The vertical, anteroposterior and transverse dimensions of the thorax can all change, and different patterns need not represent different mechanisms of gas exchange.

The respiratory diaphragm is the principal inspiratory muscle. Its contraction lowers and flattens its domes, increases the thoracic vertical dimension and displaces abdominal contents so the relaxed abdominal wall moves outwards. Its lower-rib attachments also contribute to expansion of the costal margin. The phrenic nerves, chiefly C3–C5, provide its motor supply. The external intercostal muscles elevate and stabilize the ribs, while the interchondral or parasternal portions of the internal intercostal muscles also assist inspiration. It would be wrong to give every internal intercostal fibre an exclusively expiratory function.

Elevation of upper ribs and the sternum increases the anteroposterior dimension, the pump-handle component. Outward movement of the lateral portions of lower ribs increases transverse diameter, the bucket-handle component. These are components of oblique rib rotation rather than two mechanically isolated systems. Abdominal or diaphragmatic breathing makes descent of the respiratory diaphragm particularly conspicuous; costal breathing makes chest-wall expansion more conspicuous. Both contribute in ordinary respiration, with their proportions changing with posture, mechanical loading and individual anatomy.

As the thorax expands, pleural pressure becomes more negative and transpulmonary pressure increases. The lungs expand and alveolar pressure briefly falls below atmospheric pressure, admitting air. Relaxation of inspiratory muscles lets elastic recoil reverse the flow in quiet expiration. The abdominal wall and respiratory diaphragm return towards their resting arrangement without requiring active abdominal compression for each quiet breath.^[ref]

With deeper inspiration, the scalene muscles help stabilize or elevate the upper ribs, and the sternocleidomastoid muscle can elevate the sternum when the head is fixed. With the upper limbs or shoulder girdle fixed, muscles such as pectoralis minor and serratus anterior can assist expansion in greater effort. Recruitment is graded; an anatomical list of possible accessory muscles does not mean all are active in every ordinary breath.

Active expiration uses the abdominal muscles, including rectus, oblique and transversus components, to compress the abdominal contents and drive the respiratory diaphragm upwards. The interosseous portions of the internal intercostal muscles help depress the ribs. This is important in vigorous exercise and expulsive actions. At rest, elasticity supplies much of the expiratory work; during high demand, active muscle contraction adds to it.

The observable pattern also changes with the physiological task. Exercise increases breath depth and usually frequency. A sigh is an enlarged inspiration followed by expiration; yawning includes a prolonged deep inspiratory movement. Coughing coordinates an inspiration, closure of the glottis, pressure-generating expiratory effort and sudden glottic opening. Speech imposes a controlled prolonged expiration with laryngeal regulation. Swallowing briefly interrupts ordinary airflow. Thus respiratory observation should include phase, depth, frequency and coordination, not merely count which ribs move. Fixed claims that men must breathe abdominally and women must breathe costally overstate a variable tendency. Pregnancy or a change of posture can alter mechanical contributions without replacing the basic pressure mechanism.

Chyle is intestinal lymph enriched with absorbed dietary lipids; its usual route ends in the venous circulation at the left jugulosubclavian junction. The milky appearance after a fat-containing meal is chiefly due to suspended lipid-rich particles, now called chylomicrons.

Formation begins with lipid digestion in the small intestine. Pancreatic lipase helps hydrolyse dietary triglycerides; bile salts help the digestion products reach the intestinal absorptive surface in mixed micelles. Enterocytes take up the products, reconstruct much of the long-chain triglyceride, and package it with other lipids and apolipoproteins into chylomicrons. These leave the basolateral side of the enterocytes and enter the central lacteals of the intestinal villi. Thus the particles carried in chyle are not simply intact globules of a meal passing through an open hole from the intestinal lumen.^[ref]

From the lacteals, chyle enters lymphatic channels in the intestinal wall and then collecting vessels in the mesentery. These vessels pass through mesenteric lymph nodes, with afferent vessels entering and efferent vessels leaving the nodes. Successive collectors join the intestinal lymphatic trunk or trunks. They usually communicate with the cisterna chyli, an expanded origin of the thoracic duct in the upper lumbar region, near the bodies of L1–L2 and to the right of the aorta. The cisterna chyli also receives lymph from lumbar trunks; the stream beyond it therefore contains material from more than the intestine. A conspicuous sac is not present in every individual.

