x PRACTICAL DIRECTIONS 477 



open the oviducts, and note, if present, the eggs enclosed 

 in horny egg-cases. Sketch. 



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 longitudinal 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, with- 

 out injuring the lateral vein (p. 475), remove the middle 

 portion of the pectoral arch and expose the pericardial 

 cavitv and heart. Insert a seeker, pointing backwards, along 

 the dorsal side of the heart, through the pericardia-peritoneal 

 canal which communicates between the pericardial 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 conns 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 inserting and tying the 

 cannula into the conus arteriosus : use a blue injection if vou 

 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 separating the latter from the walls of 

 the pericardium and noting the entrance of the hepatic 

 sinus (p. 475). Remove the entire heart, pin it down under 

 water, ventral side uppermost. Cut open the ventricle and 

 conus arteriosus and note 



i. Their cavities and walls; the aitriculo-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 sinu-auricular aperture is best made out by turning 

 the heart over with the dorsal side uppermost, before cutting 

 open the auricle from the ventral side. 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 



1 See also C. III. 



