22. _The cavity of the pulp_ has been always stated to exist in the tooth of the horse. In the human tooth it occupies the extremity of the fang, being situated within the substance of the ivory; and the double teeth are known to have as many cavities of this kind as there are roots to the part. The same has been assumed to be the case also with the horse, but I cannot say that investigation enables me to corroborate this opinion. I find no cavity, that is no empty space, constantly present in the fangs of the horse's teeth, and no separate or defined cavities at the roots of the molars. The bone or ivory may be deficient, more or less, towards the centre of the fang; but this space, not yet occupied by the osseous deposit, is filled by a membraneous substance which is continuous with the crusta petrosa of the root, and will ultimately be converted into bone. I cannot separate the tissues forming the pulp, from the crusta petrosa itself; and I assume the two to be continuous. The only difference I can observe is, that the tissue of the pulp is the finest; but as the vessels of the one primarily pierce through and ramify upon the other, I cannot perceive the necessity for their separation. It will be understood, that I am speaking of these parts in a fresh subject, and not alluding to a mascerated and dried specimen, the appearance presented by the last being calculated to mislead. With age, the pulp diminishes; and in a tooth extracted from a very old animal, no positive remains of it will be found--osseous matter filling up the space which the pulp once occupied.