The thoracic duct passes through the aortic hiatus of the respiratory diaphragm and ascends in the posterior mediastinum, typically between the aorta and azygos vein, behind the oesophagus. It crosses towards the left in the middle or upper thorax, continues to the root of the neck and arches to its terminal venous opening. The usual endpoint is the left venous angle, where the left internal jugular vein joins the left subclavian vein; terminal branches and the precise opening show anatomical variation. It is not normally the right venous angle or a direct entrance into the portal vein.^[ref]

The flow is promoted by intrinsic contraction of collecting lymphatic vessels, movements of surrounding muscles and respiratory changes of pressure. Lymphatic valves limit backward movement. Lymph nodes are interposed biological structures, not pumps that propel every particle independently.^[ref]

This route applies particularly to the long-chain dietary lipids carried by chylomicrons. Most absorbed glucose and amino acids, and much shorter-chain fatty acid material, follow portal vascular routes instead. Confusing these pathways would obscure the reason for asking separately about chyle. In a classical anatomical demonstration, the mesenteric channels of an appropriately fed animal appear white, and an injected or dissected preparation can trace their continuity towards the thoracic duct. The timing after a fatty meal matters: an empty-looking channel in a fasted preparation does not prove absence of a lymphatic vessel. The molecular account of chylomicrons is a modern explanation of the older gross observation.

Ordinary inspiration is active enlargement of the thorax; ordinary expiration is chiefly passive recoil after the inspiratory muscles relax. The movements increase and decrease the vertical, anteroposterior and transverse dimensions together. Their relative prominence varies with posture and the individual; there is no exclusive division into abdominal and costal kinds of breathing.

The principal inspiratory muscle is the respiratory diaphragm. Its peripheral fibres shorten towards the central tendon, lowering and flattening its domes. This increases the vertical dimension of the thorax and displaces the abdominal contents downwards and forwards, making the relaxed abdominal wall project. Its costal attachments also act on the lower ribs: with abdominal support opposing unlimited descent, shortening can lift and expand the lower costal margin. Thus the respiratory diaphragm does more than pull down a piston. Its motor supply is through the phrenic nerves, principally from cervical segments C3–C5.

The external intercostal muscles elevate the ribs and help keep the intercostal spaces from being drawn inwards as pleural pressure falls. The interchondral, or parasternal, portions of the internal intercostal muscles also contribute to inspiration. It would therefore be inaccurate to label every part of the internal intercostal muscles expiratory. Coordinated activity braces the rib cage so that the shortening of one layer moves the ribs rather than simply deforming the spaces between them.

Rotation about the posterior articulations raises the anterior ends of the upper ribs and the sternum, increasing the anteroposterior diameter: the usual pump-handle description. The more laterally directed portions of the lower ribs rise and move outwards, increasing the transverse diameter: the bucket-handle description. These are descriptions of components of an oblique movement, not two independent sets of hinges. The scalene muscles can stabilize and elevate the uppermost ribs, with recruitment varying in quiet breathing. The sternocleidomastoid muscle, when the head is fixed, is an accessory elevator of the sternum in greater inspiratory effort; it is not required for an ordinary quiet breath.

The lungs follow the expanding chest wall because their surfaces remain coupled to it through the thin fluid-filled pleural cavity. More negative pleural pressure increases the distending transpulmonary pressure; enlargement of the alveoli briefly makes alveolar pressure lower than atmospheric pressure, so air enters. At the end of inspiration, airflow stops when these last two pressures equalize. During quiet expiration, relaxation of the respiratory diaphragm and inspiratory intercostal muscles allows the stretched lungs and chest-wall system to recoil towards their resting balance. The domes rise, the ribs and sternum descend, and alveolar pressure becomes slightly positive relative to atmospheric pressure while air leaves. Active abdominal muscles and the interosseous portions of the internal intercostal muscles become important in forced expiration, rather than supplying the main force in quiet expiration.^[ref]

A simple mechanical demonstration uses two elastic bags in an airtight transparent vessel, connected through its stopper to the air, with a movable rubber sheet forming the base. Pulling the sheet down expands the vessel and inflates the bags; letting it return allows deflation. A side-tube manometer can show the associated pressure change. This illustrates expansion by lowering the floor, but its rigid vessel cannot demonstrate the movements of the ribs, the action of the intercostal muscles, or the real geometry of the pleural cavity. Observing lower-chest widening and abdominal displacement during quiet breathing supplies those missing observations without confusing the model with the anatomy.

