126 ESSENTIALS OF ZOOLOGY 



In general, the primary function of the digestive system is to con- 

 vert the complex food material into an absorbable solution which may 

 readily be taken up by the blood and in turn by the protoplasm of 

 the tissues. The digestive enzymes are indispensable agents in this 

 function. 



Circulatory System 



This system includes the blood vascular and lymphatic vascular 

 portions. The former consists of the heart, arteries, capillaries, and 

 veins. This system transports dissolved food materials, oxygen, car- 

 bon dioxide, excretions, hormones, and antibodies to and from the 

 cells over the body. In addition, the distribution of the blood helps 

 in equalizing the temperature throughout the body. Blood and 

 lymph are the two vascular fluids. 



The Heart.^ — This muscular organ is located in a division of the 

 thoracic cavity, the pericardial cavity, which is separated from the 

 remainder of the chest by the transparent pericardium. The heart 

 of the rat is quite representative of the mammalian heart with its two 

 auricles (atria) and two ventricles. The right and left sides of the 

 adult heart are quite distinct. The right atrium (auricle) receives 

 systemic blood from the precaval (superior vena cava) and postcaval 

 (inferior vena cava) veins which collect the venous blood from the 

 various organs of the body. Contraction of the right atrium forces 

 the blood through the right auriculoventricular orifice which is 

 guarded by the tricuspid valve and into the right ventricle. At the 

 same time the contraction of the left atrium, which has received 

 aerated blood from the lungs, forces that blood through the left atrio- 

 ventricular orifice which is guarded by the bicuspid or mitral valve 

 and into the' left ventricle. Immediately following the simultaneous 

 contraction (systole) of the two atria, the ventricles contract. The 

 blood from the right ventricle is forced into the pulmonary arteries 

 and carried to the lungs while the blood from the left ventricle is 

 entering the aortic arch to be distributed to all parts of the body. 

 There is no direct connection between the cavities of the two sides of 

 the heart. The principal valves of the heart are the tricuspid and 

 bicuspid mentioned above, and the two semilunar valves. There is 

 one of the latter in the proximal portion of the pulmonary artery 

 and the other in. the aorta. The function of these valves is to prevent 

 backflow of blood, in order that the blood may ])e kept in circulation 



