XI 



HEART OF THE FROG 



413 



The atrial septum at its ventricular end terminates in a concave free' 

 edge which merges dorsally and ventrally into the substance of 

 the large atrio-ventricular valve^ its line of attachment dividing the free 

 face of the valve into a larger right-hand and a smaller left-hand 

 portion. 



The wall of the ventricle shows a much greater thickness of muscle- 

 bundles than that of the atrium. They run in all directions through the 

 ventricular cavity, forming a thick open spongework which leaves only 

 a comparatively small part of the cavity at its headward end unencroached 

 upon. Towards the left this open space has on its headward side the 

 wide atrio-ventricular opening, while on the extreme right is the much 

 smaller opening leading into the conus (Fig. 182, A, h). 



U«ff. 



~-a.u.L,. 



Fig. 182. 



Heart of a Frog. B, Ventricular end of conus slit open to stiow the pocket-valves ; C, atrio- 

 ventricular valves (closed), etc., as seen in a heart cut transversely through the auricles and conus 

 and viewed from the headward side. a.v.D, Dorsal atrio-ventricular valve ; a.v.L, left ditto ; 

 a.v.R, right ditto; a.v.V, ventral ditto; C, conus; s.a, atrial septum; s.c, septum of conus; 

 i, ^, 3, pocket-valves at ventricular end of conus. 



The conus arteriosus runs obliquely from its ventricular end in a 

 headward direction and towards the left. Into the cavity of the conus 

 projects the thick spiral ridge (Fig. 182, A and B, s.c).. This describes 

 a right-handed spiral, its line of attachment to the conus wall commencing 

 at its ventricular end by being ventral, and ending up at its headward 

 end by being on the right. 



The headward end of the spiral ridge is excavated to form a large 

 pocket-valve, and there are two other smaller ones, making a circle of 

 three pocket-valves at the headward end of the conus. Another circle 

 of three pocket-valves guards the entrance to the conus at its ventricular 

 end (Fig. 182, B and C, i, 2 and 3). 



The conus arteriosus is continued into the ventral aorta ^.A) which 



