PLYMOUTH INSTITUTION. 237 



periosteum of the socket, but not united to it as some writers 

 have asserted. Internally the capsule is delicate, transparent 

 and very vascular, and encloses a gelatinous pulp on which the 

 tooth is afterwards formed. The internal surface of the capsule 

 is in contact with all the upper part of the pulp, and also of the 

 crown of the tooth, when it is formed, while within the jaw, so 

 that it forms an inverse figure of the surface of the tooth. At 

 their base the capsule and pulp are united. There is a curious 

 circumstance connected with this capsule, which, I believe, has 

 ^never before been noticed. It is that at the first, when the pulp 

 is at its base, and the tooth has scarcely begun to ascend, the 

 capsule receives its supply of blood from its connection with the 

 gum, and is consequently most vascular in that direction; but, 

 in proportion as the tooth advances tow^ards the point of its exit, 

 the vascularity diminishes in its upper part, and it begins to form 

 a new connection at the base of the socket, which was before a 

 solid thin plate of bone, but at this period is rendered pervious 

 to blood-vessels coming from the maxillary canal beneath ; thus 

 the tooth is always best supplied in that part which requires most 

 blood at the time. Ossification commences at the summit of the 

 pulp, the crown of the tooth being formed first ; and when there 

 ,are several eminences there are an equal number of points at 

 which the ossific deposit takes place, but always at the highest 

 first* It proceeds in layers, and as each layer is more 

 extended than the preceding one, the different points become, by 

 degrees, united, the crown is formed, the osseous laminse descend 

 towards the neck, and finally the root is hardened, but always 

 remains thinner than the upper portions of the tooth. In man 

 and all the animals having simple teeth, the root begins to be 

 ossified only at the moment when the tooth is ready to issue from 

 its socket, which it may be said to do from the greater degree of 

 resistance made to its growth by the lower part of that cavity, 

 than by the softer parts above. But in the animals with compound 

 teeth in which the crown becomes worn by use, the root is not 

 commenced for a long period after the tooth has appeared above 

 the gums, nor until great part of the crown has been ajready 

 abraded. Thus these animals have never an entire tooth. 

 Various opinions have prevailed with regard to the manner in 

 which the several substances of the tooth are deposited ; and there 

 i^ppears to me sufiicient reason for thinking it a much more simple 

 process than it has hitherto been supposed. 



The only opinion I have met with, on this subject, which seems 

 accordant with truth, is that of Cuvier. He considers that the 

 layers of ivory are the result of transudation -rather than of 

 ossification, from the facts, of their adhering but very little to 

 the pulp underneath, and having no apparent bood-vessels; 

 that the enamel is deposited by the internal layer of the capsule 

 by a transudation the inverse of that which gives origin to the 

 ivory ; and that when the tooth requires cement the same internal 



