ECOLOGY AND BIOLOGY OF THE PACIFIC WALRUS 



87 



Fig. 53. Diagrammatic view of development and decrement of the walrus' first upper 

 and lower permanent premolars. Transverse sections: (a) at birth, (b) at 1 year, (c) at 

 2 years, (d) at 7 years, and (e) at about 20 years of age. 



Dentin 



Deposition of dentin in the secondary teeth of the walrus takes place in two 

 locations: on the walls of the pulp chamber and within the body of the pulp. The 

 character of the dentin produced on the walls differs greatly from that deposited 

 within the pulp. On the walls, it is laid down in smooth, opaque layers; within 

 the pulp it is formed mainly as translucent, spherical globules, up to 0.5 mm in 

 diameter. The globules are comparable in location and, apparently, in structure 

 with the denticles or "pulp stones" in human teeth (cf. Johnson and Bevelander 

 1956). Whereas in man these are regarded as pathological and are associated 

 with old age, they make up a major part of each normal tooth of the walrus from 

 the first year. As these denticles gradually increase in size and number, they 

 become interconnected by slender longitudinal strands of tubular dentin (ortho- 

 dentin). Eventually, they coalesce or become amalgamated with further deposits 

 of irregular tubular dentin (Fig. 54). This amalgam of denticles and irregular 

 dentin constitutes the central "core" of each tooth, which is one of the most dis- 

 tinctive structural characters of the walrus' dentition (Penniman 1952; Cam- 

 mann 1954; Ray 1975). In general, the core makes up about 40% of the volume 

 of the tusks and up to 10% of the volume of the cheek teeth. 



The core material contains an intricate network of vascular canals, mostly 

 about 0.03 to 0.05 mm in diameter (Fig. 55). This network apparently is 

 interconnected with vessels in the pulp. I have found traces of blood in these 

 canals within a few millimeters of the pulp in the cheek teeth and up to 1 or 2 cm 

 distal to the pulp in the tusks. In most of the cheek teeth and tusks that I 

 examined in sagittal section, however, the more distal canals did not contain 

 whole blood but a paler, more homogeneous fluid resembling plasma, serum, or 

 lymph. This finding suggests that the canals usually are blocked by thrombi or 



