COMPARATIVE ANATOMY OF THE CIRCULATORY SYSTEM 207 



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 of 

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

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

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

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

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

 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 is 

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

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

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

 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 of 

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

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

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

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

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



2. The systemic veins. Systemic veins have already been defined as 

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

 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 and 

 skate will be described separately. 



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

 convenient to speak of them as if they proceeded from the heart to body 

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

 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 the 

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

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

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

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

 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 Cuvier 

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

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



