LIZARDS. 575 



which is formed by the bases of the aortic arches, and 

 exhibits a division into two parts. From the more ventral 

 part arises the left aortic arch, which curves round to the 

 left side, first giving off a short connecting vessel (ductus 

 Botallii} to the carotid arch. From the other division of 

 the truncus arteriosus, a great arterial trunk arises, and this 

 gives off the right aortic arch and the right and left carotid 

 arches. The right aortic arch sends a ductus Botallii to the 

 carotid arch of the right side, and then curves round the 

 heart to join the left arch, when the two form the dorsal 

 aorta. The carotid arches supply the head region with 

 blood. From the base of the truncus arteriosus, the right 

 and left pulmonary arteries also arise. 



From the right aortic arch as it curves round, arise the right and left 

 subclavian arteries, which carry blood to the fore limbs. A coeliaco- 

 mesenteric artery arises from the dorsal aorta and supplies the viscera. 

 Smaller vessels are also given off to the genital organs, &c., and then at 

 the anterior end of the kidneys, the aorta divides into two femoral 

 arteries which break up into a network of small vessels, supplying hind 

 limbs and kidneys, and finally, at the posterior end of the kidneys, re- 

 unite to form the caudal artery, which runs down the tail. 



The blood from the anterior region of the body is returned to the 

 heart by the right and left precaval veins or superior venae cavse. The 

 right precaval is formed by the junction of external and internal 

 jugulars with the subclavian vein ; on the left side the jugular is absent. 

 From the posterior region of the body, blood is brought back by the 

 postcaval vein or inferior vena cava. The three great veins open into 

 a thin-walled sinus venosus, which opens into the right auricle. 



The postcaval is formed by the union of two veins which run along 

 the genital organs, and receive renal veins from the kidneys. In passing 

 through the liver the postcaval receives important hepatic veins. 



From the tail region the blood is brought back by a caudal which 

 bifurcates in the region of the kidneys into two pelvics. The pelvic 

 veins give off renal portals to the kidneys, and receive the femoral and 

 sciatic veins from the hind limbs. They then unite to form the 

 epigastric or anterior abdominal, which, carries blood to the liver. 

 Except through the medium of the renal-portal system, there is no 

 connection between the anterior abdominal and the postcaval. To the 

 liver blood is carried as usual from the stomach, c., by the portal 

 vein. 



From the lungs blood is brought to the left auricle by the pulmonary 

 veins. 



A lymphatic system including a pair of lymph hearts is present. 



Respiratory System. 



The lungs are elongated oval structures which taper away 

 posteriorly. The mouth does not, as in the frog, play any 



