120 INTRODUCTION. 



When there are no false molars, as in the Rodentia and Ruminantia, the 

 form will be thus: Ox : incisors, 5; canines, ; molars, Sri ; 32. 



As the teeth of each species of Mammalia are modified according to 

 the food upon which it is destined to subsist, so, according to the characters 

 of the dental system, will be the structure of the articulation of the 

 lower jaw, and its extent of motion. Where, for example, the food is 

 exclusively vegetable, and the teeth are adapted for grinding to a pulp, 

 the lower jaw must possess a considerable degree of motion, in order that 

 the molars may perform their office with due effect :*but where, as in the 

 Tiger, the food is exclusively of an animal nature, requiring not to be 

 ground, but cut up previously to being swallowed, and where the action 

 of the molar teeth resembles that of the blades of a pair of shears, the 

 movements of the jaw will be more limited, and the articulation altogether 

 more secure. Where the teeth are weapons, and the lower jaw has to 

 bear the strain of powerful muscles, in the acts of rending and tearing, and 

 of carrying away the carcasses of slain victims, there the articulation of 

 the lower jaw must be so constructed as to bear, without dislocation, the 

 stress to which it is subjected. The Dog is less purely carnivorous than 

 the Tiger. It has been seen that the Dog has two tuberculous molars 

 behind each carnassiere, and that the first tuberculous molar above is of 

 great size and strength ; but this animal crushes bones with ease an 

 action throwing an immense strain of muscular force upon the articula- 

 tion of the lower jaw, and which it is accordingly fitted to sustain. 

 The strictness, or laxity, then, of the articulation of the lower jaw, as a 

 general rule, harmonizes with the dentition and the nature of the food, 

 upon which the teeth have to act ; regard, also, being had to the manner 

 in which they have to act upon it. The modifications, however, of this 

 articulation, are very varied: if, for instance, we compare the articu- 

 lation in question, as it appears in one group of herbivorous quadrupeds, 

 the rodents, with its construction, as observable in another herbivorous 

 group, the ruminants, a wide distinction will be found between them a 

 distinction connected with the manner in which the lower jaw is used. 



To point out the principal modifications of this articulation will now 

 be attempted ; and, in doing so, it may be premised, that the condyles 

 of the lower jaw, or, in other words, its articulating processes, are 

 received into a depression, more or less distinctly marked, at the base of 

 the zygomatic process of each temporal bone, anterior to the auditory 

 orifice, and termed the glenoid cavity. This cavity is lined with smooth 

 cartilage ; and so, also, is the condyle of the lower jaw : both, moreover, 

 are lubricated with synovial fluid, as are other articulations of the ske- 

 leton, such as the knee, elbow, ankle, &c. In Man, and most other Mam- 

 malia, this glenoid cavity is exclusively on the temporal bone; but, in 

 the marsupial animals, the extremity of the long zygomatic branch of the 



