. Elements of biology, with special reference to their rôle in the lives of animals. Biology; Zoology. 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 vessel
. Elements of biology, with special reference to their rôle in the lives of animals. Biology; Zoology. 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. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - colora
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