172 PHYLUM CHORD ATA 



The study of the digestive system will be completed after 

 the heart has been examined. 



The Heart and its Vessels. The heart of the frog is com- 

 posed of five divisions : a single ventricle, two auricles, the sinus 

 venosus, and the truncus arteriosus. Observe the pericardium, 

 which closely invests the heart. 



The ventricle is the large, conical, posterior portion of the heart ; 

 by its contractions the blood is sent forward through the trun- 

 cus arteriosus, which is the large, cylindrical vessel springing 

 from its anterior end. The truncus divides into two large vessels 

 which pass forward and leave the pericardial space. Each of 

 these vessels then divides into three arteries, called the aortic 

 arches, through which the blood is carried to all parts of the 

 body. The anterior arch is called the carotid arch; it carries 

 blood to the head. The middle arch is called the systemic arch. 

 The right and left sides of this arch meet back of the heart and 

 form the dorsal aorta, which lies just beneath the spinal column 

 and distributes arterial blood to the trunk and extremities. The 

 posterior arch is the pulmocutaneous arch ; through it blood is 

 carried to the lungs and the skin for aeration. 



In front of the ventricle are the right and left auricles ; they 

 appear dark-colored in consequence of the thinness of their walls. 

 On the dorsal side of the heart is a large, thin-walled, dark- 

 colored sac, the sinus venosus. Blood is brought to the heart 

 from the organs and tissues of the body by three large veins 

 which enter the sinus venosus : these are the right and left pre- 

 caval veins, which enter the forward end of the sinus, bringing 

 blood from the forward part of the body ; and the postcaval vein, 

 which enters the hinder end of the sinus, bringing blood from the 

 hinder part of the body. From the sinus the blood enters the 

 right auricle. Blood is brought to the heart from the lungs by 

 the pulmonary vein, which lies alongside the left precaval vein 

 and enters the left auricle ; this vein is formed by the union of a 

 right and a left pulmonary vein, which bring blood from the two 

 lungs. 



Exercise 7. Make a drawing of the ventral aspect of the heart and the 



blood vessels, so far as these have been observed, on a scale of 2 or 3. \ 



