Physical changes during puberty such as thymic involution also affect immunological response. There are also hypotheses that suggest that thymic involution is directly adaptive. It then begins to decrease in size and activity in a process called thymic involution. During involution, the thymus decreases in size and activity. Failure of head involution disrupts or deletes the salivary glands and pharynx. The operation of complement in Boolean algebras is an involution. Algebraically, classical negation is called an involution of period two. There is a canonical involutive automorphism on any superalgebra called the grade involution. If the operator is orthogonal (an orthogonal involution), it is orthonormally diagonalizable. This generalizes the concept of an involution. C*-algebras can be easily characterized abstractly by a condition relating the norm, involution and multiplication. This transposition is an involution on the set of matrices. See also 3D isometries that leave the origin fixed, space group, involution. While this ordering is reversed with the complementation involution, it is preserved under conversion. A hyperbolic involution has two fixed points.