ON THE TEETH OF MAMMALIA. 113 



which are of formidable magnitude. The development of the perma- 

 nent series is attended with great irritation of the system. The Simiae 

 suffer much at this epoch ; and, in the Dog, the second dentition is often 

 a painful process. In the human species, the first dentition would seem 

 to be the most critical ; owing, perhaps, to the comparative feebleness of 

 our race during infancy, but more to the development of the brain, and 

 the unobstructed flow of blood to that organ, which seems, at this era, 

 to be peculiarly liable to increased vascular action (the blood being 

 determined toward it in a more than ordinary ratio), ending in convul- 

 sions, or effusion within the ventricles. 



Another sort of process must now be noticed, by which a succession 

 of teeth is secured, and which is found to obtain in the Phacochoerus, and 

 Elephant, among the Pachydermata, and in the Capybara, among the 

 Rodentia. This process refers solely to the molar teeth. Instead of the 

 molars being succeeded once, and once only, by a set formed below 

 them, it appears that an oft-repeated change takes place by the gradual 

 development of a molar, posterior to the others, and which, as a sort of 

 intruder, forces them forward as it usurps their situation ; so that the 

 first molar, which is the most worn (having been the longest used), is 

 ultimately dislodged by the pressure from behind it, and falls. 



The Elephant, for example (in which animal this process is very 

 marked), will afford a good illustration of this mode of dentition. 

 The molars of this animal are four ; namely, one on each side in each 

 jaw. At first, the anterior ridge of each tooth only is apparent ; but 

 it advances, layer after layer of ivory and enamel imbedded in cortical 

 substance being added, till the tooth is perfect, and fully evolved. No 

 sooner, however, is this accomplished, than another grinder, which has 

 been preparing in a deeper socket beyond that of the first tooth, begins 

 to shew itself, and then gradually to advance ; and, while the first wears 

 by use, it is, at the same time, pushed forward by its successor, till, at 

 length, the ossification of its alveolus concurring with the efforts of the 

 posterior molar, its relics (for it is worn to an inconsiderable portion) 

 are expelled. That which has taken its place undergoes a similar fate, 

 and so on for seven or eight successive changes. Thus, the Elephant 

 has, first, one tooth only on each side (in each jaw), then two ; and, 

 afterwards, alternately, one and two, in succession, to the end of the 

 changes. On referring to the section of the skull of the Elephant (fig. 20), 

 the relative position of these teeth will be easily understood. 



As it respects the dentition of the Rodentia, it has been affirmed, by 

 Cuvier, that all those, which have but three molars, experience only a 

 single dentition ; and that a second occurs only in such as have a number 

 beyond this : that is to say, the second dentition only changes the anterior 

 supernumeraries. He also states, that the teeth of the first dentition, in 



VOL. I. Q 



