REPTILIA 



243 



ing the islands of hard horn. On the back and down the tail the 

 scales are supported by bony cores, and the principal scale rows 

 are keeled, giving a ridged effect to the middle of the back and tail. 

 The hide is used extensively in commerce. 



The tongue is flat and thick and incapable of protrusion. The 

 lungs are large and better developed than in other reptiles. The 

 feeih are large and formi- 

 dable and^very irregu- 

 larly arranged? Though 

 the mouth is provided 

 with very powerful mus- 

 cles for closing the jaws, 

 those for opening them 

 are very weak, so that 

 a man can easily close 

 with his hands and keep 

 closed the jaws of a large 

 specimen. The heart 

 (Fig. 135) and vascular 

 system is more advanced 

 in the crocodiles than in 

 any other living reptiles, 

 for the ventricle is 

 almost completely di- 

 vided by a septum into 

 a right and a left cham- 

 ber, leaving only a 

 small foramen between. 

 Thus there is practically 

 a complete separation 

 of venous and arterial blood, as in the warm-blooded vertebrates.. 

 Though both right and left aortic arches are functional, the left 

 arch as relatively somewhat reducedv 



The brain (Fig. 136) is decidedly advanced in structure for a rep- 

 tilian brain, the large cerebral hemispheres being especially note- 

 worthy. The tympanic membrane is sunk in a pit, a tendency that is 

 carried much further in the birds and mammals. It will thus be 

 seen that the crocodiles have followed part way several of the evolu- 

 tionary paths that have been carried out fully by the birds. 



FIG. 135. Heart of Crocodile with the principal 

 arteries (diagrammatic). The arrows show the 

 direction of arterial and venous currents. I. aort, 

 left aortic arch; I. am, left auricle; L aur. vent. 

 ap, left auriculo- ventricular aperture; L car, left 

 carotid; L sub, left subclavian; I. vent, left ventri- 

 cle; pul. art, pulmonary artery; r. aort, right 

 aortic arch; r. aur, right auricle; i. aur. vent, ap, 

 right auriculo- ventricular aperture; r. car, right 

 carotid; r. sub, right subclavian; r. vent, right ven- 

 tricle. (From Parker and Haswell, after Hert- 

 wig.) 



