187 



CHAPTER XXXIV. 



REPTILIA. 



The true Eeptiles and the Birds, uulike as they are in ex- 

 ternal appearance, are nevertheless related to one another by 

 various points of affinity ; so that they may well be included 

 in a single division, wliich has been termed Sauro2)sida by 

 Huxley. The Sauropsida are defined by the possession of 

 the following characters : At no period of existence are bran- 

 chice, or ivater-hrcathing respiratory organs, developed upon the 

 visceral arches; the red corptiscles of the blood are nucleated; 

 the skull articidates %vith the vertebral cohimn by means of 

 a single articidating surface or condyle; and each hcdf or 

 " ramus " of the lower jaiu is composed of several j^icces, and 

 articidates ivith the sJctdl, not directly, btit by the intervention 

 of a pecidiar bone, called the " quadrate bone" or " as quad- 

 ratum " (fig. 546). 



These being the common characters of Eeptiles and 

 Birds, by which they are collectively distinguished from 

 other Vertebrates, it remains to inquire what are the char- 

 acters by which they are distinguished from one another. 

 The following, then, are the characters which separate the 

 Eeptiles from the Birds : The blood in Reptiles is cold — thcd 

 is to say, slightly %varmer than the external medium — oioing 

 mainly to the fact that the pidmonary and systemic circida- 

 tions are ahvays directly connected together, either ivithin the 

 heart or in its immediate neighbourhood,, so that the body is 

 supplied ivith a mixture of venous and arterial blood, in "place 

 of pure arterial blood alone. The terminations of the bronchi at 



