MULTICELLULAR ANIMALS 



i8i 



exceedingly fine capillaries. It is collected again as these capillaries 

 drain together and passes by way of the pulmonary veins to the 

 left atrium. The beat or contraction of the walls forces it into the 

 left ventricle and out anteriorly through the largest vessel in the 

 body, the aorta. Shortly after leaving the left ventricle this vessel 

 arches dorsally and then turns posteriorly. At the arch of the aorta 

 large arteries to the head, face, and shoulders are given off. As the 

 main trunk passes posteriorly in the dorsal wall of the coelom 



/^ORT/i 



SUPERIOR VENA CAVA 

 PULMONARY ARTERY - 

 PULMONARY VEINS 



LEFT ATRIUM 



RIGHT ATRIUM 



LEFT VENTRICLE 

 RIGHT VENTRICLE-j^^T^ 

 INFERIOR VENA CAVA^ 



Fig. 123. — The human heart. (After Sobotta: Atlas and Textboofi of Hiinia?! Anat- 

 omy, pubUshed by W. B. Saunders Co.) 



arteries are given off to the digestive and other organs of the ccelom, 

 and to the body wall. Posteriorly it divides into large vessels that 

 supply the legs. The blood in the heart and in the vessels is kept 

 from flowing in the wrong direction by a system of valves. 



The muscle cells of the heart (Fig. 124) are different in structure 

 and in functional characteristics from smooth muscle cells of the 

 intestinal wall and from the cells of large muscles attached to the 

 skeleton. Heart muscle contracts periodically, automatically, and 

 obtains its necessary rest during the relaxation intervals between 

 contractions. The heart is connected with two sets of nerves, one 



