240 



phant, formerly presented, as an example of the growth 

 of osteo 'dentine. 



The tusk of the elephant is composed of a thin outer layer of 

 enamel, and the peculiar modification of dentine known as ivory, 

 characterized by the minute size and remarkable curvatures of 

 its tubes. The tooth grows in length by continual de[)Osition, 

 from the cone of matrix within its base, of the enamel and den- 

 tine ; occasionally, however, in old animals a thin deposit of nod- 

 ules of osteo-dentine is found within this cavity, the last effort of 

 the dentinal pulp. Injury appears to produce the same result, as 

 was seen in the specimen, which illustrated the effect of the pas- 

 saoje of a laroje wrouo^ht-iron musket-ball throuojh the dentinal 

 pulp from side to side. The ball entered probably the thin 

 socket formed by the prolongation of the premaxillary bone, break- 

 ing through the thin wall of the tusk, through the tender pulpy 

 cone, and the tooth wall of the other side, and spent its force 

 against the interior surface of the socket on the opposite side ; it 

 then fell back within the soft hollow it had formed, and became 

 imbedded in the new growth it excited. In time it was undoubt- 

 edly carried forward by the growth from the base, until it per- 

 haps cleared the socket. He inferred that the wound w^as thus 

 made, on account of the large cavity still unfilled by deposition of 

 tooth-matter from the pulp. The course of the ball was marked 

 by a hollow cylinder of osteo-dentine deposited about its track ; 

 the inner layer of ivory was driven in by the force of the ball, 

 and the efforts of the matrix never recovered their normal direc- 

 tion, for not only was its secretion deposited in irregular layers 

 and masses, but its nature was perverted, and osteo-dentine^ in- 

 stead of ivory, was ever afterward its product. , 



Prof. W. B. Rogers explained the laws of fracture of 

 a thick glass tube, exhibited by U. A. Boyden, Esq. 



A thin tube would fracture in a spiral manner, the result of 

 the circular and longitudinal tendencies to break; in a thick tube, 

 there is a third tendency to fracture from the centre to the pe- 

 riphery ; if this be combined with the other two, the resultant is 

 the imbricated fracture observed in the specimen. 



