A TURTLE 137 



Exercise 8. Draw the dorsal aspect of the heart diagrammatically, 



and the base of the great veins entering it. 



The Internal Structure of the Heart. Cut the great vessels enter- 

 ing and leaving the heart a short distance from their base and 

 remove the heart from the body. Place it in water, with the dorsal 

 side uppermost. Open the sinus venosus and wash out the blood in 

 it. Note its thin walls. Find the large opening into the right 

 auricle and the two valves guarding it. Carefully open the auricles 

 by carrying a slit around their anterior edge and wash out the 

 blood. Note the thin septum between the auricles, and the ridges 

 in their walls. Pass a probe from the sinus venosus into the right 

 auricle. Observe the opening from each auricle into the ventricle, 

 one being on each side of the septum. Probe them. 



Cut open the ventricle by carrying a slit along its anterior end 

 through the auricular openings. Note its small cavity and the in- 

 complete septum which divides it into two communicating spaces ; 

 note also the great thickness of the walls of the ventricle and the 

 muscle ridges which project into the cavity. Find the openings 

 of the large arteries into the ventricle by probing through them 

 into it. Note that the pulmonary artery and the left aorta arise 

 in the right side, and that the right aorta, from which the innomi- 

 nate artery springs, arises in the left side of the ventricle. Find, 

 by using the blowpipe, the valves which guard the mouths of all 

 these arteries. 



The circulation of the blood in the heart goes on as follows : 

 Blood is brought to the heart by the caval veins and the pulmo- 

 nary vein. The former bring venous blood from the various or- 

 gans and tissues of the body to the sinus venosus, from which it 

 goes to the right auricle, to be pumped finally into the right side of 

 the ventricle. The pulmonary vein brings arterial blood from the 

 lungs to the left auricle, from which it is pumped into the left side 

 of the ventricle. The ventricle thus receives both kinds of blood, 

 but the incomplete septum and the muscle ridges serve to prevent 

 a complete mingling of them. 



Blood leaves the heart by the three great arteries which take 

 their rise in the anterior end of the ventricle. The two leaving the 

 right side of the ventricle (the pulmonary and the left aorta) 



