AMPHIBIA 197 



by the narrow floor of the brain case, and on each side of this 

 the roof is membranous under the orbits. Behind this region 

 the roof passes into two large passages which lead each into 

 the chamber bounded externally by the tympanum. This 

 chamber is the middle ear, and the wide passage leading into 

 it from the mouth is the Eustachian tube. 



The floor is bounded by the lower jaw, or mandible, which 

 does not bear teeth. The tongue occupies the middle of the 

 floor, to which it is attached anteriorly and projects freely 

 backwards. The anterior end is rounded, the posterior end 

 is forked. The tongue, as has been said, is used in the pre- 

 hension of food, and can be shot out from the mouth and 

 retracted with the prey with such rapidity that the eye fails 

 to observe the movement. The adhesiveness is provided by 

 intermaxillary glands at the anterior end of the roof of the 

 mouth, glands against which the tongue brushes as it is being 

 launched. Beyond the tongue the longitudinal opening of 

 the glottis may be made out, and also that the cavity narrows 

 to form the oesophagus. 



The limbs are resolved into segments which may be denned 

 as anterior arm, forearm, hand ; posterior thigh, leg, foot. 

 An extra segment is provided in the posterior limb by the 

 elongation of the tarsus, or ankle, a feature associated with the 

 jumping powers. 



The hinder opening common to the intestine and the urino- 

 genital organs is the cloaca, situated at the end of the body, 

 between the hind limbs. 



INTERNAL STRUCTURE. The skeleton is founded on carti- 

 lage, but is to a large extent a bony one. Many of the bones 

 replace the cartilage more or less completely, but some of the 

 bones are developed independently of the cartilage. In the 

 case of the replacing bones the cartilage degenerates, and the 

 space it occupied is invaded by bony formation. The inde- 

 pendent bones are formed directly by the connective tissue 

 of the region being converted into bone. In both cases the 

 procedure is essentially the same. Heralded by a calcification 

 usually, a layer is developed, the periosteum, which divides, 

 and the cells are separated by a connective-tissue matrix con- 

 taining gelatin and which becomes hardened by lime salts. 



