Past exam of the medicine course of the University of Oxford / 1866 / First examination / Anatomy and physiology (No. IV.)

Original volume: Examination papers for the Degree of Bachelor of Medicine by the University of Oxford (1864–1867). Open the original paper in the scanned volume, PDF pages 59–60.

Original questions are transcribed from the public-domain edition. Credit: Wellcome Collection; original supplied by Royal College of Physicians, London. Source and public-domain rights statement.

Historical terms are interpreted in their nineteenth-century setting. Modern explanations and measurements are identified where they extend beyond the knowledge available in December 1866.

Give an account of the different ways in which the Nervous system may influence the functions of organic or vegetative life.

The nervous system regulates visceral movement, glandular secretion, blood flow and coordinated bodily responses, while also influencing nutrition indirectly through those functions. Organic or vegetative life here means activities maintaining the body, such as blood circulation, digestion, pulmonary ventilation, secretion and excretion. It does not designate an independent form of living substance.

One influence is directly motor. Autonomic nerves alter the rate and force of the heart, contract or relax smooth muscle in the intestine, urinary bladder, airways and blood vessels, and coordinate movement with the opening and closure of sphincters. Their effect need not be excitatory: an active vagus nerve can slow the heart, while other pathways accelerate it. In the modern account, sympathetic and parasympathetic effects depend on the target and its receptors; they are not respectively universal accelerator and brake systems. Some organs receive both divisions, while others do not have a simple balanced double supply.^[ref]

A second influence is secretomotor. Activity in appropriate nerves can increase salivary secretion, alter gastric acid secretion and promote sweating. This is distinguishable from moving a duct or changing its calibre. A convincing experiment collects a secretion from a cannulated duct while a nerve is stimulated, rather than inferring secretion from a change of colour or organ size. For example, stimulation of the chorda tympani can evoke fluid from the submandibular gland. The response depends on functioning secretory biological cells; the nerve communicates a command rather than supplying the material constituents of saliva.

A third influence is vasomotor. Changing the calibre of arterioles changes local vascular resistance and perfusion. The classic comparison of an ear before and after interruption of its cervical sympathetic supply illustrates loss of vasoconstrictor tone: the vessels dilate and the ear becomes warmer because more warm blood reaches it. Stimulation of the peripheral cut pathway can produce the converse vascular effect. Warmth therefore need not mean that the local nerves themselves generate more heat. Vascular control also changes the delivery of oxygen and nutrients to a gland or tissue, so it can modify function indirectly.

These outputs are organized by sensory feedback and reflexes. Distension of the intestine can evoke coordinated movement; changes in blood pressure or blood-gas composition influence heart rate and breathing. Local circuits in the enteric nervous system permit some digestive activity without an intact connection to the brain, while longer reflexes involve the spinal cord or brainstem. Central states also matter: the sight or expectation of food can initiate preparatory digestive responses, and fear can alter heart rate, sweating and visceral movement.^[ref]

Somatic motor activity also supports vegetative functions. Breathing uses the respiratory diaphragm and other skeletal muscles; swallowing and continence use coordinated skeletal muscle as well as smooth muscle. Thus organic function cannot be identified solely with the anatomical distribution of the sympathetic nervous system.

The historical idea of a direct trophic influence requires discrimination. A denervation region may change through loss of movement, altered perfusion, loss of protective sensation or changed secretion. Those observations alone do not establish a special nutritive fluid flowing down the nerve. Modern biology recognizes molecular signalling between nerves and their targets, as well as neuroendocrine signalling pathways: the hypothalamus can influence hormone release, and sympathetic input can evoke adrenal medulla secretion. These are mechanisms added to, rather than evidence for an undefined historical nutritive force. A useful experimental design measures the motor, vascular and secretory consequences separately before assigning a residual effect to direct trophic action.

What do you know of the development, relations, and functions of the Eustachian Tube ?

