AMPHIBIANS 239 



a poisonous fluid. Of these the most conspicuous are grouped 

 so as to form a swelling behind either eye. 



The internal skeleton (fig. 149) is not so bony as in a reptile, 

 consisting largely of tough membrane and gristle. The skull, 

 like that of a mammal, has a pair of condyles at the back for 

 joining on to the backbone. There is no quadrate bone, but 

 the lower jaw is hinged on to a cartilaginous projection of the 

 skull which represents it. The "hyoid apparatus" (see p. 29) 

 in the floor of the mouth consists of a small unpaired cartilage 

 in the middle line and other pieces of cartilage at the sides, 

 of which three pairs are the most conspicuous. These, as we shall 

 see later, are vestiges of structures which were once more important. 



The backbone consists of a considerable number of fairly 

 bony vertebrae, the centra of which are for the most part 

 convex in front and concave behind, just the reverse of what 

 is true for most reptiles. This is only characteristic for some 

 of the Amphibia, however. The following regions can be dis- 

 tinguished: neck, trunk, sacral, and tail. Only the first vertebra, 

 the ring-shaped atlas, can be considered as definitely belonging 

 to the neck region. It, and the hinder vertebrae of the tail, 

 bear no ribs, which are present, however, as short slender bones 

 in all the others. There is a single sacral vertebra united to 

 the pelvis by means of its ribs, and forming a division between 

 the trunk vertebrae in front and the caudal vertebrae behind. 

 A sternum is present in the form of a small plate of cartilage 

 on the under side, but this, from the way in which it is developed, 

 appears to be equivalent to two small abdominal ribs fused 

 together, and is not, therefore, of the same nature as the part 

 so called in the higher vertebrates. 



The limbs are supported by an internal skeleton, which corre- 

 sponds very closely with the patterns described on pp. 196-198. 

 Indeed it ought to be stated that these patterns are very 

 largely founded on what is seen in the Amphibia, these being 

 the simplest backboned animals possessing limbs suited for pro- 

 gression on the ground. Two deviations from the pattern may, 

 however, be noted, one being that in the fore-limb the little 

 finger is entirely absent, while the other is that in the pelvic 

 girdle there is no clear boundary between pubis and ischium, 

 the two being represented by a plate of cartilage with the hinder 

 part of which a bony plate is connected. 



