x PRACTICAL DIRECTIONS 477 



E. Circulatory 1 and Respiratory Organs, &c. 



I. After noting the spinal nerves, exposed by the removal of the 

 kidneys, the body may be cut through just behind the pectoral arch, and 

 the posterior portion thrown away. Pin down the head and anterior 

 portion of the body, ventral side uppermost, make a median longi- 

 tudinal incision through the skin from the lower jaw to the pectoral arch, 

 and dissect it away on either side as far as the gill clefts. Then, 

 without injuring the lateral vein (p. 456), remove the middle portion of 

 the pectoral arch and expose the pericardial cavity and heart. Insert a 

 seeker, pointing backwards, along the dorsal side of the heart, through 

 the pericardia-peritoneal canal which communicates between the peri- 

 cardial and abdominal cavities (p. 434) : 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. 121) : 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. 456). Remove the entire heart, pin 

 it down under water, ventral side uppermost. Cut open the ventricle 

 and conus arteriosus and note 



1. Their cavities and walls ; the auricula-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 attricle and the sinu-auricular aper- 

 ture 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] of 

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

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



1 See also C. HI. 



