A FROG 185 



The Pulmonary Veins. The common pulmonary enters the left 

 auricle. It is a very short vein and is formed by the union of the 

 right and left pulmonaries, which come from the right and left 

 lungs respectively. Each pulmonary vein lies along the medial 

 side of the lung, the right pulmonary being somewhat longer than 

 the left. Turn the apex of the heart forward and find these veins. 



Exercise 17, Draw a diagram of the entire venous system. 



The Arteries.^ All the blood in the heart leaves it through the 

 truncus arteriosus, the structure of which has already been ob- 

 served. At its anterior end the truncus divides into a right and 

 a left branch, each of which after passing through the pericar- 

 dium again divides into three branches, the carotid, systemic, 

 and pulmocutaneous aortic arches. The last named of these 

 arches, which is the hindermost in position, branches ofi from 

 the others a short distance in front of the pericardium ; the other 

 two usually remain together a short distance before separating. 



The anterior aortic arch, the carotid, passes forward and 

 dorsally a short distance and then divides into two vessels, 

 the internal and the external carotid arteries. At this point the 

 walls of the arteries are thickened and spongy, and the ovoid 

 structure thus formed is called the carotid gland; it is not a 

 gland, however, but probably acts as an accessory heart. The 

 internal carotid, the larger of the two, goes dorsally and then 

 forward and supplies the brain, the orbit, and the mucous mem- 

 brane of the roof of the mouth. The external carotid passes 

 directly forward and supplies the tongue and the muscles of the 

 lower jaw. 



The posterior aortic arch, the pulmocutaneous, takes blood 

 to the lungs and skin to be oxygenated. On each side just back 

 of the carotid gland it divides into two arteries, the pulmo- 

 nary and the great cutaneous. The former passes back a short 

 distance and then divides into three arteries which traverse the 

 walls of the lung. The latter passes first forward and dorsally 

 and then backward along the inner surface of the skin. 



1 The arterial system can be best studied after it has been injected ; this should 

 be done through the ventricle and the truncus arteriosus. 



