270 COMPARATIVE ANATOMY. 



This condition of things persists throughout life in many 

 Fishes, and is at any rate passed through in the individual 

 development of all Vertebrates. 



With the appearance of pulmonary respiration, important 

 changes take place in the primitively simple heart, which finally 

 result in the formation of a septum in both atrium and ventricle, 

 and thus lead to the presence of two atria or auricles, and two 

 ventricles : the conus arteriosus and sinus venosus become even- 

 tually incorporated in the ventricles and right auricle respectively. 

 Thus a right (venous) and a left (arterial) half can be distin- 

 guished, and a new vessel, the pulmonary artery, arises from 

 the right ventricle ; this conveys venous blood to the lungs, while 

 special vessels (pulmonary veins) return the oxygenated blood 

 from the lungs to the left side of the heart, from which it passes 

 into the general circulation of the body. 



The gradually increasing complications which occur in passing from the 

 lower to the higher Vertebrates will be more easily understood after the 

 condition of the circulation in the embryo has been considered. 



THE FCETAL CIRCULATION. 



In an early stage, the bulbus arteriosus (Fig. 215, JB) is 

 continued forwards towards the head by a long unpaired vessel 

 (branchial artery), which gives off right and left a series of 

 symmetrical transverse branches or arches (Afy, each of which 

 runs between two consecutive gill-clefts (KL\ After the first pair 

 has given off branches to the head (carotids), they all unite 

 above the clefts to form a longitudinal trunk on either side (S, S l ). 

 These branchial veins give rise further back to the right and 

 left roots of the aorta (RA, RA}. 



In all Vertebrates the aorta (A) is throughout life the most 

 important artery of the body ; it extends backwards along the 

 ventral side of the vertebral axis as a large unpaired trunk, which 

 gives off numerous branches, and forms the caudal artery in the 

 tail (Acd). 



The vitelline or omphalo-mesenteric arteries (Fig. 215, 

 Am, and Fig. 216, R.Of.A, L.Of.A), which are very important 

 up to a certain stage in development, arise from the aorta, and 

 carry blood to the surface of the yolk, whence it is returned by 

 means of the vitelline or omphalo-mesenteric veins (Fig. 216, 

 R.Of, L.Of}. In embryos of Fishes these open into a subintes- 

 tinal vein, lying on the ventral side of the alimentary tract, and 

 opening into the heart : in the higher forms this vein is repre- 

 sented by the caudal vein and the ductus venosus (Figs. 217 

 and 218, D V] ; the latter passes through the liver on its way to the 

 heart, but disappears on the formation of the portal circulation. 

 Before passing into the sinus venosus the blood becomes mixed 



