201 



The Circulation" op Mixed Blood in the Embryo Mam- 

 mal and Bird, and in the Adult Reptile, Am- 

 phibian and Fish. 



By A. G. Pohlman. 



Our conception of the course of the blood through the heart of the 

 lower vertebrates appears to be based almost entirely on the conditions 

 found in the adult of the warm-blooded forms (birds and mammals). It 

 is well known that the adult bird and mammal possess a double circula- 

 tion, i. e., a cycle in which venous blood is propelled from the heart to be 

 returned oxygenated (pulmonary circulation), and one in which arterial 

 blood is expelled to be returned venous (systemic circulation). The 

 afferent and efferent vessels are in no way connected save through a capil- 

 lary system, and for this reason the heart may be divided into a right 

 or venous and a left or arterial heart. While the greater part of the 

 seventeenth century was occupied with the Harvey doctrine, the eighteenth 

 century found men equally engaged with the course of the blood through 

 the fetal heart. Three distinct theories were suggested before the begin- 

 ning of the nineteenth century — one based on alleged physiological ne- 

 cessity, a second on the anatomical relations found in the fetal mammalian 

 heart, and a third on the logical deductions from the differences between 

 the fetal and adult circulatory conditions. The differences between the 

 fetal and adult heart in mammals are, briefly, the right auricle receives 

 the precavals (venous) and the post-caval vein (V. cava inf.), which is 

 arterial ; a communication between the right and left auricle is present 

 (foramen ovale), and a connection is found between the heart efferents 

 (pulmonary artery and aorta) in the ductus arteriosus. 



The theory based on physiological necessity (von Haller-Sabatier) 

 was this : if the left heart in the adult is arterial, then the chances are it 

 must also be arterial in the fetus ; hence the oxygenated blood in the post- 

 caval vein must pass through the foramen ovale into the left heart. It 

 was further inferred that because the ductus arteriosus short-cut the 

 venous blood from the pulmonary artery into the descending aorta, the 

 vessels arising from the aortic arch would convey a better quality of blood. 



