ECOLOGY AND BIOLOGY OF THE PACIFIC WALRUS 



89 



Fig. 56. Passage of carbon ink 

 through the central canals in the 

 tusk of a 2-year-old Pacific wal- 

 rus. Direction of flow was from 

 the pulp chamber (upper) toward 

 the distal tip (lower), which was 

 excised and immersed in saline 

 solution. (Photo by G. C. Kelley) 



dentin occur in the teeth and tusks of some other mammals (Ray 1975), in none 

 that I have seen was the quantity at all comparable to that in Odohenus. Because 

 of its vascularity, the globular dentin resembles to some extent (but is not 

 homologous with) the vasodentin or osteodentin in the teeth of some lower 

 vertebrates. The functional value of its vascular system is unknown, but the 

 presence of a vascularized tusk in the narwhal (Monodon monocerus), the only 

 other marine mammal with grossly comparable dental specialization, suggests 

 some unifying adaptive purpose, possibly related to the marine environment. 



The dentin deposited on the walls of the pulp chamber in teeth of Odohenus 

 contains an abundance of fine tubules, in which are protoplasmic processes from 

 the odontoblasts. The homologous processes in human teeth are believed to have 

 a sensory, rather than circulatory function (Scott and Symons 1961). The tubules 

 radiate outward from the central core of each tooth and appear to be 

 interconnected to some degree with those in the core. The tortuous disarray of 

 the tubules within the core identify this material as irregular secondary dentin, 

 as distinguished from the regular secondary dentin deposited on the walls of the 

 pulp chamber. In the regular secondary dentin, the tubules are only slightly 

 sinuous and tend to be parallel. 



The tubules in the regular secondary dentin appear somewhat S-shaped in 

 longitudinally sectioned teeth. Slight secondary curvatures occur where the 

 tubules are intersected by incremental "lines." These incremental lines are of two 

 major types: those indicated only by secondary curvature of the dentinal tubules 

 and those accentuated further by differential calcification. Lines of the first type 



