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The breast contains mammary glandular tissue, ducts and supporting connective tissue and fat. During lactation, secretory structures produce milk that reaches openings at the nipple. The amount of supporting tissue and the state of the glandular tissue vary with age and reproductive physiology.

Breast milk is milk produced by human mammary glands during lactation. Its nutrients and other constituents support the infant while its composition changes with physiological context. It is a biological secretion rather than one invariant chemical mixture.

Cow's milk is the milk secretion of cattle. Its fat, protein and dissolved constituents form a dispersed mixture whose composition varies among animals and across lactation. A typical composition is therefore a description of a sample population rather than a fixed chemical formula.^[ref]

For a crown-based usage, a member of the clade comprising the last common ancestor of living monotremes and therians and all its descendants. hair and lactation characterize living members, but recognition of many early fossils depends on jaw and dental characters.

Absent menstruation is a sign with several causes; treatment must distinguish physiological absence, failure of menstrual cycling and obstruction of outflow. The word “suppression” does not establish that retained blood is poisoning the body or that producing a bleed will cure the underlying problem.

First distinguish primary amenorrhoea, where menstruation has never begun, from secondary amenorrhoea, where established cycles have stopped. Ask about the previous pattern, possibility of pregnancy, breastfeeding, age, illness, nutrition, exercise, medicines and associated symptoms. Pregnancy, lactation and menopause can explain physiological absence. Pregnancy must be considered before any proposed menstrual stimulant, with assessment for complications when pain or abnormal bleeding accompanies the missed period.^[ref]

A second group has impaired hormonal signalling or ovarian function. Severe illness, inadequate energy intake, major exertion and stress can suppress the hypothalamic–pituitary–ovarian axis. Other possibilities include thyroid disease, hyperprolactinaemia, primary ovarian insufficiency and chronic anovulation. These modern categories separate processes that an 1866 clinician might group under debility, constitutional disease or disordered ovarian activity. Anaemia and wasting deserve investigation and treatment when present, but are not universal explanations for every missed period.

A third group has an anatomical explanation. Cyclical pain with absent visible menstruation can indicate retained menstrual material behind an imperforate hymen or other outflow obstruction. Congenital absence or abnormal development of reproductive organs is different again. Acquired scarring of the uterine cavity may interrupt bleeding after instrumentation or disease. The response to obstruction is diagnosis-specific correction by an appropriate specialist; neither a “tonic” nor a drug intended to provoke uterine contraction repairs an absent or blocked passage.^[ref]

In period terms, treatment of a genuine associated illness, improved nourishment and protection from exhausting conditions could be sensible. Warm baths and drugs called emmenagogues were also employed, but their use should not be mistaken for proof of efficacy, or attempted before excluding pregnancy. Menstrual absence should not automatically be interpreted as the cause of mental or systemic illness.

Modern correction: investigate the relevant physiological pathway and treat its cause. Selected investigations include a pregnancy test, prolactin, thyroid-stimulating hormone, ovarian and pituitary hormone measurements and pelvic ultrasonography. For functional hypothalamic suppression, restore energy availability and address contributing illness or disordered eating; simply imposing withdrawal bleeds can conceal persisting energy deficiency. Hormone replacement may be appropriate for ovarian insufficiency, while thyroid, prolactin-related and anatomical disorders need their own pathways. Management of chronic anovulation includes reproductive goals and protection of the endometrium, rather than just the appearance of regular bleeding.^[ref][ref]

cetaceans illustrate a sequence of inherited mammalian features, terrestrial specializations and subsequent aquatic transformations, followed by divergence in feeding and sensory anatomy. This is a branching phylogenetic tree, not a procession of named fossils that can all be identified as direct ancestors. Here Cetacea includes living whales, dolphins and porpoises; mammals means the crown group containing living monotremes and therians, whereas mammaliaforms also includes close extinct relatives outside that crown group. Dates below are approximate millions of years ago, abbreviated Ma.

The starting condition is a terrestrial, non-mammaliaform cynodont, itself a therapsid within Synapsida. Along the mammalian stem group, enlargement of the dentary and reduction of the other lower-jaw bones accompanied more differentiated teeth, increasingly precise dental occlusion and elaboration of a secondary palate. These changes belong to a feeding and respiratory complex: a larger tooth-bearing bone carries the mastication load, differentiated teeth process food, and a bony partition separates nasal and oral passages. They were acquired in a mosaic; neither a full modern mammalian jaw nor the aquatic body plan appeared at the initial divergence from other cynodonts.

