110 THE VITAL FUNCTIONS. 



dry and tough materials, from which but little nutriment 

 can be extracted; such as the bark, and roots, and even the 

 woody fibres of trees, and the harder animal textures, which 

 would appear to be most difficult of digestion. They are 

 all animals of diminutive stature, whose teeth are expressly 

 276 formed for gnawing,nibbling, 



and wearing away by conti- 

 nued attrition, the harder tex- 

 tures of organized bodies. 

 The Rat, whose skull is de- 

 lineated in Fig. 276, belongs 

 to this tribe. They are all 

 furnished with two incisor teeth in each jaw, generally very 

 long, and having the exact shape of a chisel; and the molar 

 teeth have surfaces irregularly marked with raised zig zag 

 lines, rendering them very perfect instruments of tritura- 

 tion. The zygomatic arch is exceedingly slender and fee- 

 ble; and the condyle is lengthened longitudinally to allow 

 of the jaw being freely moved forwards and backwards, 

 which is the motion for which the muscles are adapted, and 

 by which the grinding operation is performed. The Beaver, 

 the Rat, the Marmot, and the Porcupine, present exam- 

 ples of this structure, among the omnivorous rodentia: and 

 the Hare, the Rabbit, the Shrew, among those that are 

 principally herbivorous. 



The Quadrumana, or Monkey tribes, approach nearest 

 to the human structure in the conformation of their teeth, 

 which appear formed for a mixed kind of food, but are 

 especially adapted to the consumption of the more esculent 

 fruits. The other orders of Mammalia exhibit intermediale 

 gradations in the structure of their teeth to those above de- 

 scribed, corresponding to greater varieties in the nature of 

 their food. Thus, the teeth and jaws of the Hyena are 

 formed more especially for breaking down bones, and in so 

 doing exert prodigious force; and those of the Sea Otter 

 have rounded eminences, which peculiarly fit them for 

 breaking shells. 



