458 Mabel Bishop 



off anteriorly a short trunk, the cephalic, which in turn divides into the 

 right and left common carotids. The main difference to be noted in this 

 condition and that found in normal pig (also in Teras XII) is that in 

 the latter no anterior aorta exists, since the sub-clavians arise indepen- 

 dently from the aortic arch. The common carotids pass to the head in 

 normal manner, giving off en route the customary branches. 



The systemic aorta of Teras XV arises as an aortic trunk from the 

 basal portion of the left ventricle, and from the first is directed slightly 

 dorsally and posteriorly, thus beginning immediately at its origin to arch 

 gently backward. It curves over the root of the left lung and crosses the 

 trachea and oesophagus diagonally to the right. About opposite the third 

 rib the arch is completed, the aorta resumes a median position and con- 

 tinues backward as the posterior aorta, lying along the mid-ventral sur- 

 face of the vertebral column deep within the dorsal mediastinum and 

 giving off in its course the usual intercostals. The aortic trunk is con- 

 spicuously large (diameter 15 mm.), but tapers gradually as it is con- 

 tinued backward into the posterior aorta whose uniform diameter is 

 11 mm. 



The pulmonary artery arises from the basal end of the right ventricle, 

 but owing to the position of the latter, the origin of the pulmonary trunk 

 is immediately posterior to that of the systemic aorta, and not anterior to 

 it as it would be normally. Therefore the convexity of the heart between 

 the anterior margins of the auricular appendages is pronounced in Teras 

 XY by the systemic aorta, and not by the pulmonary artery. Also 

 as a result of the position of the right ventricle, the pulmonary artery 

 of Teras XV courses parallel to the aorta on its posterior side instead of 

 accompanying it on its right side, and is covered in part by the right 

 auricular appendage, normally by the left. About a centimeter and a 

 half from its margin the pulmonary artery bifurcates into a right and left 

 branch, each of which redivides into smaller branches at the root of the 

 lung and enters it together with the bronchi. Near the crotch of the 

 bifurcation each pulmonary branch gives off twigs to the bronchus. The 

 trunk of the pulmonary still retains a well defined ductus Botalli, six 

 millimeters in diameter, which unites with the posterior aorta a few 

 millimeters posterior to the anterior aorta. Between the two the aortic 

 arch presents a left lateral constriction, analogous to the "aortic isthmus" 

 in human beings.^ The aortic trunk is short and at its posterior ex- 



^Quain, Elements of Anatomy, Vol. II, Part II, Angeiology, p. 384. The 

 diameter of the aorta at the isthmus in Teras XV is 9% mm., and imme- 

 diately anterior and posterior to it 12 mm. 