The two pneumogastric nerves are the right and left vagus nerves, the tenth cranial nerves: mixed sensory, branchial motor and parasympathetic pathways from the medulla oblongata to the neck, thorax and abdomen. A useful description separates their apparent origin at the surface of the brain from the origins and terminations of their component fibres.

Rootlets emerge in the groove behind the inferior olive, between the glossopharyngeal nerve above and the accessory rootlets below. Branchial motor fibres arise chiefly in the nucleus ambiguus; preganglionic parasympathetic fibres arise in the dorsal motor nucleus of the vagus and, particularly for cardiac control, the nucleus ambiguus. Visceral sensory fibres have their cell bodies chiefly in the inferior vagal ganglion and terminate centrally in the nucleus of the solitary tract; somatic sensory fibres associated with the superior vagal ganglion project to the spinal trigeminal nucleus. Consequently a sensory fibre is not accurately described as originating in a motor nucleus simply because its peripheral nerve emerges nearby.^[ref]

Each vagus nerve leaves the skull through the jugular foramen, with the glossopharyngeal nerve and accessory nerve. It bears the small superior vagal ganglion at the foramen and the larger inferior vagal ganglion below it. Connections include communications with adjacent cranial nerves and the cervical sympathetic nervous system. Traditional descriptions assign fibres joining the vagus nerve from the cranial part of the accessory nerve to the latter peripheral nerve; their modern classification is more nuanced and does not change the peripheral branches being described. In the neck the vagus nerve descends in the carotid sheath, behind and between the internal jugular vein and the internal, then common, carotid artery.

Its meningeal and auricular branches supply small sensory territories of the cranial covering and external ear. Pharyngeal branches contribute to the pharyngeal plexus, supplying much of the soft palate and pharynx; the stylopharyngeus muscle belongs to the glossopharyngeal nerve and the tensor veli palatini muscle to the mandibular division of the trigeminal nerve. The internal branch of the superior laryngeal nerve supplies sensation above the vocal folds; its external branch supplies the cricothyroid muscle. The recurrent laryngeal nerve supplies the other intrinsic laryngeal muscles and sensation below the vocal folds. The right recurrent laryngeal nerve loops beneath the right subclavian artery; the left loops under the aortic arch beside the ligamentum arteriosum. Both return towards the larynx near the groove between trachea and oesophagus.^[ref]

In the thorax, both vagus nerves pass behind the roots of the lungs, contrasting with the anterior course of the phrenic nerves. Cardiac branches join the cardiac plexus; pulmonary branches join the pulmonary plexus. The trunks break up into the oesophageal plexus, then collect principally as an anterior trunk derived from the left vagus nerve and a posterior trunk derived from the right. Through the oesophageal hiatus they reach the stomach; hepatic and coeliac branches distribute to abdominal viscera. Their parasympathetic territory includes foregut and midgut derivatives, extending into the proximal colon, rather than every abdominal and pelvic organ.^[ref] This extensive distribution explains why the level of experimental section is indispensable information.

The main consequences of section can be organized by the interrupted functions:

A discriminating historical experiment would record respiratory movements and heart rate before and after section, distinguish unilateral from bilateral section, and compare an intact airway with an airway bypass. A tracheal cannula can separate obstruction at the glottis from changes in the respiratory rhythm, while the central-versus-peripheral stimulation comparison separates incoming reflex traffic from outgoing motor effects. Later separate-lung recordings confirmed that a change following unilateral section could affect both lungs, supporting a central reflex interpretation rather than simple local paralysis.^[ref] The gastric distinction is also borne out by measurements of persistent acidity after surgical vagotomy, despite major reductions in stimulated gastric acid secretion.^[ref]

Respiratory mechanics relates the movements of the thorax and respiratory diaphragm to pressure changes and airflow.

Relations between the surface of the thorax and its walls, lungs, pleura and mediastinal contents. Respiratory phase, posture and individual anatomy alter projected boundaries.