Arteries in Monsters of the Dicephaliis Group 457 



parison with that of Teras XII is more anterior in position. A per- 

 pendicular lateral line from apex to middle of the base of the organ 

 passes through the second intercostal space. Thus it is seen that its 

 longitudinal axis is nearly at right angles to the median axis of the body, 

 while that of the double pig is directed antero-posteriorly. The apex 

 of the heart lies a little to the left of the mid-line of the body against 

 the sternal wall, which at this point presents externally a slight bulging. 



The exterior of the heart appears in general quite normal, but its 

 interior and the great vessels arising therefrom present some interesting 

 variations, traceable to early embryonal conditions as will be shown later. 

 The right ventricle is small in proportion to the size of the heart, occupy- 

 ing about a third of the ventricular mass. The left ventricle is cor- 

 respondingly larger, occupying the remainder of the ventricular portion. 

 It is a curious fact that the ventricles (and the auricles) of Teras XV 

 have seemingly changed position with those of a normal heart, for in 

 the Teras the right ventricle has the position and extent of a normal left 

 ventricle, and vice versa for the left ventricle (and auricle). The right 

 ventricle has a very thick wall, equal to that of the left ventricle. Pos- 

 sibly the greater extent, and therefore the greater capacity, of the right 

 ventricle and the greater thickness of its walls are compensations for its 

 lack of size. It has an extremely posterior position, so much so that in 

 a median longitudinal section it is not visible at all, but shows only in 

 cross section. The altered position of the ventricles suggests an unusual 

 twisting during the early development of the organ, but the heart gives 

 no other evidence of such a proceeding. The right auricle is propor- 

 tionally smaller than the left, and each opens into its respective ventricle 

 in normal manner. A partially obliterated foramen ovale is still present. 

 So far as position and extent are concerned, it would be very easy to 

 mistake the right ventricle for a normal left and the left for a normal 

 right, but the auricles and the great blood vessels that communicate 

 with them respectively leave no room for error in identification. In the 

 following description I shall use Chauveau's nomenclature, since the 

 origin of the arteries from the heart differs somewhat in lamb from that 

 in pig, and a slightly different nomenclature is used. The differences are 

 those characteristic of ruminant and non-ruminant animals. 



In a normal lamb the first part of the systemic aorta is termed the 

 aortic trunk. This arises from the basal portion of the left ventricle and 

 very soon divides into an anterior and posterior aorta. The anterior 

 aorta is short and terminates in the right and left brachial (sub-clavian) 

 arteries. The right subclavian, near its divergence from the left, gives 



