THE CIRCULATORY SYSTEM 31 



dark color. Remove the skin and pick away the tissue and 

 muscle from the region between the anterior end of the 

 pericardial cavity and the lower jaw. Dissect away and 

 expose the vessels, starting just anterior to the heart. 



a. The ventral aorta is a continuation of the conus 

 arteriosus and is on the median ventral line. Trace it out 

 on the left side and expose the afferent branchial arteries. 

 The first pair of these is given off at the point where the 

 conus arteriosus passes into the ventral aorta. It divides 

 into three branches, which supply the fourth, fifth, and 

 sixth visceral arches and extend into the interbranchial 

 septa, giving off branches to each of the demibranchs. 



It is necessary for dissectional reasons to point out here 

 the coronary arteries which supply the heart walls with 

 aerated blood. They are yellow in color and should not be 

 injured more than is necessary in tlie following work. 

 Their connections will be pointed out later. 



The second pair of afferent branchial vessels is formed 

 by the bifurcation of the ventral aorta at its anterior end. 

 Follow the aorta forward almost to the lower jaw where 

 the second pair originates. 



Trace the left one. It divides into two vessels which 

 penetrate the interbranchial septa of the second and third 

 visceral arches. The embryonic first afferent artery of the 

 first arch, the spiracle, is not persistent in the adult skate 

 probably because there is no functional gill on this 

 arch. 



Drawing 16. Draw the ventral aorta and the afferent branchial 

 vessels. 



5. The Internal Structures of the Heart. 



Review the drawing of the heart, number 13. The heart 

 is a modified tube bent dorsoventrally into the various 

 chambers which have been mentioned before. Because of 



