The thalamus forms the largest part of the diencephalon, which consists of the thalamus, epithalamus, hypothalamus, and subthalamus. The two halves of the thalamus, situated within the two cerebral hemispheres vary in size and there is marked intra-individual variation as well as consirable species differences. The sizes differences of the thalamus in humans are mainly due to variations in the dorsally sited pulvinar region that has been shown in a number of clinical studies.