132 Dr. Thomson on Caries, or Decay of the Teeth. 



The author, however, considers the cause of caries to be external, and 

 not to depend upon inflammation. To make his views apparent, it is 

 necessary to give a short sketch of the development of the teeth, from 

 the pulp upwards. 



The teeth diner much in formation from the bones in general, 

 having for their basis a pulp similar in shape to the tooth to be produced, 

 instead of the usual base, cartilage. "Wo can trace the fonnation of these 

 pulps so early as the fourth month of animal existence ; and as the forma- 

 tive process goes on, they are each gradually enclosed in a cell produced 

 by small processes of bone, which may be observed shooting across from 

 each side of the groove in the jaw in which the pulps are first found, and 

 which gradually form these cells. Each pulp is covered by a membrane 

 firmly attached to the gum and to the pulp at its base. When the pulps 

 have been injected, we find that they are filled with vessels, as also the 

 membrane by which they are enveloped. The pulps derive their vessels 

 from the artery which passes through the jaws and the membranes, from 

 the gums. The bone of the tooth is formed from the pulp, and the 

 enamel from the investing membrane. The bony portion is formed in the 

 following manner ; when the ossification commences, the bone is deposited 

 in the extreme points of the pulp from the vessels. In the incisors it 

 begins upon their edges, and in the molars upon the points of their 

 grinding surfaces, usually four in the lower jaw, and in the upper five. 

 These soon extend over the surface, and eventually the whole pulp is 

 covered. The deposition of the bone continues from without inwards, 

 and this goes on till the tooth becomes complete. When the body is 

 formed, the pulp elongates and takes the form of the fang proper to each 

 particular tooth. Bone is then deposited, and it becomes smaller till it 

 terminates in a point ; when there are two or more fangs, the pulp divides^ 

 and the ossification proceeds accordingly. The cavity gradually decreases, 

 till at last it contains merely nervous and muscular matter, which is after- 

 wards to give life and sensation to the tooth. The enamel is collected 

 from the investing membrane, and is deposited on the ossific points in the 

 shape of a fluid. This is at first of a consistence, not firmer than chalk ; 

 it, however, soon grows hard, and seems to undergo a process similar to 

 that of crystallisation, for it takes a regular and peculiar form. The 

 enamel, when broken, appears to be composed of a great number of small 

 fibres, all of which are arranged so as to pass in a direction from the centre 

 to the circumference of the tooth, or to form a sort of radii round the 

 body of the tooth. This is the crystallised form which it acquires some- 

 time after its deposit. Now, as the process of formation goes on, new 

 particles being deposited, the lamellae thus formed, meet at last, in the 

 centre, and should a child, for example, be of an unhealthy constitution, 

 we find invariably that these plates do not join in the centre, but leave 

 minute divisions of a crucial nature on the crowns of the teeth, thereby 

 giving access to any acid matter that may have an affinity for the bony 

 portions of the teeth. In the author's opinion, all simple decay arises 



