622 MR. 0. H. o'donoghue on the 



II. The Primary Carotid (Carotis primaria, Ratlike) is a short 

 trunk which quickly divides into two unequal branches : — 



i. The Thyroid Artery is the smaller of the two, and, in addition 

 to supplying the thyroid gland, it sends a twig to the 

 right thymus glands. This is the sole remnant of the 

 right common carotid. 



ii. The Left Common Carotid (Arteria carotis communis, 

 Rathke, Arteria cephalica, Schlemm) runs along the left 

 side of the oesojjhagus and trachea, to which it sends three 

 or four slender branches, until it reaches the posterior 

 region of the head. Here it divides into intei^nal and 

 external branches, and supplies the whole of both sides of 

 the head. (The distribution of this vessel in the head will 

 be dealt with later.) 



III. The Vertebral Artery (Arteria vertebralis, Rathke, Arteria 

 collai'is, Schlemm) arises from the anterior dorsal part of the right 

 carotid arch and runs forward, a little to the right of the vertebral 

 column about half-way to the head. It gives from three to 

 seven branches to the parietes and one or more to the oesophagus 

 before disappearing into the vertebral musculatui-e in the mid- 

 dorsal line. 



IV. Five Parietal Arteries are then given off. The first three 

 are very slender and close together, while the remaining two are 

 of the same size as the regular parietal arteries. 



V. One or two small (Eso2)hageal Arteries run to the oesophagus. 

 After this the two systemic arches unite to form a single 



vessel, the dorsal aorta. 



The Left Ductus Botalli is not completely closed up in the 

 course of development, and its proximal portion is to be found in 

 the adult animal as a cul-de-sac rvinning cranially from a point 

 low down on the root of the right carotid arch *. This remnant 

 varies somewhat in size in different individuals, and although 

 it is always more or less short, is of nearl}'^ the same calibre as 

 the right carotid. It is completely hidden by the left auricle, but 

 is readily seen if that body be carefully removed. From its 

 somewhat bluntly conical end comes off a thin strand of tissue 

 which runs forward into the left sj'stemic arch at the point where 

 it bends over to run backwards. This represents the closed part 

 of the left pulmonary arch, and is therefore the left ligamentum 

 Botalli, such as has been described by Brenner (11) and Hoch- 

 stetter (21). I find myself in agreement with the former author 

 also when he states that he was unable to find a right ligamentvim 

 Botalli in Tropidonotus natrix. 



According to Brandt (10), quoted also by Hoffman (23), there 



* Such a saccular appendage appears to be present generally in those snakes with 

 one lung suppressed, and has been recorded bj' Hochstetter (21) in Tropidonotus 

 natrix, T. tesselatus, Coluber cescuJapH, Coronella Icevis, Vipera berus, and 

 Cerastes vipera. 



