CIRCULATORY SYSTEM OF THE OllASS-SNAKE. 623 



is present in the Grass-Snake a solid strand of tissue joining the 

 primaiy carotid to the transverse pai-t of the left aortic arch and 

 called by him the ligamentum caroticum. This, he states, in 

 exceptional cases may remain open and may then be described as 

 a ductus caroticum, and is a vestigeal structure somewhat similar 

 to the ductus Botalli. In the hearts I have examined no trace of 

 this vessel or cord could be found, and, indeed, no such connection 

 exists in the course of embryonic development, as a glance at text- 

 figs. 88 and 89 will show. A connection between the carotid and 

 left aortic arches is present in the embiyo, but in the adult it 

 would run fi'om the dorsal part of the left systemic arch along the 

 wliole length of the neck up to the point of origin of the internal 

 carotid (text-fig. 89). This, however, does not fit in with Brandt's 

 description of the ligamentum caroticum. 



(b) Posterior Vessels. 



The Right Puhnonary. Artery (Ai'teria pulmonalis, Schlemm) 

 arises separately from the ventricle and leaves the heart the most 

 dorsal of the three arterial roots. It runs backwards alongside 

 the esophagus almost parallel with the right systemic over which, 

 however, it passes ventrally, and then runs dorsal of the post- 

 caval vein to the anterior end of the lung. As it passes along 

 the right border of that organ it gradually gets smaller and 

 smaller until it disappears as a distinct vessel at the level of the 

 posterior end of the liver, although the lung is continued on for 

 some distance. 



In correlation with the suppression of the left lung no left 

 pulmonary artery is found at any time. 



The Right and Left Aortic Arches unite posterior to the heart to 

 form the dorsal aorta. 



The Dorsal Aorta runs in the body-cavity just ventral to the 

 vertebral column, back to the level of the cloaca. Just posterior 

 to this it leaves the body-cavity and enters the hfemal canal, and 

 in this is continued along the tail as the Caudal Artery. During 

 its course through the coelom it gives oft' a number of branches. 



The Parietal Arteries form a numerous and more or less regular 

 series of branches going to the body-wall, of which there are 

 about tv/elve up to the point of origin of the superior mesenteric 

 artery. These arteries enter the body-wall in the mid-dorsal line, 

 a characteristic of most colubrine snakes, and do not split into 

 two before so doing, as in the pythonine snakes [cf. Beddard, 

 4 and 1). 



The (Esophageal and HejxUic Arteries. — In front of the liver 

 the dorsal aorta gives ofi' two or three slender branches to the 

 cesophagus. After these come a series of common trunks, about 

 fifteen in number, which divide into two branches, one going to 

 the liver and one to the oesophagus or posteriorly to the stomach. 

 The last of this series is considerably larger than the others and 

 has more branches, some of which go to the anterior end of the 

 stomach. 



