The spinal cords of three men, nine cats, three dogs and four rabbits were chosen for the present investigation. It was found that the thoracic segment of the spinal cord is longer than the other segments in man and animals. The volume of the gray substance in the lumber enlargment are larger than that in the cervical enlargment in man, cat, dog and rabbit, The cross—section areas of the white substance of the spinal cord in man increase clearly at the cranial level but that of the rabbit does not.Based on the measurements we found that the move of the gray substance anteriorly in man and cat may be the mark of evolution of the mammals. Perheps the spinal eord of eat is a satisfectory specimen in neuro anatomical and neurophysiological laboratory.