696 



MANUAL OF PHYSIOLOGY. 



The aortic bulb likewise, by a projection from the inner wall 

 of the cavity, becomes divided into two canals, the anterior of 

 which remains in continuity with the right ventricle, and the 

 posterior canal is continuous with the left ventricle. The anterior 

 thus becomes the pulmonary artery, and the posterior the perma- 

 nent aorta. 



The primitive circulations of a human embryo may be divided 



FIG. 284. 



FIG. 284. Development of the heart in the human embryo, from the fourth to the 

 sixth week. A. Embryo of four weeks. (KOlliker, after Coste.) B. Anterior and, C, 

 posterior yiews of the heart of an embryo of six weeks. (KOlliker, after Ecker.) a. 

 Upper limit of buccal cavity. 6. Buccal cavity, c. Lies between the ventral ends of 

 the second and third branchial arches, d. Buds of upper limbs, e. Liver. /. Intestine. 



1. Superior vena cava. ]'. Left superior vena cava or connection between the left 

 brachio-cephalic vein and the coronary vein. 1". Opening of inferior vena cava. 



2. 2'. Right and left auricles. 3. 3'. Right and left ventricles. 4. Aortic bulb. 



FIG. 285. Human embryo of about three weeks. (Allen Thomson.) uv. Yelk sack. 

 dl. Allantois. am. Amnion. ae. Anterior extremity, pe. Posterior extremity. 



into two, which differ in their time of appearance and in the 

 accessory organs to which they are distributed. Though they 

 may, for the sake of clearness, be described as two independent 

 circulations, they are not strictly so, as they exist for a short 

 time coincidently, and arise in connection with one another from 

 the same heart. 



