Owen had maintained that the structural difference between man and the highest apes was so great that it could only be rightly expressed by placing man in a separate sub-class of the class "Mammalia"--the hairy vertebrate animals which have warm blood and suckle their young. He pointed chiefly to the large size of the brain in man, the existence on each side of its central cavity of a little internal swelling called the "hippocampus minor," in the fanciful language of anatomists, and of the overlapping (within the skull) of the cerebellum by the hinder part of the large brain-hemispheres, or cerebrum. He called the sub-class (in which he proposed to place man alone) the "archencephala," or that of the highest developed brains (Greek, "archi," chief, and "encephalon," a brain), and proposed three other sub-classes, to contain the other orders of mammals (the Gyrencephala, Lissencephala, and Lyencephala), grouped according to three grades of complexity of the brain. Huxley denied the justification of this special grouping, by which man was placed in a separate and highest sub-class apart from the apes and monkeys. He pointed out that every bone and every part recognized by the anatomist in the higher apes is present in man (though other mammals present no such identity with him or them), and that there are only three little muscles belonging to the hand and the foot which are present in man and not present in the higher apes. He showed that the term "four-handed," or "quadrumanous," as applied to the apes and monkeys, is misleading, inasmuch as, though modified in the proportions of the digits and the mobility of the great toe, the foot of the apes has the same bones and muscles as the foot of man, and differs in structure from their hand as the foot of man differs from his hand, whilst the true hand of the apes agrees in structure with the hand of man.