Early mammaliaforms show how jaw support and hearing became reorganized. Morganucodon, around the Triassic–Jurassic transition and Early Jurassic, retained the old articular–quadrate jaw joint alongside a dentary–squamosal joint. In the mammalian condition the dentary–squamosal articulation supports the jaw, while the articular and quadrate correspond to the malleus and incus; the angular bone contributes to the ectotympanic. This is biological homology supported by comparative anatomy and biological development, not merely similarity in function. Hadrocodium, about 195 Ma, records further changes in jaw attachment and the braincase consistent with increased separation of mastication and hearing. Its tiny skull does not preserve an entire living-style ear apparatus; the reconstruction depends partly on bony attachment evidence. ^[ref]

The inherited mammalian package also includes diphyodonty, hair and lactation, although the latter two leave much poorer evidence in these particular fossils than do teeth and jaws. Precise adult dental occlusion is easier to maintain when repeated replacement ceases. The lineage subsequently passed through Theria, Eutheria and the ancestors of living placental mammals. These nested relationships matter: a Cretaceous eutherian is not automatically a member of the living placental crown group, and neither the first known fossil nor the date of a tooth innovation fixes the origin of an entire group.

The immediate terrestrial context of whale evolution is within Artiodactyla, broadly circumscribed to include Cetacea. Living hippopotamuses are the closest living relatives of cetaceans, not their ancestors. Some early whale teeth suggested relationships with extinct mesonychians, but dental resemblance alone was misleading. Early whale skeletons preserve the characteristic artiodactyl double-pulley astragalus, providing a powerful independent anatomical test of the relationship inferred from molecular evidence. The relevant comparison is the detailed construction of the ankle, not simply whether an animal has hooves or eats meat. ^{[ref] [ref]}

Early Eocene pakicetids, roughly 50 Ma, combine a largely terrestrial, weight-bearing skeleton with diagnostic whale ear-region anatomy, notably the thickened medial wall of the auditory bulla called the involucrum. Thus identification as a whale does not imply an obligately marine animal. Ambulocetus, roughly 48 Ma, has a different combination: robust limbs still useful around shore, large feet and an axial skeleton suited to substantial movement in water. Reconstruction of amphibious locomotion uses limb proportions, joint surfaces and vertebral anatomy together; it does not require assuming that its swimming exactly resembled an otter's. These are successive grades represented by branching relatives, not proof that a particular pakicetid gave birth to the known Ambulocetus specimen. ^[ref]

Middle Eocene protocetids further document reduction of terrestrial support and increasing commitment to swimming. Changes in the connection between pelvis and vertebral column, limb proportions and the position of external nostrils occurred in different combinations. The ankle of Rodhocetus is especially informative about ancestry even while other parts of its skeleton document aquatic specialization. It would be unjustified to draw a preserved tail fluke onto Rodhocetus: a reconstructed swimming mode must distinguish actual preserved anatomy from inferred soft parts. ^[ref]

Aquatic hearing changed alongside locomotion. Enlargement of the mandibular sound-reception region and changes in the middle ear support a transition towards receiving underwater sound through the lower jaw, with increasing acoustic separation of the ears from the skull. Early whale lineages display different intermediate combinations, so the presence of an involucrum alone does not imply a modern dolphin hearing system. Reconstructing soft sound-conducting tissues requires comparison with living anatomy; the fossil preserves their bony context rather than the entire pathway. ^[ref]

By the later Eocene, basilosaurids were fully aquatic. The forelimbs functioned as flippers, the vertebral column and tail supplied propulsion, and the hind limbs were far too small to support terrestrial walking. Nevertheless, the reduced limbs of Basilosaurus retain recognizable limb and foot elements. Loss of a locomotor role is not equivalent to absence of all function; reproductive or muscular roles are compatible with reduction, although a specific behaviour cannot be read directly from a bone. The transition therefore involved changing proportions and connections as well as losses. ^[ref]

Finally, the living branches acquired different specializations. Odontoceti retain teeth and evolved specialized sound production and echolocation; Mysticeti evolved baleen and lack functional adult teeth. fossil toothed mysticetes and degraded enamel-related genes show that modern toothlessness is derived. Palatal vascular features have been used to infer an intermediate combination of teeth and baleen, but such foramina do not preserve the baleen itself, and the exact sequence of feeding behaviours is less secure than the transition from toothed ancestry. ^[ref] The resulting history explains why a whale can have a mammalian jaw and reproductive system, an artiodactyl ancestry, a radically modified swimming skeleton, and a feeding apparatus that evolved after the aquatic transition. The strongest reconstruction combines nested anatomical characters with independently dated fossils, while treating soft-tissue and behavioural reconstructions as hypotheses.