THE HEART AND ITS VESSELS. 



279 



In correspondence with the different function which each 

 portion has to perform, the walls of the atrium are thin, while 

 those of the ventricle are much stronger, its muscles giving rise 

 in the interior to a network and also usually to a series of 

 large trabecula3; this holds good throughout the Vertebrata 

 (Fig. 222, C, A). 



Between the ventricle and atrium, at the margins of the atrio- 

 ventricular aperture, membranous valves are present ; of these 

 there are usually two, but this number may be increased to as 

 many as six (Fig. 222, C, a, a). Numerous valves, arranged in rows, 

 are present in the muscular truncus or conus arteriosus 

 (Fig. 222, C, Ca, V) ; these are most numerous in Elasmobranchs and 

 Ganoids. There is a tendency however for the posterior ones, or 

 those which lie towards the ventricle, gradually to undergo reduc- 

 tion. The most anterior row always persists, and corresponds to 

 the single row of valves between the ventricle and bulbus in 

 Teleostei. Together with the reduction of these valves, the conus 

 arteriosus of Teleosteans also becomes reduced, so that the non- 

 contractile bulbus arteriosus usually lies close against the 

 ventricle (Fig. 222, B, Ba). 



ce 



RA 



FIG. 223. DIAGRAM OF THE ARTERIAL SYSTEM OF FISHES. 



ff, heart ; c, c 1 , anterior and posterior cardinal veins ; a, branchial arteries ; R, 

 capillaries of the branchial vessels ; b, branchial veins ; ce, circulus cephalicus ; 

 ca, carotids ; RA, root of the aorta ; A, dorsal aorta ; E, artery to viscera 

 (cceUaco-rnesenteric) ; N, renal arteries. 



The heart of Fishes contains venous blood only, which it forces 

 through the branchial arteries (Fig. 223, a) into the capillaries 



