414 ZOOLOGY FOR MEDICAL STUDENTS chap. 



is exceedingly short in the frog. Its cavity is divided into a ventral 

 systemic and a dorsal pulmonary portion by a horizontal partition. 

 This is continued at its cardiac end into the cavity of the large pocket- 

 valve, and it lies in the same plane as the front end of the spiral septum 

 seeing that the attachment of the latter to the conus wall is on the right 

 side. If this description has been followed it will be realized that the 

 dorsally placed pulmonary cavity of the ventral aorta is continuous with 

 that part of the conus cavity which at its headward end is dorsal but 

 at its cardiac end, owing to the spiral twist of the septum, on the right. 

 Consequently we will call this part of the conus cavity " pulmonary " 

 and the portion on the other side of the septum " systemic." This 

 systemic cavity of the conus is continuous at its headward end with the 

 systemic cavity of the ventral aorta. 



'^■' '^-Vt its headward end the ventral aorta becomes paired and each 

 branch gives off in the adult frog three aortic arches — III (Fig. 182, A, c), 

 IV {s) and VI (j>c) — which are, however, firmly bound together for some 

 distance so as to look in an ordinary dissection like a single vessel. An 

 important detail is that arch VI is continuous with the dorsal or pulmonary 

 cavity of the ventral aorta, while arches III and IV are continuous with 

 the ventral or systemic cavity. 



Having mastered the structural details of the heart it is possible to 

 follow its mode of functioning. The two auricles become gradually 

 distended with blood : the right auricle with venous blood received 

 from the tissues by the sinus venosus, the left with arterial blood from 

 the pulmonary vein. When the atrium has become fully distended its 

 walls contract and the blood is forced through the atrio-ventricular 

 opening into the ventricle — the venous blood from the right auricle 

 tending to occupy the right side of the ventricle, the arterial blood from 

 the left auricle tending to occupy the left side. The deep recesses in 

 the spongy wall of the ventricle help to keep the two kinds of blood 

 from mixing. When the ventricle has become distended with blood it 

 too contracts. The first result is the closure of the atrio-ventricular 

 opening, the valves being forced up into what in the diagram would be 

 a position at right angles to the page — the connective tissue threads 

 (" chordae tendineae ") checking their movement when the opening is 

 closed (Fig. 182, C). The ventricular cavity is now completely enclosed 

 except as regards the opening into the conus, the blood rushes forwards 

 through this, flattening the pocket-valves against the conus wall. It 

 first fills the systemic part of the conus into which the opening leads, 

 and distending the wall of the conus lifts it well clear of the edge of the 

 spiral septum, so that the pulmonary cavity becomes also filled with 