The Eustachian tube, or auditory tube, connects the middle ear with the nasopharynx. Its principal functions are ventilation and pressure equalization, clearance of middle-ear secretions, and protection against inappropriate entry of nasopharyngeal material.

Developmentally, its lumen and epithelial lining arise chiefly from the proximal part of the tubotympanic recess, an extension of the first pharyngeal pouch. The expanded distal part contributes to the tympanic cavity. The neighbouring first pharyngeal cleft is associated with the external ear passage and must not be substituted for the pouch as the origin of the auditory tube. Supporting cartilage and connective tissue develop from surrounding mesenchyme. At birth the tube is shorter and more nearly horizontal; craniofacial growth lengthens it and changes its orientation. The developmental account distinguishes formation of the channel from later growth of its surrounding structures.^[ref]

In an adult the auditory tube is roughly 3.5–4 cm long. From its tympanic opening in the anterior wall of the middle ear, it runs forwards, downwards and medially towards its pharyngeal opening in the lateral wall of the nasopharynx, behind the nasal cavity. Its lateral third is bony, lying in the temporal bone; the medial two thirds are chiefly cartilaginous with a membranous part. The narrow junction is the tubal isthmus. The cartilaginous portion is normally collapsed, so an open passage for sound or reflux does not continuously connect pharynx and middle ear.

The bony part is closely related medially to the canal of the internal carotid artery; the canal for the tensor tympani muscle lies above it. The medial cartilaginous part rests towards the skull base. Its pharyngeal end makes the tubal prominence, or torus tubarius, near the soft palate. The tensor veli palatini muscle acts on the cartilaginous and membranous tube and is the main active dilator. The levator veli palatini muscle passes beneath it and participates in the palatal and tubal functional arrangement. Swallowing and yawning intermittently open the tube.^[ref]

Opening permits gas transfer to compensate for a difference between middle-ear and ambient pressure. If the tube remains obstructed, absorption of gas from the cavity can make middle-ear pressure fall; movement of the tympanic membrane and sound transmission then change. Clearance is helped by the mucociliary lining, which carries material towards the nasopharynx. Closure between openings limits reflux and transmission of nasopharyngeal pressure disturbances. A tube that remains persistently open therefore also functions abnormally; more openness is not invariably better.

The physiological point can be demonstrated by observing the tympanic membrane while swallowing during a change of external pressure, or by recording the membrane's mechanical response over a controlled range of ear-canal pressures using tympanometry. These observations examine pressure communication and membrane mobility, not merely whether air can be forcibly driven through a passage. The relatively immature geometry and opening mechanism in young children helps explain their vulnerability to middle-ear fluid retention, but geometry alone is not an exhaustive account of infection risk. The anatomical structures were recognizable in 1866; the modern measurement method should not be mistaken for apparatus supplied by the examination.

Write an account of the origin, course, and functions of the chorda tympani, the buccal, the musculospiral, and the obturator nerves.

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]

What are the physiological limits within which the size of the liver may vary, and what are the conditions which produce such variations ?

The liver has a substantial normal range of size, determined by age, body dimensions, nutritional state and its variable contents; there is no universal absolute minimum and maximum valid for every healthy person. One must also distinguish true organ volume from the position of its lower border, which moves with the respiratory diaphragm during breathing.

An ordinary adult liver is on the order of 1.5 kg and roughly a few percent of body mass, but neither figure is a diagnostic boundary. As an explicitly modern example, an autopsy study of adult men reported a mean hepatic mass of 1561 g and observed values from 838 to 2584 g in its selected population. Those are sample observations, not a declaration that every liver inside those limits is healthy or that every one outside is diseased. A newer large-sample study similarly found an association of liver mass with body mass; much of an apparent sex difference disappears when body size is taken into account.^[ref][ref]

Age changes the proportion as well as the absolute size. The fetal and infant liver is relatively large; the adult organ is smaller in proportion to the whole body. Its fetal role in haematopoiesis and subsequent changes of growth and function make an adult mass standard inappropriate for an infant. Constitutional body size and normal anatomical shape also vary: a long lower projection is not necessarily the same thing as an enlarged total organ.

