Circulatory System 



GENERAL RELATIONS OF HEART AND CIRCULATORY CHANNELS 



The heart is always ventral to the alimentary tube and anterior to 

 the abdominal cavity (Fig. 10). It pumps the blood cephalad into 

 arteries which convey it to all parts of the animal (Fig. 64). The 

 arteries give off successively smaller branches which ultimately lead 

 into the narrowest vessels, the capillaries. The caliber of a capillary 

 may be only slightly larger than the diameter of a blood-cell. The 

 capillaries, usually forming a network or plexus, bring the blood into 

 intimate relation to the cells of the various tissues. A certain amount 

 of the fluid part of the blood, the plasma, diffuses through the capil- 

 lary walls into minute intercellular spaces. This escaped plasma 

 brings to each cell the nutriment and oxygen necessary for the cell's 

 existence and picks up the waste substances which are given off by the 

 cell. The intercellular fluid is called lymph. It contains "white blood- 

 cells" (leukocytes) but no red blood-corpuscles (erythrocytes). 



Passing through the capillaries, the blood emerges into small veins 

 which unite to form progressively larger veins. These ultimately con- 

 verge into a relatively small number of main venous trunks which 

 return the blood to the heart, entering at its posterior end. At their 

 region of junction with the heart, these main veins usually join to 

 form a capacious thin-walled venous sinus (Fig. 65) which opens by 

 a single aperture into the posterior chamber of the heart. The sinus 

 is sometimes described as a part of the heart, but in a strict sense it 

 is not. 



The lymph drains away from the intercellular spaces into a system 

 of lymph-vessels which, for the most part, parallel the courses of the 

 veins and finally open into some large vein near the heart, thus return- 

 ing the lymph to the main circulatory channels (Fig. 66). Along the 

 course of the lymphatics are occasional lymph-nodes or "glands" 



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