11 



In front of the pectoral girdle lies a thin walled sac, the peri- 

 cardial sac. Open it by a median ventral incision. Remove about 

 one-half inch of the middle of the pectoral girdle, being careful not 

 to cut the thin-walled part of the heart lying dorsal to it. The 

 pericardial cavity is a pear-shaped chamber containing the heart, 

 and lined by the smooth pericardium which is morphologically 

 equivalent to the peritoneum. At the anterior extremity of the 

 chamber the pericardium is seen to be reflected backward over the 

 surface of the heart, thus forming its smooth outer coat. 



The heart may be considered as a bent tube, enlarged in certain 

 regions to form the chambers. Anteriorly and ventrally is a short, 

 thick-walled tube, the conus arteriosus; this leads out of the peri- 

 cardial sac anteriorly, while posteriorly it opens into a large mus- 

 cular chamber, the ventricle. Dorsal to the ventricle, and project- 

 ing on either side of it is the thin walled auricle. Dorsal to both 

 ventricle and auricle is the extremely thin-walled sinus venosus. This 

 is triangular in shape, the apex opening into the posterior side of 

 the auricle, the base attached to the posterior wall of the pericardial 

 cavity; the lateral angles are drawn out into the ducti cuvierii, 

 which receive veins from the anterior and posterior parts of the 

 body. From the conus arteriosus springs a smaller vessel, the 

 ventral aorta, which passes forward between the gill pouches. Take 

 note of the small arteries passing over the surface of the conus and 

 along the inner ends of the gill pouches, and take care not to cut 

 them or their branches in the subsequent dissection. 



Two pairs of arteries leave the ventral aorta as it emerges from 

 the pericardial sac. The aorta then passes forward some distance 

 and finally divides into two branches which pass to either side. Fol- 

 low the branches of the aorta outward on the left side and demon- 

 strate their courses. The anterior branch quickly divides into two, 

 the anterior of these passing along the base of the first demibranch. 

 The posterior enters the septum between the first and second pouches, 

 and supplies the second and third demibranchs. The middle branch 

 of the aorta passes directly to the fourth and fifth demibranchs. The 

 posterior branch divides almost as it leaves the aorta, its branches 

 supplying the remaining demibranchs. There is considerable varia- 

 tion in this branch of the aorta. It usually divides as stated, but it 

 frequently passes some distance toward the gills before dividing, 

 and in a considerable number of cases two vessels arise directly 

 from the aorta instead of one. 



The arteries carrying blood from the ventral aorta to the gills 

 are named the afferent branchial arteries. Observe the relation of 

 these vessels to the gills. 



VENOUS SYSTEM. All the blood of the body is conveyed to the 

 sinus venosus. The sides of the sinus venosus are extended as large 

 vessels, already referred to as the ducti cuvierii. Open the sinus 

 and ducti by a transverse ventral incision. The ducti pass directly 

 into the lateral veins. Near the middle of the posterior wall of the 

 sinus is an aperture of varying size, the opening of the hepatic 

 sinus; there are rarely two openings in Squalus, always two in 

 Eugaleus. A large opening on the posterior wall of each ductus 

 leads into the posterior cardinal vein. On the anterior wall of the 

 ductus, near the sinus venosus, is a small aperture, that of the in- 

 ferior jugular vein. Lateral to this is frequently a somewhat larger 

 opening of the anterior cardinal vein. This is absent, however, in 



