What is a kymograph used for?

A kymograph is an instrument specifically designed to record variations or undulations of physiological phenomena, such as changes in pressure, sound, or movement over time. This device plots such variations on a moving graph, allowing for detailed analysis of dynamic processes.

The use of kymographs has historically included various applications, including but not limited to studies in respiration, blood pressure changes, and muscle contractions. By obtaining a visual representation of these variations, researchers can better understand the underlying biological functions and rhythms. This makes option C the most appropriate choice, as it accurately describes the primary function of a kymograph.

The other options relate to medical procedures and anatomical interventions, which do not align with the purpose of a kymograph. For example, options that discuss examination or excision of vertebrae pertain to surgical or diagnostic techniques rather than the recording of physiological data, emphasizing the specificity of the kymograph's function in the context of recording undulatory or variable processes.