224 COMPARATIVE ANATOMY 



The heart may be said to originate in two large venous 

 vessels, the ductus Cuvieri, which run transversely towards the 

 middle line in close contact with the fifth cerato-branchial 

 cartilages. In the posterior wall of the pericardial cavity these 

 venous trunks dilate, and, meeting in the middle line, form a 

 transverse thin-walled tube known as the sinus venosus. By 

 their union in the middle line the limbs of the sinus venosus 

 form a single vessel which runs forward in the median ventral 

 line below the ventral members of the branchial cartilages. 

 The posterior end of this vessel, immediately succeeding to 

 the sinus venosus, is dilated, bent upon itself to form a 

 figure S, and its walls are thickened by muscular tissue. It 

 is, in fact, the heart, and it should be remembered that the 

 heart is, essentially, nothing more than a muscular dilatation 

 of a ventral median blood-vessel. 



The sinus venosus, then, opens by a single median aperture, 

 guarded by a membranous valve, into the posterior division of 

 the heart or auricle. The auricle of the dogfish is a single 

 triangular sac, its apex pointing forward and its posterior 

 angles produced into lappets or auriculae. Its walls are 

 muscular, but not thick, and it lies in the dorsal moiety of the 

 pericardial cavity. The ventricle is an almost globular sac, 

 with very thick muscular walls lying ventral to the auricle, 

 with which it communicates by a single opening guarded by 

 valves. The anterior end of the ventricle is continued for- 

 wards as a stout muscular tube, the conus arteriosus. This 

 in turn passes through the anterior wall of the pericardium 

 and runs forward on the ventral wall of the throat, as the 

 so-called cardiac aorta. The interior of the conus arteriosus 

 is provided with two rows of semilunar valves, each row consist- 

 ing of three watch-pocket valves so disposed as to admit of 

 the free passage of the blood from the heart to the cardiac 

 aorta, but prevent any reflux in the contrary direction. 



The cardiac aorta runs forward nearly as far as the lower 

 ends of the cerato-hyal cartilages, where it divides into right 

 and left branches, and each of these almost immediately sub- 

 divides into two branches, the anterior of which runs along the 

 hyoid arch, supplying the anterior demi-branch of the first gill- 

 pouch with blood, the posterior runs along the first branchial 

 arch supplying the posterior demi-branch of the first gill- 

 pouch, and the anterior demi-branch of the second gill-pouch. 



