420 



REPTILES 



xv. 14- 



The soft parts of the crocodiles are of special interest because croco- 

 diles are, except the birds, the only living creatures closely related to 

 the great group of dinosaurs. The heart (Fig. 245) shows a complete 

 division of the ventricle, but there are still two aortic arches. The 

 truncus arteriosus is divided in a spiral manner to its base, so that 

 the aortae cross and the right arch opens from the thick-walled left 



car int. 



car. cxt 



car. int. 



I 1 1 1 „ car. ext 



v.c. 

 Che Ionian 



Crocodile J 



Fig. 244. Fig. 245. 



Diagram of heart and arterial arches of a chelonian and of a crocodile, seen from below. 

 Lettering as Fig. 215, p. 379. (From Ihle, after Goodrich.) 



ventricle, while the left opens with the pulmonary arteries from the 

 weaker right ventricle. The left arch would therefore contain venous 

 blood, but an aperture, the foramen of Panizza, connects the two 

 arches near the base and presumably the higher pressure in the left 

 ventricle ensures that the left arch receives at least some oxygenated 

 blood. Possibly, however, the pressure in the right ventricle is in- 

 creased when the crocodile dives and the blood flows through the 

 foramen from right to left. 



The lungs are well developed, having a system of tubes ending in 

 sacs. A transverse partition separates off a thoracic from the main 

 abdominal cavity. This 'diaphragm' is not itself muscular, but is 

 continued into a diaphragmatic muscle attached to the abdominal 

 sternal plates. This muscle, innervated by abdominal spinal nerves, 



