CHAPTER VI 

 THE CIRCULATORY SYSTEM 



In vertebrates the circulatory system is composed of two 

 series of tubes, the lymphatic and the blood-vascular sys- 

 tems, which inclose fluids that circulate. The blood-vascu- 

 lar system is commonly termed the circulatory system. It 

 consists of a set of branching continuous tubes which form 

 a closed system and are unconnected with any other system. 



1. The Pericardial Cavity. 



Remove the skin from the ventral surface of the region 

 posterior to the mouth. Start with an incision at the pec- 

 toral girdle and work forward between the gill-slits of the 

 two sides. Remove with the forceps the layers of visceral 

 muscle found below the skin until a membrane, the parietal 

 pericardium, in the posterior median region is uncovered. 

 Remove this membrane and expose the pericardial cavity, 

 within which is the heart. The cavity may be better re- 

 vealed if the tissue along the anterior face of the girdle 

 is removed. This should be cut away where necessary, care 

 being taken not to injure the heart. 



The pericardial cavity is a part of the coelom. The other 

 part, the pleuroperitoneal cavity has been studied. The 

 two parts are separated by a membrane, the transverse 

 septum, which forms the posterior wall of the pericardial 

 cavity. 



The septum is not complete since an opening, the 

 pericardio-peritoneal canal, which will be pointed out later, 



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