378 REPTILES xiv. 5- 



columella auris with a small cartilaginous element (extra-columella) 

 at its outer end. The columellar system usually conducts vibrations 

 from the tympanum, lying behind the quadrate, to the fenestra ovalis 

 and inner ear. In some forms, e.g. snakes, however, the tympanum is 

 absent and the outer end of the columella is applied to the quadrate. 

 These animals may be deaf to air-borne sounds but sensitive to 

 ground vibrations, transmitted through the bones of the jaw. 



The hyoid apparatus consists of a basal plate, which projects into 

 the tongue, and three pairs of ascending horns. These represent the 

 remains of the hyoid and branchial arches. 



6. Feeding and digestion 



Food is seized either by the teeth or, in some specialized lizards 

 such as the chameleon, with the elongated tongue. The teeth are 

 situated along the edges of the jaws and often also on some of the 

 bones of the palate. Typically, they are all of the same conical shape, 

 but may be slightly serrated, or modified to form crushing plates, 

 poison fangs, and other devices. As a rule, tooth succession is con- 

 tinuous throughout life, though exceptions to this are found among 

 the lizards. Salivary glands are well developed in some forms; in 

 snakes and one genus of lizards (Heloderma) some of them are modified 

 to form poison-glands. The tongue is very variable, being hardly 

 movable in some reptiles (e.g. crocodiles) but long, forked, and highly 

 mobile in others (e.g. snakes). 



Digestion proper begins in the stomach. The alimentary canal is 

 built on the typical vertebrate plan, with a tubular stomach, rather 

 short small intestine, and wider large intestine, leading to a short 

 caecum. 



There is a well-marked cloacal chamber in all reptiles, subdivided 

 into a coprodaeum for the faeces, and a urodaeum for the products of 

 the kidneys and genital organs. These two chambers open into a final 

 common proctodaeum, closed by a cloacal sphincter. This division 

 of the cloaca is associated with the necessity for the retention of water, 

 the cloacal chambers serving for water reabsorption from both the 

 faeces and urinary excreta (p. 380). 



7. Respiration, circulation, and excretion 



The typical method of respiration is a backward movement of the 

 ribs, produced by the muscles attached to them. There is no complete 

 separation of the thorax from the abdomen, but a partial diaphragm 

 may be present. The glottis is a slit at the back of the mouth and leads 



