THE FISH. 79 



the walls of this cavity, the pericardial cavity, out 

 to the sides. 



The red, angular portion of the heart, which in the 

 natural position of the fish lies lowest and hindmost, 

 is the ventricle; the darker, more irregular portion 

 lying (in the natural position) above the ventricle, is 

 the auricle; the larger blood-cavity back of the auri- 

 cle, and extending across the body-cavity, above the 

 false diaphragm, is the venous sinus ; in front of 

 the ventricle is the light-colored conical arterial bulb. 

 This narrows forward into an artery which gives off 

 branches on both sides, one to each gill. Make a 

 drawing of the heart and arterial bulb. After passing 

 through the gills, the blood-vessels re-unite to form 

 the dorsal aorta, which passes backward just under- 

 neath the spinal column. From above the gills 

 branches also run forward to the head. 

 11. Insert the finger into the fish's mouth and depress the 

 tongue. Observe the thin membrane which forms 

 the floor of the mouth on each side of the tongue ; 

 cut through this membrane close to the inner border 

 of the lower jaw on each side and across the band of 

 muscle which is attached to the point where the two 

 halves of the lower jaw unite ; continue the cut back- 

 ward on each side between the gill-cover and the 

 branchiostegal membrane, and wholly separate them. 

 Turn back the whole flap thus loosened, and again 

 examine the gills; note the joints in the gills : with 

 the forceps seize the tongue, raise and lower it, to see 

 the action of these joints in the gill-arches. 



Observe, where the gills unite above and below, 

 patches of closely set teeth, the superior and inferior 