Reversible differences within one person arise because the liver is a store and a vascular organ:

A meal therefore changes hepatic activity and splanchnic blood flow without requiring a large immediate increase in total liver volume. In a modern paired study of 20 healthy volunteers, single-photon emission computed tomography before and after a standardized meal showed a nonsignificant 1.5% decrease in estimated liver volume. The authors judged the change to be within methodological error. This is a useful check against assuming that greater digestive activity proves gross swelling.^[ref]

The experimental approach is to compare like with like: specify age and body mass, use a fixed interval after food, standardize hydration, posture and respiratory phase, and distinguish a volume measurement from a palpable edge or a single linear dimension. Magnetic resonance imaging can measure volume while magnetic resonance spectroscopy measures glycogen or lipid signals; the two observations answer different questions. The size of an excised organ additionally depends on how much blood remains in it. The physiological point appropriate to the 1866 question is reversible variation with growth, alimentation and vascular filling. Modern imaging results refine that point; they are not numerical standards that the historical exam could have expected.

Trace the course taken by the chyle from its formation up to the spot where it is poured into the blood-vascular system.

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.

Enumerate the different muscles, membranes, and vessels which you would meet with in dissecting from the external integument down to the level of Cowper's glands.

Cowper's glands are the paired bulbourethral glands, deep to the perineal membrane on either side of the membranous urethra. The relevant dissection is of the male anterior perineum, or urogenital triangle. A broad exposure reveals structures at different lateral positions; they are not all piled one above another along a single central line.

From superficial to deep, the principal layers and contents are:

The superficial muscles, erectile bodies and pudendal branches identify the shallow compartment; the perineal membrane separates it from the gland-bearing deeper region.^[ref] The deep space contains the sphincter complex and male bulbourethral glands, with their ducts traversing the membrane.^[ref]

The main internal pudendal vessels themselves run laterally in the pudendal canal, in relation to the fascia over obturator internus, before their terminal distribution. A median approach to a gland does not necessarily expose their whole trunks. The deep dorsal vein of the penis has a distinct anterior route beneath the pubic arch and should not be confused with the small accompanying perineal veins. These positional qualifications are necessary for an accurate enumeration of the vessels one may meet.

Nineteenth-century descriptions often arranged the deep structures as a urogenital diaphragm enclosed between two neat fascial layers, the superior and inferior layers of the triangular ligament. Modern dissection finds a more complex sphincteric and connective-tissue arrangement. The historically named layers explain the older vocabulary, but the perineal membrane and surrounding muscles should not be made into a fictitious uniform sandwich. Nor does reaching the bulbourethral glands require dissecting through the whole pelvic diaphragm above them.

Give some account of the organisms known as “Infusoria,” “Vibriones,” and “Bacteria.” What relations have they been supposed by different authorities to hold to the production and to the products of Fermentation and Putrefaction ?

The names are historical microscopic groupings, and the central dispute is whether the organisms cause the chemical changes, arise as their products, or merely grow in the altered liquid. Those are different causal propositions. Seeing small moving bodies in a fermenting infusion does not by itself choose among them.

Infusoria were organisms encountered in infusions of organic material. Many recognizable examples are now classified as ciliates or other unicellular eukaryotes, with a cell nucleus, organized feeding structures and locomotory cilia or flagella. Historical usage could extend much more widely than a modern taxonomic group. Some organisms found in an infusion feed on the bacteria already growing there; their later appearance can thus reflect ecological succession rather than production of the first chemical change.

Vibriones commonly denoted minute actively moving, often elongated, curved or sinuous forms. It does not follow that every historical vibrion belongs to the modern genus Vibrio. Bacteria, in some nineteenth-century classifications, meant a narrower set of minute rod-like bodies than the present bacterial domain. In modern terms bacteria are prokaryotes, lacking a membrane-bound cell nucleus; they generally multiply by binary fission. Shape and apparent movement alone do not establish their species. Brownian motion of nonliving particles also must not be mistaken for independent locomotion.

