CIRCULATORY SYSTEM OF THE GRASS-SNAKE, 621 



of Ratbke), which remains short as in most snakes *. After the 

 common carotids have split into internal and external carotids, 

 each internal vessel gives off a branch which enters the neural 

 canal with the iirst spinal nerve and joins the basilar artery on 

 the ventral side of the nerve-cord. These branches quickly 

 widen out and, forming as they do an anastomosis between the 

 two internal carotids, make it possible for the right common 

 carotid to degenerate. This it does completely t from the point 

 where it divides into internal and external branches down to 

 close to its union with the left common carotid, but the last part 

 of it remains and is to be found in the adult as a small artery 

 supplying the Thyroid gland (text-fig. 89). 



The Systemic Arch undergoes veiy little change during 

 development. 



The Pulmonary Arch degenerates almost completely on the 

 left side. In conjunction with the suppression of the left lung 

 in Tropidonotus, we find that the pulmonary branch of the 6th 

 arch is only developed on the right side. In the adult only one 

 pulmonary artery is to be found. 



(B) Adult Form, 

 (a) Anterior Vessels. 



The Left Aorta bends dorsally around the cesophagus and 

 trachea and then postei'iorly to unite with the right aorta in the 

 mid-dorsal line. During this course it gives off two very small 

 branches to the oesophagus, but none whatever to the parietes. 



The Right Aorta takes a corresponding course on the other side, 

 during which it gives off the following branches : — 



I. The Left and Right Coronary Arteries arise behind the two 

 semi-lunar valves which guard the base of the aorta. The right 

 coronary artery runs in the groove between the auricle and 

 ventricle, and is the chief supply of the dorsal surface of the 

 heart. The left passes around the base of the pulmonary artery 

 and spreads out over the ventral side of the heart. 



* In some snakes it is absent altogether, so that the conimou carotids come off 

 separately from the sj^stemic arch, e. g. JBoa. 



t It is interesting to note, however, that in some variations oi T. natrix this 

 does not occur, and so the two common carotids persist in the adult. The first 

 specimen of T. natrix that I examined was in this condition, although in all other 

 respects it appeared a perfectly normal adult male. The two common carotids, left 

 and right, sprang from a common stem, the primary carotid, and were about equal 

 in calibre. On the left side the carotid pursued a normal course. The abnormal 

 right carotid passed ventral to the oesophagus, just behind the thyroid gland, to 

 which it sent a small branch, over to the right side of the neck. From here up to 

 the posterior end of the skull it followed a similar course to its fellow^ on the left. 

 Unfortunatel.v the vessels of this specimen were not injected, so that the relation of 

 the persistent right carotid to the basilar artery could not be ascertained. However, 

 this apparent anomaly in the arterial system is quite readily understood in the light 

 of the developmental history of these vessels. Only one other example of this 

 peculiar abnormality seems to have been described before, and that by Van Bemmelen 

 (9), but in this case the right carotid was only a fine tube. 



