Arteries in Monsters of the Dicephalus Group 449 



The heart and anterior arteries of Teras XII present a striking simi- 

 larity to those of a normal pig of about the same age. With the exception 

 of a difference in size the hearts are identical. As in the normal pig 

 studied, the heart of XII lies within the mediastinum in ordinary rela- 

 tion to surrounding parts, and extends from about the 3d rib to the 7th. 

 It is a little larger, but the Teras as a whole exceeds the normal animal 

 somewhat in total length; the former, although a little younger, meas- 

 ures 36.5 cm. from tip of either snout along the mid-dorsal line to root 

 of tail, the latter 34 cm. In bulk the bodies are about equal. 



The longest lateral dimension of the heart, that is, from apex to dorsal 

 margin of the left auricle, is 43 mm. ; in the normal specimen it is only 

 35 mm. The diameter of the organ, measured on its anterior surface 

 across the ventricular border of the auricles, is 30 mm. while the same 

 diameter in the normal pig is but 35 mm. As would be expected, the 

 cavities of the two hearts are in proportion to their external dimensions ; 

 otherwise they present no appreciable differences. In both, the foramen 

 ovale and ductus Botalli are still present. 



The great vessels that issue from the base of the heart are normal 

 in relationship, and it is not until the origin of the external carotids 

 is reached that any striking differences are observable [Plate V]. The 

 right subclavian together with the right and left common carotids 

 arise from the crest of the systemic aorta by an innominate a centimeter 

 long. The left subclavian arises independently from the convexity of 

 the arch about 2 mm. from the innominate. Xear its beginning each 

 subclavian gives off a vertebral artery and the other customary branches 

 for this region. The common carotids pass in normal manner to the 

 componental head, each giving off an inferior thyroid soon after leaving 

 the innominate, and farther headward the superior thyroid. Slightly 

 anterior to the superior thyroid, the common carotid gives rise to the 

 internal carotid and the occipital by a common trunk from its dorsal 

 wall. The continuation of the parent artery now becomes known as the 

 external carotid. It is from this point anteriorly that variations from 

 the normal arrangement of vessels are obvious. Topographically the 

 arteries in this region have responded to the changed structural condi- 

 tions, but physiologically they maintain a normal value to the tissues 

 supplied by them, a statement which will be proved in a later para- 

 graph. 



For a full appreciation of the arterial condition in the dicephalons 

 region a brief comparison of its bony structure with that of a normal 



