178 



ELEMENTS OF BIOLOGY 



digestion pass into fine vessels of the hepatic portal vein and are 

 rectified in the liver (p. 163). 



Propulsion of Blood. The propulsion of the lymph about 

 the body is effected in part by gravity, but chiefly by the motions of 

 the body and the pressure and expansion of the various tissues and 

 organs that are muscular and active. The blood within the blood 

 vessels is propelled by a muscular organ, the heart. In the annelid 

 worms (Fig. 116) certain segmental blood vessels serve as hearts 

 and exhibit the property of periodic contraction and relaxation, 

 which motions propel the blood. In the Arthropoda, for example the 



INLET 



PERICAROIUM 



HEART 



ARTERY 



ARTERY 



VEIN 



Fig. 118. — Diagram of the heart of a lobster. Veins drain into the pericardial sac. 

 Inlet valves in the heart allow this blood to enter the ventricle, or heart proper. 

 When the walls of the ventricle contract the inlet valves close and the blood is forced 

 out through the arteries, as indicated by the arrows. 



lobster (Fig. 118), a quite different contractile organ or heart is 

 located in the dorsal region of the thorax. It is enclosed in a cham- 

 ber, the PERICARDIAL CAVITY, that rcccivcs the blood, which has been 

 oxygenated in the gills, and by a system of one-way valves and 

 muscular contraction and relaxation expels the blood into well- 

 formed blood vessels, the inlet valves closing and the outlet valves 

 opening with each contraction. When it relaxes the appropriate 

 valves open as it again fills. 



The Vertebrate Heart. In the vertebrates the heart is always 

 ventral to the digestive canal. In the fishes it consists of three cham- 

 bers set in series (Fig. 119). The most posterior chamber receives 

 the blood from all regions of the body. The beat or contraction 



