IX PRACTICAL DIRECTIONS 461 



the canal which communicates between the pericardial and abdominal 

 cavities : it opens on the ventral side of the gullet by two apertures. 

 Make out — 



1. The form and relations of the chambers of the heart [sinus venosus, 

 auricle, •ventricle and conus arteriosus). 



2. The ventral aorta, to expose which the muscles in front of the 

 pericardium must be carefully removed ; but before doing so, it is better 

 to inject the ventral aorta, cutting a small hole in the ventricle, and in- 

 serting and tying the cannula into the conus arteriosus : use a blue 

 injection if you have already used red for the dorsal aorta. 



3. The five afferent branchial arteries (compare Fig. 112): trace 

 these outwards, and note their distribution. Sketch your dissection. 



II. — Cut through the ventral aorta at its junction with the conus 

 arteriosus and through both ends of the sinus venosus, carefully separ- 

 ating the latter from the walls of the pericardium and noting the 

 entrance of the hepatic sinus (p. 458). Remove the entire heart, pin 

 it down under water, ventral side uppermost, and cut open the ventricle 

 and conus arteriosus. Note — 



1. Their cavities and walls ; the auriculo-ventricular aperture and 

 valves ; the valves in the conus arteriosus, of which there are two sets, 

 consisting of three in each set. Sketch. 



2. The cavity and walls of the auricle and the sinu-auricular aper- 

 ture, which are best made out by turning the heart over, with the dorsal 

 side uppermost, before cutting open the auricle. Sketch. 



3. Insert a seeker into one of the cut distal ends (still left in situ) oi 

 the sinus venosus, and slit it up so as to expose 'Che. precaval sinus of the 

 same side ; by means of a seeker find the apertures into it of the 

 following veins or sinuses (compare Fig. 113)— a, the jugular ; b, the 

 injerior jugular ; c, the cardinal; and d, the lateral vein. 



III. — Insert the scissors into each external gill-cleft of one side, one 

 by one, and extend them by cutting dorsally and ventrally, so as to 

 expose the gill-pouches, communicating with the pharynx by the internal 

 gill-clefts. Make out — 



1. The branchial filaments, and observe that there are four complete 

 gills on the first four branchial arches, and a half-gill or heviibranch on 

 the posterior face of the hyoid arch. Note also ih.e pseudobranch on the 

 anterior side of the spiracle. 



2. The structure of the gills. Remove two entire gills ; dissect one, 

 and cut the other across transversely (compare Fig. 109), noting the 



