An optical section through the visual apparatus in a specified direction. The two principal meridians of regular astigmatism have different powers and are perpendicular.

A contact lens is an optical device placed on the front surface of the eye. Its shape and refractive properties alter image formation, for example to correct refractive error. Unlike an implanted lens, it remains external to the internal ocular chambers.

Miosis is constriction of the pupil. In the human eye, contraction of the sphincter pupillae muscle reduces the opening in the iris. Pupil size is distinct from lens focus, even though pupil constriction and accommodation can occur together.

A nictitating membrane is a movable third eyelid present in various vertebrates. It can protect or moisten the exposed eye while allowing some visual access. Its presence and mobility vary among lineages, so it is not a universal feature of every eye.

The ciliary body is the anterior continuation of the vascular middle coat of the eye. It includes the ciliary muscle and processes involved in aqueous-fluid production. Connections to the lens through zonular fibers link its muscular action to accommodation.

The choroid is the vascular and pigmented layer between the retina and sclera. It supports the outer retina and helps limit stray light. Its position distinguishes it from the retinal vessels supplying other parts of the sensory tissue.

The posterior chamber is the narrow aqueous-filled region behind the iris and in front of the crystalline lens and related structures. Aqueous fluid passes from it through the pupil. It is not the large vitreous-filled space behind the lens.

The vitreous body is the transparent gel occupying most of the space behind the crystalline lens and in front of the retina. It contributes to the eye’s internal optical and structural environment. It is distinct from the more freely circulating aqueous fluid in the anterior eye.

Place each operator before its operands. With known operator arities and unambiguous tokens, the abstract syntax tree is recovered recursively without precedence rules or parentheses. A binary application is encoded by its operator, the first subtree's encoding and the second subtree's encoding: this is a preorder traversal of the expression tree.

Place each operator after its operands. A binary application encodes its first subtree, then its second subtree, then the operator, a postorder traversal. A stack machine evaluates the tokens in one pass, popping the right operand before the left one. Reversing a Polish notation token list also reverses operand order: prefix subtraction - a b would become b a - rather than the required a b -.

To discover a positive unknown integer $m$ by comparisons, test the thresholds $2^k,2^{2k},\dots$ for fixed $k\ge 1$. If the first threshold not smaller than $m$ has index $j$, fewer than $2^{kj}$ candidates remain and binary search needs at most $kj+O(1)$ comparisons. Since $j\le {\log }_2(m)/k+1$, the total is at most $(1+1/k){\log }_{2}m+O_k(1)$. Choosing $k\ge 1/ϵ$ makes the leading constant at most $1+ϵ$; the additive constant may depend on $ϵ$.

A chitinous exoskeleton is an external covering whose structural material includes chitin, commonly embedded in a protein matrix. In arthropods, regional differences in thickness and stiffness allow strong plates to coexist with flexible joints. Chitin is one component rather than a claim that the covering is chemically pure.

A diagram of an entity-relationship model, including relationship cardinalities and, when needed, subtype and identification constraints.

An entity type whose instances inherit the identifier, database attributes and relationships of a more general entity type. A total disjoint specialization requires every parent instance to belong to exactly one listed subtype.

Communication in which nervous system activity influences hormone release or neurosecretory products influence distant targets.

The endocrine signalling system linking hypothalamic and pituitary activity with ovarian function. Suppression at one level differs from outflow obstruction.

Hyperprolactinaemia is an elevated circulating prolactin concentration. Physiological state, altered regulation and other causes can contribute. The concentration alone does not identify a unique structural lesion of the pituitary.

Prolactin is a peptide hormone produced chiefly by the anterior pituitary, with an important role in milk production. Its release and physiological actions are regulated in relation to reproductive state and neural input. Milk synthesis and milk ejection are different processes and should not be attributed to one hormone interchangeably.

Thyroid disease includes disorders of thyroid structure, secretion or both. Enlargement, inflammation and altered hormone output are different dimensions. A goitre therefore does not by itself establish hypothyroidism or excess secretion.

Thyroid-stimulating hormone is a pituitary hormone regulating thyroid activity. Its concentration is interpreted together with thyroid hormone measurements and the control pathway involved. Reduced thyroid output and impaired upstream stimulation can produce different combinations of measured values.^[ref]