Clans 4. ReptiUa. 



419 



a rule the left is rudimentary or absent. The Snakes are also peculiar, 

 in that, whilst the anterior portioa of the lung is like that of a Lizard's ; 

 posteriorly, it is simply a smooth unfolded sac which receives blood, 

 not from the pulmonary, but from one of the systemic arteries; 

 this portion is entirely without respiratory sig-oificauce. The 

 Chameleons (and some other Laoertilia) are characterised by 

 the possession of thin-walled, finger-like sacs extending from the 

 lungs between the viscera. The animal can fill these with air so 

 that the size of the body is noticeably increased. Inspiration, 

 in the majority of cases, is effected as follows : the capacity of the 

 trunk is increased by movements of the ribs, so that the aii- in 

 the elastic lungs is rarified, and external air rushes in through the 

 nares to equalise the pressure ; expiration is brought about 

 by reversed movements of the ribs. In the Chelonia, where the 

 ribs are immovable, inspiration results from the contraction of a 

 special diaphragm-like muscle in the 

 ibody-cavity (c/., the Mammalia). 



In the Crocodilia the inner nares open, as 

 aheady noticed, far back in tlie mouth. On 

 "the back of the tongue (Fig. 346) there is 

 a projecting, stiff, transverse fold, which, when 

 the mouth is opened, Hes against the palate 

 and separates a posterior cavity into which the 

 internal nares open above, the tracbea below. 

 In consequence of this arrangement the animal 

 can he in the water with its mou.th open wait 

 ing for prey, and if only the snout with the 

 •external nares is above the sui-faoe can breathe 

 quietly. 



In consequence of the development 

 of a neck, the heart in Reptiles is 

 further from the head than in Amphibia 

 and Fish. The atrium is separated 

 into a larger right, and a smaller left, 

 auricle, of which the latter receives the 

 blood from the lungs, the former, that 

 from the rest of the body. The ventricle 

 shows only the beginning of a division 

 into two, for the septum is still in- 

 complete ; in the Crocodilia alone the 

 right and left ventricles are completely 

 separated, and connected only with the 

 right and left auricles respectively, so 

 that the arterial pulmonary, and the 

 venous systemic blood are entirely 

 .separated within the heart. The conus is either rudimentary or 

 .absent, so that the arterial trunk springs direct from the ventricle. 



£ £ 2 



Kg. 347. Diasium of the 

 heart and artei-ial arches of a 

 Crocodile. a right, a! left 

 auricle, v and •"' right and left 

 ventricles, 1, 1' carotids (arterial 

 arch 1) ; 2, 2' right and left aortic 

 arches (arterial arch 2) ; c thin part 

 of 2', after giving off the vessel m 

 to the intestine ; 4, 4' pulmonary- 

 arteries (arterial arch 4), ao aorta. 

 — Orig. 



