The relation existing between the animal and plant functions is brought to light in another way. When the plant was building tissue it released oxygen and exhaled it into the air. At the same time, by means of leaves, it gathered in the carbonic acid to use in plant building. Of course this was got from the air. The animal in performing its functions and in building its tissue inhales oxygen from and exhales carbonic acid gas into the air. Thus it is that animals take up what is unnecessary to the plant and the plant uses what is waste and poison to the animal. CHAPTER III The Teeth As An Indication of Age When a colt is born the first and second temporary molars, three on each jaw, are to be seen. These are large when compared with the size of those that later replace them. In from five to ten days after birth the two central incisors or nippers make their appearance. In three or four weeks the third temporary molars appear, followed within a couple of months by an additional incisor on each side of the first two, both above and below. The corner incisors appear between the ninth and twelfth months after birth. This makes the full set of teeth--twenty-four in number. There is now no change in number, although there is considerable change taking place all the time; the incisor teeth, in rubbing against each other, are more or less worn, giving rise to the expression “losing the mark.” The two molars present at birth remain until the animal is about three years old, at which time they fall out of their sockets by the protrusion of the second set, or permanent molars. This change from temporary to permanent teeth takes place usually without difficulty and without trouble. The permanent teeth push their way up from below crowding those in view. While this pushing and crowding is going on the temporary teeth are losing ground, for the reason their roots are being absorbed, and a time comes when the cap only is left attached to the gums. This cap drops out and the new or permanent tooth soon is established in its place. LOSING OF TEMPORARY TEETH According to the observation of Mayo, the temporary incisors are replaced by permanent teeth as follows: “The two central incisors are shed at about two and a half years, and the permanent ones are up ‘in wear’ at three years. The lateral incisors are shed at three and a half and the permanent ones are up and in wear at four years. The corner incisors are shed at four and a half and the permanent ones are up and in wear at five.