. The biology of the frog . n and golden colors, which are present in varyingproportions. The changes in color which the skin mayundergo under certain conditions will be discussed in a laterchapter. 68 THE BIOLOGY OF THE FROG CHAP. CHAPTER IV PRELIMINARY ACCOUNT OF THE INTERNAL STRUCTURE Mouth Cavity. — If the mouth of the frog is held widelyopen, the following parts will appear. In the roof of thel mouth there is a pair of rounded prominences causedby the eyes, as has alreadybeen mentioned. Aroundthe margin of the upper jawis a row of fine, sharp, closelyset teeth which are conical inshape an
. The biology of the frog . n and golden colors, which are present in varyingproportions. The changes in color which the skin mayundergo under certain conditions will be discussed in a laterchapter. 68 THE BIOLOGY OF THE FROG CHAP. CHAPTER IV PRELIMINARY ACCOUNT OF THE INTERNAL STRUCTURE Mouth Cavity. — If the mouth of the frog is held widelyopen, the following parts will appear. In the roof of thel mouth there is a pair of rounded prominences causedby the eyes, as has alreadybeen mentioned. Aroundthe margin of the upper jawis a row of fine, sharp, closelyset teeth which are conical inshape and curved inwardmore or less at the tip. Ex-ternal to the teeth is a fleshyfold, or upper Up, and on theinner side is a groove, thesulcus marginalise which re-ceives the lower jaw when itis closed. Anteriorly thisgroove is crossed on eachside by a low elevation, thepulviriar rostrate; immedi-ately behind the tip of thejaw the sulcus is deepenedagain, forming the mediansub rostral fossa; on eachside of the pulvinars are the. Fig. 8. — Mouth of the frog widelyopened. E, Eustachian tubes;G, glottis; /, lower jaw ; L, lat-eral subrostral fossa; M, mediansubrostral fossa; A7, posteriornares ; O, oesophagus ; P, pulvi-nar rostrale; S, opening of thevocal sac; T, tongue; tp, tuber-culum prelingnale. iv THE INTERNAL STRUCTURE 69 lateral subrostral fossce, which are mere deepenings of thesulcus marginalis. The lower jaw is entirely devoid of teethand is held tightly pressed against the upper jaw, the mandib-ular muscles being normally in a state of tonic contraction;the tip of the lower jaw is flexible and is capable of being ele-vated or depressed independently of the rest of that structure,the joints of the movable part, pars mentalis, lying underthe pulvinars of the upper jaw. The elevation at the extremetip of the lower jaw {tuberculum prehnguale) fits into themedian subrostral fossa, and there is a slight depression oneither side of this tubercle corresponding to the
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