RAT. 59 



(dorsal or ventral) of the oesophagus does the aorta lie? 

 On which side is the heart, and on which side does the 

 aortic arch pass? Insert the vessels now made out in the 

 sketch, which should now represent the principal vessels 

 of the systemic circulation. 



Dissect the aortic arch loose from the surrounding 

 tissue, lift it up, and see dorsal to it the pulmonary arteries 

 going to the lungs. From what part of the heart do they 

 arise? Tip the heart to the animal's right and see the 

 pulmonary veins, which bring the blood back from the 

 lungs to the heart. On which side, with reference to the 

 pre- and postcava, do they enter the heart? The pul- 

 monary arteries and pulmonary veins belong to the pul- 

 monarv circulation. Add them to the sketch. 



/ 



Cut through the cavse, pulmonary vessels, and aorta, and 

 remove the heart.* The heart is conical in shape, with a 

 broader anterior base and a more pointed posterior apex. 

 On the base on either side will be found small lobes- 

 the auricles. Split the heart parallel to the horizontal 

 plane of the animal with a sharp scalpel, keeping in mind 

 which side of the organ was originally right and which 

 left. Make out two pairs of cavities (usually containing 

 clotted blood, which should be carefully removed). Which 

 of these have the thicker walls the right or the left? 

 The basal cavities are the auricles, the apical the ventricles. 

 Which parts, auricles or ventricles, would you suppose to 

 play the greater part in forcing the blood through the 

 circulation? Study the connections between auricles and 



/ 



ventricles. Do the two auricles connect with each other? 

 Is the same true of the ventricles? Notice wiiat vessels 

 enter the left auricle. Where do the pre- and postcavse 



* The larger heart of a cat, sheep, or pig will show these points 

 much better. 