Three positions illustrate the debate available in 1866:

The decisive experimental question is not whether fresh air is present, but what accompanies it. In a curved-neck flask, a boiled nutrient liquid remains in communication with air while much airborne dust settles in the neck. An otherwise comparable preparation can be exposed by breaking the neck or bringing trapped material into contact with the liquid. The comparison tests whether access of contaminating material, rather than air as such, permits growth and putrefaction. Heat treatment, the preparation's composition, possible surviving resistant forms and accidental contamination all affect interpretation. A negative culture under one condition is not a universal proof about the origin of life.^[ref]

Modern fermentation is an enzyme-mediated metabolic process; putrefaction commonly involves microbial degradation of nitrogenous organic matter, with mixed and sometimes foul-smelling products. Organisms can be causal agents, and their products can subsequently feed other organisms. Thus the two directions of influence need not be mutually exclusive in a mixed infusion. The mere coexistence of bacteria, ciliates, gas and odour supplies much weaker evidence than controlled inoculation, time-resolved microscopy and chemical measurement.

A later qualification is essential. Eduard Buchner's cell-free fermentation experiments, first reported in 1897 and described in his 1907 lecture, showed that an extract of yeast could carry out alcoholic fermentation without intact living yeast cells. This supports chemical machinery made by organisms; it does not restore the earlier indiscriminate decomposition-by-contact hypothesis. These later findings must not be put into an 1866 candidate's mouth.^[ref]

Explain what is meant by the Botanical Terms “Stomata,” “Primordial Utricle,” “Cambium Layer;” and give a short history of the physiology of the sap.

Stomata are regulated pores of the plant epidermis; the primordial utricle is an older name for living peripheral cell contents; the vascular cambium is a tissue that produces secondary conducting tissues. These are respectively an opening apparatus, part of a cell's living contents, and a region of dividing cells.

A stoma is an aperture bounded by two guard cells, commonly with associated subsidiary cells. Changes of guard cell turgor pressure alter the aperture. Stomata permit entry of carbon dioxide for photosynthesis and loss of water vapour through transpiration, while also allowing other gas exchange. Most leaves have many on their lower surface, but distribution varies, so absence from the upper surface is not a definition. A pore is not itself a minute leaf blood vessel.

The primordial utricle was distinguished in nineteenth-century microscopy as the protoplasmic layer inside the cell wall, especially conspicuous around the large central vacuole of a mature plant cell. In modern vocabulary this includes peripheral cytoplasm bounded externally by the cell membrane; the vacuolar boundary is the tonoplast. The old term should not be reduced to a synonym of the rigid cell wall or assumed to designate an independently living bag. In plasmolysis, an appropriate external hypertonic solution draws water from the cell and the living contents withdraw from the wall, making the distinction visible.^[ref]

The vascular cambium is a lateral meristem, characteristically situated between the xylem and phloem of a woody stem. Its divisions produce secondary xylem towards the inside and secondary phloem towards the outside; radial derivatives also contribute to vascular rays. Activity connects the growth of the conducting system with thickening of the axis. It is not simply a film of nutritious liquid that spontaneously turns into wood. The cork cambium is a different lateral meristem, concerned with the protective outer covering.^[ref]

The course of sap can now be followed from soil to growing tissue. Root cells acquire mineral ions, and water enters along a water potential gradient. Water and dissolved minerals move into the xylem and upwards through the root and stem to the leaves. In a transpiring plant, evaporation from moist internal leaf surfaces and diffusion out through stomata maintain a pull on the continuous water columns. Cohesion between water molecules and the mechanical support of xylem walls allow tension to be transmitted. Root pressure can drive exudation or guttation under some conditions, but it is not a sufficient universal explanation for supply to the tops of tall trees.

