VASCULAR SYSTEM. 



573 



Embedded in the mesentery below the stomach lies the 

 rounded spleen. A whitish thyroid gland lies on the ventral 

 surface of the trachea a short distance in front of the heart. 



Vascular system. — The heart is completely enveloped by 

 the pericardium, and is three-chambered, consisting of two 

 thin-walled auricles and a muscular ventricle. From the 

 ventral surface of the ventricle arises the conspicuous 

 truncus arteriosus, which is formed by the bases of the 



J?,! 3, 



Fig. 249. 



-Heart and associated vessels of a lizard. 

 After Nuhn. 



A., Right auricle; jugulars (/.), subclavians (Sc.v.), and inferior 

 vena cava (I.V.C.) enter it. V., ventricle ; tr., truncus arteri- 

 osus ; 1, first aortic arch giving off carotids ; 2, second aortic 

 arch; p. a., pulmonary artery; Sea., subclavian; Ao., dorsal 

 aorta. ^4!., left auricle ; pulmonary veins (/.».) enter it. In 

 the lizard described, the left jugular is not developed. 



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 connect- 

 ing 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 



