284 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



for the most part, in the abdominal cavity, which is separated 

 from the thoracic cavity by a firm partition, the diaphragm^ and 

 lined by the thin peritoneum, which forms suspensory mesenteric 

 folds for the viscera. 



The small mouth, guarded by flexible lips, leads into a large 

 mouth-cavity, the roof of which is elongated, and presents a 

 central part, the palate, bounded by the margin of the upper jaw. 

 The anterior half of the palate, marked by firm transverse ridges, 

 and the soft flexible posterior half, are called respectively hard 

 and soft palates. The latter ends in a free notched edge, on each 

 side of which is a small thickening, the tonsil, indented by a pit. 

 The floor of the mouth, bounded by the margins of the mandible, 

 has the long muscular tongue attached to the greater part of its 

 extent. This exhibits in front a free tip, in the neighbourhood 

 of which are numerous small papillce that also extend back some 

 distance on the upper surface. There is a hard elongated area 

 above and behind, and, on each side of the posterior end of this, 

 a circumvallate papilla, consisting of a small projection encircled 

 by a groove. An oval elevation, the papilla foliata, across which 

 numerous oblique ridges run, is present on each side of the tongue. 

 The hair covering the body is continued into the sides of the 

 mouth-cavity for a short distance.. 



The teeth, definite in number, are imbedded in sockets, or 

 alveoli, and are of three kinds. There are two sets of them, the 

 first, milk teeth, very transitory, and succeeded by permanent teeth, 

 which continue to grow throughout life as they are worn away 

 at their ends. This is called growth from " permanent pulps." 

 The front of the jaws is occupied by cutting teeth (incisors) ; the 

 remainder are grinding teeth (premolars and molars) placed a 

 good deal further back. 



In the front of the upper jaw two strongly curved incisors are found, 

 grooved in front, and with chisel-shaped edges. Two very small incisors 

 of similar shape are placed behind them. All the upper incisors are 

 imbedded in the pre-maxillse. Next follows a gap destitute of teeth 

 (diastema) on each side, posterior -fcc- which are six prismatic grinding 

 teeth, imbedded in the maxilla. The first three, premolars, are preceded 

 by milk-grinders, the last three, molars, have no predecessors. The 

 grinding teeth are much flattened, and, with the exception of the first 

 and last, which are small, each possesses a deep groove on its outside, 

 from the end of which a transverse ridge runs nearly across the flattish 

 grinding surface or crown. 



Two incisors similar to the large upper ones, but less curved and not 



