COMPARATIVE ANATOMY OF THE CIRCULATORY SYSTEM 20' 



B. THE CIRCULATORY SYSTEM OF ELASMOBRANCHS 



The following account applies to both species of dogfish and to the skate. 



1. The chambers of the heart. — The heart when first formed is a simple 

 straight tube, but it soon becomes bent upon itself in the shape of the letter S 

 and its wall becomes differentiated into a number of chambers. The heart o 

 the elasmobranchs is in this condition, consisting of four chambers. The peri 

 cardial cavity has already been exposed in the preceding dissection. Spread it 

 walls apart. Identify the chambers of the heart as follows. On raising thi 

 heart a triangular chamber will be seen extending from the heart to the transversf 

 septum, its two corners buried in the septum. This is the sinus venosus, th< 

 most posterior chamber of the heart. Each corner of the sinus venosus is con 

 tinuous with a large vein, the duct of Cuvier or common cardinal vein, which i; 

 inclosed in the transverse septum and will be seen later. Anterior to the sinu; 

 venosus is the atrium or auricle, a large thin-walled chamber expanded on eacl 

 side of the heart and appearing as if paired. Between the two sides of th< 

 auricle rests the ventricle, a thick-walled, heart-shaped chamber, the most con 

 spicuous portion of the heart from ventral view. The pointed posterior end o 

 the ventricle is known as the apex, the broad anterior end, the base. From th< 

 base of the ventricle a thick- walled tube runs forward and penetrates the anterio: 

 wall of the pericardial cavity. This is the conus arteriosus, the fourth and mos 

 anterior chamber of the heart. The blood circulates through the chambers o 

 the heart in the following order : sinus venosus, auricle, ventricle, conus arteriosus 



2. The systemic veins. — Systemic veins have already been defined a: 

 those veins which enter the heart. All systemic veins in vertebrates open int< 

 the sinus venosus or its equivalent, that is, they enter the phylogenetically poste 

 rior end of the heart. Owing to differences between them, the dogfishes anc 

 skate will be described separately. 



In dissecting the veins, they are followed away from the heart, and it is oftei 

 convenient to speak of them as it they proceeded from the heart to bod] 

 structures. The student must, however, always bear in mind the fact tha 

 they convey the blood from the parts of the body to the heart. 



Dogfish: Insert one blade of a fine scissors in the sinus wall and slit thi 

 ventral wall of the sinus venosus open in a crosswise direction. The cavity o 

 the sinus is thus exposed and should be washed out thoroughly under a strean 

 of running water. All of the systemic veins open into the cavity of the sinu 

 and the openings may now be identified, with the cut edges of the sinus wal 

 spread well apart. Each lateral wing of the sinus which lies buried in the trans 

 verse septum receives a very large opening, the entrance of the duct of Cuvie 

 or common cardinal vein. The natural relations of this entrance are best observe^ 

 on the intact right side. On the left side carry the slit in the sinus laterally t 



