Arteries in Monsters of the Dicephalus Group 407 



distance, it then crosses ventrally to the right aorta and enters the lung. 

 The left pulmonary branch passes more directly to its lung, but courses 

 parallel to the left aorta for a portion of the way. The artery arising 

 apparently next to the pulmonary is the left aorta and the third vessel 

 to take origin from the ventricle is the right aorta ; each crosses the other 

 close to the heart. Thus it is seen that in turtles (and all other rep- 

 tiles) two aortic arches are present, in mammals only one. 



The right aorta arches over the right pulmonary artery and bronchus, 

 and receives a communicating branch from the left aorta about opposite 

 the fourth dorsal vertebra. The left aorta passes round the left bronchus 

 and pulmonary artery to a dorsal position and ends by dividing into 

 three main trunks, a gastric artery, superior mesenteric, and a gastro- 

 duodenal artery. A portion of the gastric artery continues as the com- 

 municating branch to the right aorta. In the plate these branches have 

 been omitted in order to render the illustration more simple. 



The right aorta is covered close to the heart by an innominate artery, 

 which it gives off. The innominate divides almost immediately into the 

 right and left subclavians, and the right and left common carotid 

 arteries. The subclavians are at first ventral to the carotids, but soon 

 diverge. Each gives off a minute thyroid, an ascending cervical, a 

 vertebral, axillary and brachial arteries. The remainder of the sub- 

 clavian continues posteriorly along the edge of the carapace in the 

 hollow of the marginal plate as the internal mammary, and anastomoses 

 with the epigastric. It was impossible to follow all of these subclavian 

 branches in Teras XX, but otherwise Plate YII demonstrates a perfectly 

 normal arterial distribution to each component, the only connection be- 

 tween the duplicated parts being through a short communicating branch 

 between the inner subclavians (Plate VII, x). 



If the vascular condition in the double lamb be recalled and a com- 

 parison be made between it and the two-headed turtle, it is at once 

 obvious that in the latter the division of the component parts of the 

 heart and anterior arteries has progressed farther. In the lamb only the 

 bulbus arteriosus had divided, but in the turtle the division has extended 

 through the common ventricle, auricle and sinus, thus dividing the heart 

 into two identical halves, or in other words, forming two distinct hearts, 

 each destined to give a normal arterial supply to an anterior moiety of 

 the monster, and retaining only a slight communication with each other 

 through a small branch from the inner subclavians. 



