90 



NORTH AMERICAN FAUNA 74 



appear to be products of two superimposed rhythms of dentin production, one at 

 interv'als of about 0.025 mm and the other at about 0.3 mm. Because of the 

 regularity of these lines, they resemble the fine contour lines of rhythmic growth 

 in the dentin of human teeth. The incremental lines of the second type, caused by 

 differential calcification, are more irregular in quality and arrangement. I 

 presume that they are correlated with general systemic disturbances, such as 

 nutritional deficiencies and other physiological stresses of major importance (cf . 

 Massler et al. 1941). The incremental lines that mark the beginning and end of 

 each annual deposit of dentin are of this second type and are the most clearly 

 expressed. The most prominent second type line is the "neonatal line," which (as 

 in the teeth of other mammals) separates the dentin formed before birth from 

 that formed after birth. Its homologue in man is believed to reflect an 

 interruption of growth during the physiological stresses of the late prenatal and 

 early postnatal period (Scott and Symons 1961:188). In the walrus, this 

 interruption of growth is expressed also on the external surface of the teeth, 

 where it appears as a slight depression just proximal to the anatomical crown 

 (Fig. 53). 



Dental Pulp 



In the incisors, postcanines, and lower canines, the dental pulp initially is 

 large, relative to the amount of calcified tissue, and the proximal end of the pulp 

 chamber is ver\' broad. The breadth of this opening (apical foramen) and the 

 volume of pulp increase for 1 or 2 years after birth. Thereafter, the volume 

 declines steadily in the cheek teeth, ending in closure of the apical foramen and 

 complete filling of the pulp chamber by dentin (Fig. 53) usually in the fifth or 

 sixth year after birth. All further accretionary growth of the cheek teeth is by 

 addition of cementum alone, on the exterior of the root. Cementum is deposited 

 over the apex of the root, after closure of the apical foramen. Even so, vascular 

 connections are maintained between the dentin and the periapical tissues 

 through minute, transcemental canals about 0.02 mm in diameter (Fig. 57). In 

 the upper canines or tusks, the dental pulp does not diminish greatly in volume 

 until old age, and the apical foramen remains wide open. 



Cementum 



The cementum of walruses is entirely of the "cellular" type, though there is 

 some variation in quantity of cells or lacunae within and between layers. The 

 least number of lacunae per unit of volume is found in the first layer (primary 

 cementum), deposited soon after birth. Somewhat more are present in the dense, 

 translucent layers that comprise the annual incremental lines; the greatest 

 quantity is present in the opaque cementum between them. 



The cementum deposited between the annual incremental lines in the teeth of 

 immature walruses is mosdy from 0.2 to 0.5 mm thick. Annual deposits in very 

 old adults may be one-tenth as thick (Fig. 58). Each successive annual increment 

 tends to be somewhat thinner than its predecessor (Mansfield 1958fl). 



The continual deposition on the root of each incisor, postcanine, and lower 

 canine results in a cumulative increase in thickness of cementum with increasing 

 age (Fig. 53). Since the annual layers tend to be successively thinner, the rate of 

 increase in cumulative thickness declines with increasing age (Mansfield 1958a). 



