This paper deals with the activity of a group of cells in the cat's spinal cord and with some sensory phenomena in man. These cells in the cat's cord receive afferents from the skin and respond to all stimuli to which the cat's skin is sensitive. The discovery of a system of cells on which many types of peripheral fibers converge should be interpreted with great caution. The behavior of a cat shows that it is capable of differentiating between various types of skin stimuli, including various degrees of pressure, temperature, and "itch-producing" stimuli. In addition to these differentiated responses, the animal shows reactions such as the flexor reflex and the startle response, in which the modality of the stimulus seems largely irrelevant and only the intensity and rate of increase of the stimulus are important. Therefore, one expects to find convergent central pathways in which cells respond to several