S7^' 



THE FROG. PHYLUM CHORDATA 



x:a. 



The openings between these chambers are guarded by certain 



v-ilves or folds of the hning of the heart. Two simple lips of the 



opening between the sinus and right auricle are the smu-auricular 



* valves ; these allow blood 



to flow into the auricle, but 

 when it tends to flow the 

 other way fold over and 

 meet to oppose it. The edge 

 of the auricular septum is 

 cleft and projects into the 

 ventricle as two flaps, the 

 auriculo-ventricular valves. 

 Each of these is connected 

 with the walls of the ventricle 

 by fine cords, the chordae 

 tendinese, and thus, while 

 blood can pass from auricles 

 to ventricle, its reflux is 

 prevented by its raising the 

 valves, which are kept from 

 turning back into the auricle 

 by the chordae tendinese. The 

 opening from ventricle to 

 truncus is guarded by three 

 semilunar valves, shaped like 

 watch-pockets, which are 

 spread out by any reflux of 

 blood, so that by meeting one 

 another they stop it. The 

 truncus arteriosus is divided 

 internally by a second ring 

 of semilunar valves into two 

 unequal parts, a long conus 

 arteriosus next the ventricle, 

 and a short ventral aorta. The 

 conus is incompletely divided longitudinally by a spiral valve into 

 an aortic part, which begins dorsally and curves round by the 

 right to become ventral, and a pulmocutaneous part, which begins 

 ventrally and curves round by the left to become dorsal. The 

 ventral aorta is completely divided into a dorsal and a ventral 

 ^ber by a septum which ends towards the heart by cutting 



Fig. 288. — A ventral view of the heart of a 

 frog, opened to show the internal 

 structure. The ventral wall of the 

 truncus, ventricle, and auricles has 

 been removed, with part of the spiral 

 valve. 



au.v., Auriculo-ventricular valves ; c.a., carotid arch ; 

 c.ao., cavum aorticum ; c.pu., cavuin pulmocu- 

 taneum ; ch.t., chordae tendineae ; l.au., left auricle ; 

 o.p.v., opening of pulmonary vein ; o.pc. opening 

 of dorsal division of the aorta ; pea., pulmo- 

 cutaneous arch ; r.au., right auricle ; s.au., sinu- 

 auricular opening with valves ; si., first row of 

 semilunar valves ; si'., second row of semilunar 

 valves ; sl'.i, the semilunar valve from which the 

 spiral valve starts ; s/'.2, small semilunar valve at 

 end of cavum pulmocutaneum ; si'. 3, a small part 

 of a large semilunar valve, of which the rest extends 

 across that portion of the front wall of the truncus 

 which has been removed ; sp.v., spiral valve ; sy.a., 

 systemic arch ; tr.a., wall of truncus arteriosus; 

 tr"., one of the two bundles of arteries into which 

 the truncus divides ; v., ventricle. 



