the cutting Teeth of the Rat. 



391 



better understood by reference to a drawing which, at the 

 time, I made of the head. (flg. 50. a and b.) The in- 



-. 



50 



cisors of the lower jaw (a) made a turn over the snout, in- 

 clined to the right side, and gradually tapered to their 

 extremity. The external incisor was much longer than the 

 internal one, and had grown into the eye, so as totally to 

 destroy the sight. The two incisors in the upper jaw (Z>) 

 passed down to the left of those in the under jaw, and made 

 a short turn round into the roof of the mouth. I did not 

 take notes at the time, and my sketch does not show whether 

 they had perforated the bones. From the appearance of the 

 animal, I should say that this extraordinary growth had not 

 much impaired the animal's power of mastication, nor pre- 

 vented its obtaining food. I should think that this species 

 would, in fact, suffer much less from this disease than the 

 more herbivorous Rodentia ; as, for instance, a rabbit, which 

 must employ the fore teeth in cropping the herbage. 



In accounting for this growth, I agree with Mr. Jenyns 

 (Vol. II. p. 134.) that it is caused by the want of apposition, 

 in some way, between the upper and lower teeth, which pre- 

 vents that attrition and loss of the growing substance of the 

 teeth which is necessary. The growth of the incisors in this 

 order varies from that generally followed in the growth of 

 teeth. All teeth are formed originally from a vascular pulp, 

 which is of the form of the future tooth, and through which 

 the vessels and nerves enter. On the upper surface of this 

 pulp the growth of the bony portion commences^ by the 

 secretion of a thin layer, under which another layer is se- 

 creted ; the pulp diminishing with the increase of bone, till 

 the tooth is fully formed, when it becomes a vascular mem- 

 brane lining the' tooth. Differently to this, in the incisors of 

 the Animalia rodentia the pulp does not diminish, but goes on 

 secreting the tooth, which is formed of a hard external layer 

 of enamel, and a softer bony layer. The latter, being more 

 quickly worn away than the enamel, gives to the incisor the 



c c 4 