In leaves, photosynthesis incorporates carbon dioxide into organic compounds. Soluble products, especially sucrose, enter the phloem and move from sources to sinks, such as growing roots, buds, fruits or storage organs. Thus elaborated sap is not invariably a single downward stream: the direction depends on where a source and a sink lie. The modern pressure-flow hypothesis explains bulk movement through differences of hydrostatic pressure associated with loading and unloading solutes. Neither the ascending nor the assimilate stream forms an exact analogue of an animal's closed blood circulation.^[ref]

A short experimental history makes these distinctions concrete. In Vegetable Staticks by Stephen Hales (1727), Stephen Hales measured water use and perspiration by plants and attached tubes and pressure gauges to cut vines. Exudation into a sealed vertical tube demonstrates a positive pushing pressure generated below the cut. The corresponding observation after the bleeding season differed, showing that root pushing and the supply to a leafy, transpiring shoot need not have the same explanation. Weighing a potted plant while limiting evaporation from the soil provides another balance: water lost from the system must be compared with its supply and changes of storage.^[ref] A cut shoot taking up water is informative, but its uptake alone does not establish all the forces acting in an intact rooted plant.

For the phloem, removal of a ring of bark can interrupt assimilate transport while leaving a substantial wood pathway intact; accumulation above the interruption distinguishes the two transport systems, although injury complicates any inference about a natural circulation. In an 1866 account, terms such as crude and elaborated sap could summarize these routes. The modern quantitative water potential and pressure-flow hypothesis accounts extend beyond that historical vocabulary.

What are the distinctive characters of the classes Vermes and Arachnida ? Specify parasitic members from each of these classes.

The historical Vermes is a broad assemblage of worm-like animals; Arachnida is an arthropod group with a fundamentally different appendage-bearing plan. Vermes has no single set of characters defining an equivalent modern natural clade, so it is necessary to state the scope of the comparison.

Animals called Vermes typically have elongated or flattened bodies and lack paired jointed walking legs of the arthropod type. Many show bilateral symmetry. Their internal organization is highly variable: some possess a complete alimentary canal, some a blind-ending one, and Cestoda lack an alimentary canal entirely. Body cavities, body coverings and segmentation also vary. Modern Annelida, Nematoda and flatworms are consequently distinct groups; the segmented body of an annelid and the chain of reproductive proglottids in a tapeworm should not be identified as the same organization. It would be inaccurate to characterize all Vermes as either segmented or unsegmented, or all as having no digestive tract.

Parasitic examples within that historical assemblage include Ascaris lumbricoides, Enterobius vermicularis, Trichuris trichiura and Trichinella spiralis among Nematoda; Taenia solium and Taenia saginata among Cestoda; and Fasciola hepatica or Schistosoma haematobium among Trematoda. These illustrate intestinal adults, tissue larvae and blood- or bile-duct inhabitants rather than one obligatory habitat. Leeches provide annelid examples, including blood-feeding temporary external parasites. A parasitic mode of life alone cannot unite these forms into a natural zoological class.^[ref][ref]

An adult typical arachnid has a chitinous exoskeleton, jointed appendages, and a body organized into a prosoma and opisthosoma, though their boundaries may be inconspicuous or fused in some groups. The prosoma bears chelicerae, pedipalps and four pairs of walking legs. There are no antennae and no wings. These features distinguish the usual arachnid plan from the three pairs of adult insect walking legs and the antenna-bearing insect head. Respiration may use book lungs, arthropod tracheae, body-surface exchange or combinations according to the group; book lungs are not a universal requirement.^[ref]

The strongest parasitic examples are mites and ticks. Sarcoptes scabiei, the itch mite, burrows in the superficial skin and has eight legs in its nymphal and adult stages; its larva has six. The Demodex mites occupy hair follicles and associated glandular sites, with elongated bodies that should not make them nematode worms. Ixodes and Argas include blood-feeding ticks. Their compact or fused body regions do not abolish their arachnid appendage plan. The six-legged larva of a tick or mite is a developmental exception to the adult eight-leg character, not evidence that it becomes an insect.^[ref][ref]

A final useful negative example is the louse: although a familiar ectoparasite, it is an insect, not an arachnid. Conversely, a free-living spider is an arachnid without being a parasite. The comparison therefore requires structural characters as well as a separate specification of which members are parasitic.

