2 02 CHARACTERS OF VERTEBRATE ANIMALS 



constructed on a brand-new pattern especially suited to their re- 

 quirements, but are simply a modification of the more ancient 

 arrangement found in fish-like animals. In both Mammals and 

 Birds the modification is very perfect, and there is a complete 

 separation of pure and impure blood; but in Reptiles and 

 Amphibia the separation is less complete, and the relative slug- 

 gishness of these creatures is partly due to the fact that their 

 bodies are to a large extent supplied with mixed blood, i.e. 

 with blood which, being a mixture of pure and impure blood, 

 is only imperfectly oxidized. 



Not only does the Lizard possess an hepatic portal system (see 

 p. 41), by which the blood of the digestive organs is carried to 

 the liver, but there is also a renal portal system in connection 

 with the kidneys, by which some of the impure blood from the 

 hinder part of the body is supplied to those organs, the function 

 of which is, as elsewhere, to get rid of nitrogenous waste. 



Breathing Organs and Organs of Voice (fig. 131). — The 

 arrangements connected with respiration are here much simpler 

 than in either Mammals or Birds, which is what might be ex- 

 pected, as cold-blooded animals have not the same need for 

 thorough and rapid oxygenation of the blood, though at the same 

 time it must be remarked that Lizards exhibit less complication 

 than is usual among Reptiles. 



On the floor of the mouth behind the tongue there is a slit-like 

 glottis leading into the organ of voice or larynx that forms the 

 beginning of the wind-pipe, which runs back and forks into two 

 bronchi, one for each lung (see p. 46). These air-tubes are 

 supported by gristly rings, and thus prevented from collapsing. 

 Nothing is present equivalent to the syrinx or song-box of a Bird 

 (see p. 149). The lungs are spindle-shaped bags, the linings of 

 which are raised into a honey-combing of ridges, by which the 

 surface exposed to the air is considerably increased. 



Nervous System, and Sense Organs (fig. 131). — Only a few 

 points call for special attention. As previously remarked (p. 192), 

 the flatness of the head is an indication of the relatively small size 

 of the brain. At the front end of this organ are the club-shaped 

 olfactory lobes, which supply the fairly well developed organs of 

 smell, and pass back into the smooth cerebral hemispheres, the 

 relatively small size of which as compared with a Bird (see p. 149) 

 is an index of inferior intelligence. Still farther back are two 



