. 224 



low cone filled by the pulp, sinks more deeply into the jaw, and 

 shallow transverse walls of division spring up between the teeth 

 and form the separate sockets. These, however, never close up 

 and grasp the shaft of the tooth as in the mammalia generally, 

 for such a firm settin"; would be unnecessary for teeth which meet 

 no opponents 'to clash against, but fall within soft pockets. The 

 bottom of the pits, as well as their septa, are traversed by numer- 

 ous large canals, through which the long-continued activity of the 

 dental pulp or matrix is nourished and sustained. After many 

 years this loses its vitality, and, not possessing the life-long and 

 unwearied generative power of the rodent's incisor, tapers to a 

 narrow cleft by the contraction of the fang, and solidifies. A 

 small opening, however, is always left, by which the mature tooth 

 may still keep up a low vital connection with the vascular system. 

 This process of development once complete is never repeated. 



The structure of the tooth oi Physeter consists of a hollow cone 

 of dentine surrounded by a coating of cement, which, when first 

 formed, according to Owen, is tipped by a thin cap of enamel. 

 The cement, as in all the cetacea, is very thickly deposited, and 

 especially so on the anterior or convex surface. This is to coun- 

 teract the attrition by which this face is constantly worn down, as 

 may be seen on the summits of all the specimens in the cabinet. 

 This action, which the dentine resists longer, though according to 

 analysis of a more organic nature, gradually reduces the tooth to 

 a stump after the reparative power of the matrix is exhausted. 

 The tubes of the dentine are arranged at an acute angle with the 

 central axis of convergence, and follow the sub-curves of direction 

 which the tooth assumes. This is the normal development. Not 

 unfrequently, however, a new element enters into their compo- 

 sition, of which traces are often visible in their early stages of 

 growth. This is the production of a deposit within and arising 

 from the dentinal pulp, called osteo-dentine, a substance more 

 resembling cement or true bone than the dentine itself; it is a 

 modification of this latter element, in which its cellular matter is 

 arranged in concentric layers, and furnished with radiated cells 

 like the corpuscles of bone. As above stated, and as was well 

 illustrated by a section of an immature tooth, isolated nodules of 

 this material are often found imbedded in the early-formed den- 

 tine tissue. Generally, however, it is in more mature teeth, and 



