Again, in 1883, “Mark Twain” published his charming _Life on the Mississippi_, a very valuable human document. It contains a well-thought-out story of an identification by means of a thumb-print on a system supposed by him to have been invented by a French prison doctor. His _Pudd’nhead Wilson_, in which a still better study of the subject occurs, did not come out till 1894, the year in which the sitting of Mr. Asquith’s Committee on Identification of Habitual Criminals had set journalists agoing again on the theme of “thumb-prints.” Prior to that year a great deal had been written on the subject, the facts being chiefly taken from the correspondence in _Nature_, to which reference has been made. CHAPTER II SWEAT-PORES, RIDGES AND FURROWS [Illustration: SECTION OF SKIN, SHOWING SWEAT-GLANDS, DUCTS AND PORES _a_. pore open. _b_. pore closed. _d_. sweat duct. _e_. sweat gland.] The front or palmar surface of human hands, and the corresponding solar or plantar surface of the feet, are marked with alternate ridges and furrows, lying for the most in nearly parallel rows, but often again at certain points on palm or sole, curving, splitting, twisting, or joining to form patterns of much intricacy. The ridges, called technically _rugæ_ (sing. _ruga_), are punctuated at very frequent intervals with small openings, which are the mouths or pores of the sweat ducts connected with certain glands which lie deep in the lower strata of the skin. The furrows or _sulci_ (sing. _sulcus_) are almost devoid of any such apertures. There are probably some two or three millions of those tiny sweat pores in a human body, which afford an evaporating surface, according to the anatomist Krause, of about eight square inches. The sweat is a watery, slightly saline fluid, with slight--very slight--traces of grease, some small cell-like particles, and some carbonic acid and other gaseous matters, which exhale from the skin. The more oily secretion of the skin comes from a different set of openings with their associated glands, the sebaceous glands, which are associated with the hairy surfaces of the body. In Ludwig Hopf’s work, _The Human Species_, the subject is discussed fully. When the palmar surface leaves a distinctly greasy impression, this greasiness must have been acquired from outside or from transmitted exudation from the back or dorsal surfaces, or other parts of the body. Those skin ridges, apart from any relation they may have either to the sweat-pores or to the special nerves of touch and temperature which lie near them, serve a useful purpose in helping the horny hands of toil to grasp its tools firmly. They occur in a few other parts of animals somewhat near to us in the scale of being. A striking example is that on the palmar surface of the prehensile or grasping tail of the Spider Monkey (_Ateles ater_), which it uses in climbing almost like a hand.