CHAPTER II. 

 THE VASCULAR SYSTEM OF THE FROG. 



THE vascular system is a closed system of tubes or vessels 

 filled with blood, and ramifying through all parts of the body : 

 its main parts are : (1) the heart, which by its contractions is 

 continually driving the blood round and round the system of 

 vessels: (2) the arteries, which are the vessels taking the 

 blood from the heai-t to all parts of the body : (3) the veins, 

 which carry the blood from those parts back to the heart : and 

 (4) the capillaries, a system of very small vessels connecting 

 the arteries and veins together. 



A. The Heart. 



Pin down the frog on its back under water and open tJte body 

 cavity as before, taking special care to preserve the anterior ab- 

 dominal vein. (See p. 18.) In freeing the pectoral girdle from 

 the underlying muscles take care not to injure the neighbouring 

 bloodvessels. 



Open the pericardial cavity and dissect the pericardium from 

 the heart and the roots of the great vessels, examine and draw the 

 heart in situ, showing its several divisions. 

 1. The divisions of the heart. 



i. The auricles form the anterior and dorsal division 

 of the heart : they are thin-walled and appear 

 dark in colour owing to the blood being seen 

 through their walls. On close examination the 

 division into right and left auricles can be seen. 

 ii. The ventricle is posterior to the auricles : it is paler 

 in colour owing to the greater thickness of its 

 walls ; and is conical in shape, with the apex 

 pointing backwards. 



iii. The truncus arteriosus is a cylindrical body arising 

 from the right anterior border of the ventricle, 

 and running obliquely forwards across the 

 auricles. 



