THE VASCULAR SYSTEM 1 53 



demonstrated that the Purkinji fibers were parts of the bundle 

 of His and in 1908 Retzer proposed the name sinoventricular 

 system for the entire apparatus and suggested that it is neuro- 

 muscular in nature. Miss Lydia Dewitt in 1909 succeeded in 

 making a very complete reconstruction of the entire system in 

 man, dog, sheep, calf and cat. The system is of interest because 

 it seems to have to do with the coordination of atrial and ventri- 

 cular rhythm and because of its relations to certain pecu- 

 liar pathological conditions, such as Stokes-Adams disease. 

 Whether it may originate rhythmic impulses in the heart is 

 still a problem for investigation. 



The atria are very small when not injected, and may by 

 the beginner be cut away with the pericardium and surround- 

 ing adipose tissue. The right atrium receives three veins, the 

 superior vena cava, inferior vena cava and coronary sinus, all 

 of which enter its dorsal aspect. The slit-like opening of 

 the coronary sinus is guarded by the semilunar valve of Thehesiiis^ 

 valvula sinus coronarii (Thebesii). The portion of the atrium 

 into which the above veins open is the sinus venosus. At the 

 dorsal part of the septum which divides this atrium from the 

 adjoining one is an oval depression, the fossa ovalis, where, 

 in an embryo, the two atria communicated. The left atrium 

 is in contact with the right dorsally, and receives three pul- 

 monary veins (Fig. 77). 



The right ventricle is smaller than the left. It makes a half 

 spiral turn about the left and terminates at its cranial end into 

 the pulmonary artery. The portion between the atrioventri- 

 cular opening and the pulmonary artery is known as the conus 

 arteriosus. Between it and the pulmonary artery are three 

 pocket-like semilunar valves which prevent the return of the 

 blood. Between each valve and the wall of the artery is a 

 pulmonary sinus, or sinus of valsalva. The left ventricle gives 

 origin to only one important vessel, the aorta, which arches 

 dorsally around the left atrium, and at the apex of the arch 

 gives off two branches, the innominate, and the left subclavian. 



