LACERTA 



109 



effect is to increase the volume of the thoracic cavity and of the 

 lungs. Air then rushes in. 



Vascular System . — The heart consists of sinus venosus, 

 two auricles and a single ventricle. The truncus arteriosus 

 has been split into three right down to its base, so that the 

 ventricle opens directly into three arteries. The more ventral 

 of the three opens into the right side of the ventricle, and leads 

 to the lungs, dividing as it goes into two pulmonary arteries. 

 These no longer connect with the lateral dorsal aorta or systemic 

 arches. The pulmonary circulation is therefore distinct. 

 The other two vessels are the right and left systemic arches 

 (corresponding to the systemic arches of the newt and the 4th 

 arterial arches of fish). The right systemic arch springs from 

 the left side of the ventricle, and the left arch from the right 

 side of the ventricle. These vessels run up the 4th visceral 

 arch and join dorsal to the gut to form the dorsal aorta. Now, 

 the left side of the ventricle is occupied mostly by arterial 

 (oxygenated) blood from the left auricle and pulmonary veins ; 

 consequently the right systemic arch receives pure blood, more 

 or less. But the left arch and the pulmonary vessel are on the 

 right side of the ventricle, which contains venous blood from 

 the right auricle, sinus venosus, and the veins of the body. 

 In addition, there is an incomplete septum dividing the 

 ventricle, so that while the pulmonary artery receives venous 

 blood as would be expected, the left systemic arch receives 

 mixed blood. The carotid arches spring from the base of the 

 right systemic arch and therefore receive pure blood, as indeed 

 they need, for they supply the brain. The carotid arches run 

 dorsally in the 3rd visceral arch and when dorsal to the gut run 

 forwards into the head as the internal carotid arteries. In 

 addition, however, the carotid arteries connect back with the 

 systemic arches by what are really remnants of the lateral 

 dorsal aorta. These connexions are known as the ductus 

 caroticus. 



The arteries given off to the viscera are on the whole similar 

 to those of Triton. 



The venous system is likewise similar to that of Triton, but 

 it is necessary to mention three new points. In correlation 



