VIII.] THE SEPTUM OF THE BULBUS ARTERIOSUS. 215 



below (ventral), to the right and above (dorsal). The septum 

 separating these two channels, according to Dr Tonge (Proc. 

 Roy. Soc. 1868), commences as an outgrowth from the wall 

 between the fourth and fifth pair of arches, and is so arranged 

 that the channel from the left ventricle communicates with 

 the third and fourth pairs of arches only, and that from the 

 right ventricle with the fifth pair only. 



According to Dr Tonge's view the two channels after the completion of the 

 septum do not communicate with each other at any point, and are completely 

 shut off from each other at their ends. Von Baer believed that the two 

 channels (into which the bulbus arteriosus is divided by the septum) opened at 

 their distal terminations into a common trunk, but that the direction of the 

 openings of the two channels caused the two streams of blood from them to 

 enter into different arterial arches. According to him the stream from the 

 channel communicating with the left ventricle is directed so as entirely to 

 miss the last pair of arterial arches, and to fall into the third and fourth pairs, 

 while that from the right ventricle enters the fifth pair alone. 



One result of this arrangement is that all the blood which 

 passes to the anterior extremity of the body, comes from the 

 left ventricle of the heart. 



At about the seventh day an entire separation begins to 

 take place between the arterial roots which come respectively 

 from the right and left chambers of the heart. The root 

 from the right chamber (Fig. 66, R.P.A.) remains connected 

 with the fifth pair of arches. The root from the left ventricle 

 is connected with the third and fourth pairs of arches. 



According to Von Baer the right arterial root is connected with the fourth 

 arch on the left side, and the fifth arch on the right side. He also believed 

 that the fifth arch on the left side has by this day altogether disappeared. 

 According to his view, therefore, the pulmonary arteries are derived from 

 both the fifth and fourth pairs of arches. Rathke (Denlschriften der AJcademie 

 zu Wien, 1857, -Bd. xin.) however altogether denies this, and states that both 

 pulmonary arteries are derived from the fifth pair of arches alone. We have 

 in our account followed Rathke's statements. 



The lower part of the body still receives blood from both 

 the right and left ventricles, since the blood which enters 

 the fifth arch still flows into the common dorsal aorta. 



As the lungs however increase in size, a communication 

 is set up between them and the fifth pair of arches in the 

 shape of two vessels which, springing one from the arch of 

 each side, grow downwards towards the lungs. At first 

 small and narrow, these pulmonary arteries, for such they 

 are, grow rapidly larger and larger, so that more and more 

 of the blood from the right ventricle is carried to the lungs. 



