98 VERTEBRATE ZOOLOGY 



The Heart and its Vessels. The heart of the frog is composed 

 of five divisions,— a single ventricle, two auricles, the sinus venosus, 

 and the truncus arteriosus. Observe the pericardium, which closely 

 invests the heart. Do not remove the heart or its vessels. 



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

 by its contractions the blood is sent forward through the truncus 

 arteriosus, which is the large cylindrical vessel springing from its 

 anterior end. The truncus is made up of two portions,— a basal 

 portion, which is called the bulbus cordis, and an anterior portion. 

 The latter at once divides into two large vessels which pass for- 

 ward 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 aorta descendens, or 

 dorsal aorta, which lies just beneath the spinal column and dis- 

 tributes 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 precaval 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. 



