A 16-year-old boy shows somatically a diffuse cerebral process. With a mental retardation of 5 years 9 months, a retardation in reading of 4 years 5 months and in arithmetic of 6 years 8 months, this boy has a definite defect in the finger schema. In addition, he shows a disability in the grasp of spatial relations. A comparative investigation, using other mentally deficient children with relatively either exceptionally high or low arithmetic achievement, gave evidence of a relation between deficiency in the finger schema and a retardation in the development of number concept.Abstract Teaser