84: 



ANIMAL PHYSIOLOGY. 



posed of a structure called dentine, and this is covered in 

 the crown with a cap of enamel, and in the root with crusta 

 petrosa. 



54. Dentine has a matrix 

 yielding gelatin, and impreg- 

 nated with mineral matter in 

 slightly greater proportion than 

 bone; but, instead of lacunae, 

 it contains a multitude of 

 closely set tubes, which radiate 

 from the pulp-cavity in an 

 undulating nearly parallel 

 course, getting smaller as they 

 approach the surface, and giv- 

 ing off branches. Each tube 

 contains an albuminoid fibre, 

 which, at least in the young 

 state, would appear to be an 

 offshoot from a corpuscle at its 

 Fig. 44. MOLAR TOOTH, verti- inner or pulp-extremity, com- 

 cal section; letters same as in parable with a bone-corpuscle; 

 % 43. and as the dentine grows from 



without inwards, invading the pulp-cavity, these corpuscles 

 travel inwards also. Crossing the substance of the dentine, 



Fig. 45. DENTINE. At a, the section is parallel to the tubes; at b, 

 it cuts them across; c, granular layer (of Purkinje) resulting from 

 the presence of small spaces connected with the extremities of 

 the dentine tubes; d, crusta petrosa. 



faint lines may be often seen, called contour lines (Owen), which 

 consist of chains of irregular spaces in the matrix, filled with 

 less refractive material, and possibly caused by the influence 

 of the irregular pressure to which the teeth are subjected. 