What are the main points of difference and what the main points of interdependence existing between the Animal and Vegetable Kingdoms ?

The main differences concern cellular construction and the usual modes of obtaining food, movement and development; the main interdependence concerns transfers of matter and energy through ecosystems. The nineteenth-century animal–vegetable division is useful as a comparison of familiar forms, but it is not a complete modern classification of life.

A typical plant cell has a cell wall, a large central vacuole and, in photosynthetic tissue, chloroplasts. An animal cell lacks a cellulose wall and chloroplasts and has a different extracellular and tissue organization. Both possess a cell membrane, cytoplasm, a cell nucleus and, in their usual aerobic cells, mitochondria. Roots and other non-green parts of a plant need not contain functional chloroplasts; an erythrocyte's loss of its cell nucleus is likewise not a definition of all animal cells.^[ref]

Most green plants acquire carbon through photosynthesis, taking carbon dioxide from their surroundings and mineral nutrients and water chiefly through roots. Familiar animals obtain preformed organic material by feeding and digesting it or absorbing material derived from a host. Animals generally have conspicuous coordinated movement, and many have nervous systems and muscles; higher plants regulate growth and movement without such organs. These are useful generalizations with exceptions. Some plants depend on other organisms for carbon; some animals are permanently attached; and neither lack of locomotion nor a green appearance alone settles every microscopic classification. Fungi and bacteria should not be forced into one of these two kingdoms in a modern account.

One major interdependence is nutritional. Photosynthesis converts light input into chemical energy stored in organic material. Plant material feeds herbivores and, through them, carnivores; it also enters detrital food webs. An animal that eats an animal can therefore remain indirectly dependent on primary production by plants, algae or other photosynthetic organisms. Chemical elements pass between organisms, but energy is progressively dissipated as heat; it is not recycled in the same closed manner as an element.^[ref]

A second relation is gas exchange. In its simplest overall form, oxygenic photosynthesis may be represented byAerobic utilization of the resulting glucose has the complementary overall balanceThese balanced equations show complementary material transfers without making the underlying pathways literal reversals of one another. Plants also perform cellular respiration, by day as well as by night. They can therefore release carbon dioxide while consuming oxygen; a sufficiently illuminated leaf may simultaneously show net uptake of carbon dioxide because photosynthesis exceeds respiration. Stating that plants only purify air and animals only spoil it would erase this essential distinction between gross processes and net exchange.

Other relations close material cycles. Animals return organic matter through excretion and death; microbial decomposition and mineral transformations return nutrients in forms available for plant uptake. Most atmospheric nitrogen is not directly available to ordinary plants merely because animals have breathed or excreted near them: nitrogen fixation, ammonification and often nitrification involve particular microorganisms. Plants and animals also interact through pollination, seed dispersal, grazing, parasitism and shelter. Such relations may benefit both partners, one partner at the other's expense, or different partners under different conditions; interdependence is not universal mutual benevolence.

A closed illuminated aquarium containing plants and animals gives an instructive experiment, provided gas exchange with air, illumination, nutrient supply and organismal amounts are specified. Dissolved oxygen and carbon dioxide should be followed over light and dark periods. Survival alone is an ambiguous readout: microorganisms, stored food and air exchange may support it temporarily. A more controlled comparison includes illuminated and dark plant vessels and animal-only vessels. The expected alternation of net gas exchange makes the physiological distinction visible while exposing why no simple rule guarantees indefinite balance in any arbitrarily stocked closed jar.