320 Dr Allen Thomson on the Vascular System 



cd by any fold in the paries, like that of the auricle, but re- 

 sembles rather some of the larger columnae carneae of the adult 

 heart, except in the softness of the bundles of fibres of which it 

 is composed. It forms acrescentic arch, the convexity of which 

 looks downwards, and is attached on the lower and right side, 

 and anterior and upper side, of the common ventricle. On the 

 sixth day, in the goose, the septum increases in size, and can 

 be displayed by making a vertical section of the ventricles, as 

 in Fig. 20. d. C. The cavity of the right ventricle is now consi- 

 derably increased, and the septum is found to occupy the apex, 

 and whole anterior part of the ventricle, but is scarcely, if at 

 all, attached to the posterior surface. Here, as well as at the 

 openings of the auricles and bulb of the aorta, the septum is 

 quite free, and a hair may with ease be introduced from the left 

 into the right ventricle (Fig. 20. C). After six days and a half 

 incubation, the septum is still stronger than before (^;,Fig. 21. C), 

 and rises higher up in the ventricle, till very soon it reaches the 

 entrance of the bulb of the aorta. Under the mouth of the 

 bulb, a communication continues to exist for about a day or 

 more longer, until the roots of the aorta proper and pulmonary 

 arteries, are formed by the junction of the opposite walls of this 

 vessel. Before this takes place, however, the bulb is much 

 shortened. At the back part of the ventricles, again, where 

 they communicate with the auricles^ the septum increases till it 

 reaches the level of the opening of communication: here it 

 becomes united with the anterior and posterior prolongations 

 of the septum of the auricles; and some time after, the 

 valve which closes the foramen ovale being formed, the whole 

 heart is divided into two cavities, no longer communicating 

 with one another. Before the union of these septa has taken 

 place, while the four cavities of which the heart consists yet 

 communicate freely with one another, the valves of the auri- 

 culo-ventricular orifices are partly formed (i ?, Fig. 23). In 

 the goose of the sixth day, these valves consist of two folds 

 of the internal wall of the auricle, which hang down into the 

 ventricle, one on the anterior, the other on the posterior edge of 

 the opening. As development proceeds, and the union of the 

 septa takes place, each of the depending plates is divided into 

 two, leaving the half on each side for the formation of the valve. 



