4 i2 REPTILES xv. 10 



by additional intervertebral articulations known as the zygantra and 

 zygosphenes. 



The skull is highly modified, permitting, in all except a few bur- 

 rowing forms, an enormous gape and the swallowing whole of large 

 prey. The premaxilla is small and usually toothless, and the bones of 

 the upper jaw are loosely attached to the rest of the skull. The two 

 halves of the lower jaw are united only by ligaments. The sharp 

 recurved teeth are carried on the palate bones as well as on the maxilla 



pro. / WSW5" 

 sm. pi. 



Fig. 236. Diagram of skull in ophidians. 

 d. dentary; ec. ectopterygoid ; jr. frontal; mx. maxilla; ». nostril; na. nasal; 0. orbit; pa. 

 parietal; pf. postfrontal; pi. palatine; pm. premaxilla; prf. prefrontal; pt. pterygoid; q. quad- 

 rate; s. squamosal or supratemporal; sf. upper temporal fossa (this is shown diagram- 

 matically, as it occurs in many lizards; in Lacerta it is largely covered by an extension of 

 the postfrontal — see Fig. 213); sm. septomaxilla. (Modified from Goodrich.) 



and dentary. The brain-case is strong and compact, the brain being 

 protected from mechanical injury during swallowing by the massive 

 parasphenoid and by flanges of the frontals and parietals, which lie 

 between the orbits, so that there is no interorbital septum. 



In the normal ophidian kinetic mechanism the upper jaw as a whole 

 is raised as the result of forward rotation of the lower end of the freely 

 mobile quadrate, which is attached to the back of the pterygoid. The 

 well-developed protractor muscles of the pterygoid and quadrate play 

 an important part in the process. In the viperid snakes a further 

 elaboration of this mechanism is seen, the maxilla being very short 

 and able to rotate on the prefrontal so that the fangs can be erected 

 (Fig. 237). A slip from one of the muscles is attached to the poison 

 gland and helps to expel the venom as the snake bites. 



The respiratory system and viscera of snakes are also much modi- 

 fled. The glottis can be protruded so as to keep the airway clear while 

 prey is being swallowed, and in some forms a part of the trachea is 

 specialized for respiration as a tracheal lung. The left of the two paired 



