Within ray theory, travel times are sensitive to the medium only along the ray path. If the arrival time is defined to be the point where the observed seismogram crosses a nonzero threshold, time domain computations using Born scattering in a homogeneous unbounded medium show that the sensitivity is large within a considerable volume, and most intense near the source and receiver. Details of this volume or ''tube'' depend on the threshold value, the source time function and the instrument response. Generally, as the threshold value gets larger, the volume of sensitivity and magnitude of sensitivity get larger. If the source time function is oscillatory, the region of the highest sensitivity can be quite complex. Due to the high sensitivity near the endpoints, ray-theoretical tomography is likely to alias near-source and near-receiver structure to produce spurious structure along the ray path. ¿ American Geophysical Union 1993