18. _To distinguish an upper from a lower molar tooth_ is not difficult. The presence of the infundibula in the first would alone enable it to be pointed out; but there are other indications that are worthy of a passing notice. The table of the upper molars is much broader, being compared to those of the lower jaw as nine is to five. Then the form of the table is different in each. The table of the lower tooth is indented on either side, the indentation extending downwards along the fang. The table of the upper molars present two narrow prominences, and two broad grooves on the outer side; and also exhibits one broad prominence upon the centre of the inner side--all of these developments being likewise contained along the fang. The slant of the tables also differs. In the upper jaw, the outer edge is the lowest. In the lower jaw, the tables incline in the opposite direction. 19. _The teeth of the horse are very firmly implanted in the jaws._ The fangs are of a length which they who have only seen the teeth extracted from the mouth of a human being would hardly imagine. The length of fang of course gives the teeth some security; but this is not the only provision made for that end. The teeth radiate towards a common centre, being widely separated at their roots, but approximating at the crowns. They also incline laterally from the perpendicular, so that the force applied to them never acts in a direct line, or is concentrated upon one particular point. 20. _The molars of the lower jaw are the active agents of mastication_, for motion, during this process, only takes place in that part of the head in which those teeth are placed. The inferior molars are the instruments that grind--the upper molars are simply the surfaces upon which the food is ground. The whole of the molars, however, are never simultaneously employed. The horse can chew but on one side at a time, for as the inferior maxillary bone is considerably narrower than the superior, if the teeth on one side are brought into apposition, those of the opposite side are necessarily separated. Any one who has observed a horse feeding, will have remarked that the animal, during the act, continually abducts and adducts, the lower jaw not causing it to describe a circle, as does the cow, but urging it first to one side and then to the other. It will be seen, therefore, that the whole force of those strong muscles, which close the mouth of the horse, is employed on a portion of the jaw at the same moment, and the power then exerted must obviously be very great. Recognizing this fact, the reason for all that has been pointed out--the shelving tables--the grooved or indented sides--the double inclination, and the great length of the fangs is at once perceived, for the action is a wrenching one; and the greater extent of, and more uneven, surface presented by the molars of the upper jaw, facilitates the comminution, without imposing weight upon the muscular activity.