THE CIRCULATORY SYSTEM 33 



Drawing 17. Make a drawing of the heart showing its internal 

 structure with arrows to indicate the course of the blood. 



6. The Thyroid Gland and Pericardio-peritoneal Canal. 



The thyroid gland, the secretion of which seems to regu- 

 late growth, may now be located as a brownish tissue just 

 behind the lower jaw and immediately ahead of and in the 

 angle of the bifurcation at the anterior end of the ventral 

 aorta. 



The pericardio-peritoneal canals may also be located 

 now. These canals, it will be remembered, connect the two 

 parts of the coelom, the pericardial and the pleuroperi- 

 toneal cavities. In the center of the posterior wall of the 

 sinus venosus there is an opening of moderate size. Probe 

 into this. It leads into a canal, the pericardio-peritoneal 

 canal, which passes through the center of the hepatic sinus. 

 It forks into two canals in the ventral wall of the esophagus 

 and opens into the pleuro-peritoneal cavity by small open- 

 ings. Refer back to Chapter IV. 



7. The Dorsal Aorta. 



The dorsal aorta and the efferent branchial arteries will 

 be found on the roof of the mouth and pharyngeal cavity. 

 With the forceps pick off the mucous membrane and expose 

 the arteries which are filled with yellow starch paste in- 

 jected from the tail. There are three main pairs of efferent 

 branchial arteries. The blood comes from the gills and is 

 collected by the union of the efferent arteries to form a 

 dorsal aorta which extends backward just ventral to the 

 vertebral column. 



Trace the efferent branchial arteries towards the gills, 

 removing the cartilage where necessary. Each efferent 

 artery is formed at the dorsal angle of the internal gill- 

 slits by the union of two arteries. A large post-trematic 



