26 THE SKATE 



the heart in the order mentioned. The internal structures 

 of the heart will be considered later. 



Drawing 13. Sketch the heart from the ventral viewpoint. 



3. The Venous System. 



Veins are those blood vessels through which blood is re- 

 turned to the heart. There are three kinds of veins in the 

 vertebrates: (1) The systemic veins which empty directly 

 into the heart; (2) the portal veins which carry the blood 

 to a system of capillaries (e.g., in the liver), where it is 

 brought together in systemic veins and delivered to the 

 heart; (3) the pulmonary veins in vertebrates with lungs 

 which bring the aerated blood back to the heart from the 

 lungs. 



A. The Systemic Veins. 



The dissection of the veins is the most difficult part of 

 the entire study of the skate. Follow the directions very 

 closely and most of the system will be clearly unfolded. 

 Refer to the figure to aid in your organization of this 

 system. Many of the veins of the skate are not sharply 

 defined vessels, but are large spaces in the tissue, without 

 very definite walls and are more correctly called sinuses. 



The systemic veins enter from both posterior corners of 

 the dorsal side of the heart by way of the sinus venosus, 

 which has already been pointed out. The central portion 

 of the sinus venosus, which receives the two tubes from 

 either corner, is attached to the transverse septum by a 

 sheet of connective tissue which should be broken away 

 carefully. Trace out the right side of the sinus venosus, 

 which is buried in the transverse septum, until it disappears 

 dorsal to the pectoral girdle cartilage. Follow the sinus 

 laterally by carefully shaving away the cartilage and the 

 surrounding tissue. With fine scissors cut open the ventral 



