22 ZOOTOMY. 



beneath the notochord ; it receives the efferent branchial 

 arteries from the gills. 



XIV. Remove the outer (left) wall of the auricle, and 

 observe 



75. The comparatively thin walls of the auricle, 

 strengthened by a network of pectinate muscles. 



76. The auriculo-ventricular valve (Fig. 4, see 

 description), composed of two membranous flaps, and 

 guarding the oval opening leading from the auricle to the 

 ventricle. 



77. The sinu-auricular valve (Fig. 4, x), also com- 

 posed of two flaps, and guarding the large aperture which 

 leads from the sinus venosus into the auricle. 



XV. Pass a guarded bristle through the sinu-auricular 

 valve upwards along the sinus venosus as far as it 

 will go : also make an aperture in the left cardinal 

 vein, pass a probe forwards, and open the vein 

 along it. 



78. It will be found that the cavities of the two cardinals 

 unite with one another by an oval aperture (Fig. 4, x) just 

 above the dorsal end of the sinus venosus, and that, at the 

 same place, they both communicate with the sinus. 



79. The blood from the anterior part of the body, except that re- 

 turned by the inferior jugular, is brought back by the jugular veins 

 (Figs. 4 and 6, ju), which lie one on either side of the notochord, 

 and, uniting each with the corresponding cardinal, pour their blood 

 into the sinus venosus. The jugular is best made out by passing a 

 guarded bristle forwards from the cardinal and gradually dissecting 

 down to it. 



80. The hepatic vein (Fig. 4, h.v) may also be seen at this 

 stage ; leaving the liver, it pierces the pericardium and enters the 

 sinus venosus at the same point as the inferior jugular. 



XVI. Open the ventricle and note 



81. The thick fleshy walls and small cavity. 



