172 ZOOTOMY. 



walled chambers, situated anterior to the ventricle, to the ba^ 

 of which they are united. 



199. The three great arteries, springing from the 

 base of the ventricle, and passing forwards between the 

 auricles, closely bound together with connective tissue : 

 when the latter is dissected away they are seen to have a 

 twist to the left. Of the three, the pulmonary artery 

 (Figs. 43 and 44, p.d), lying to the animal's left, and the 

 left aorta il.ao) to the right, are situated ventrally at their 

 origin, while the right aorta {r.ad) is dorsal to the other 



Fig. 43. — Lacerta viridis. The heart, from the dorsal side (X 3). 



cr, carotid artery : e.jii, external, and i.jti, internal jugular vein : Lao, 

 left aorta : l.au, left auricle : p.a, pulmonary artery : pr.c, riglit, and 

 pr.(^, left, precaval vein: pt.c, postcaval vein : /.z', pulmonary vein: 

 r.ao, right aorta : r.au^ right auricle : s.au, sinu-auricular aperture : sxl, 

 subclavian vein : s.v, sinus venosus, cut open to show the sinu-auricular 

 aperture : v, ventricle. 



two ; traced forwards the pulmonary artery is seen gradually 

 to assume a dorsal position, the left aorta becoming ventral 

 and to the left, the right aorta ventral and to the right. 



200. The sinus venosus {s.i), a chamber situated to 

 the dorsal side of the right auricle, and seen by turning the 

 heart aside ; it receives the three great veins ; the right 



