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AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



constructed as described on p. 202. The veins contain numerous 

 valves, which are flap-like projections of their inner coat. 



The capillaries are networks of fine tubes (cf. p. 202), con- 

 necting the ultimate branches of arteries and veins. 



Course of the Circulation (cf. p. 247). The impure blood of 

 the body is returned by the caval veins to the right auricle, 

 thence passing into the right ventricle and into the lungs through 

 the pulmonary artery. The oxygenated blood is returned to the 

 left auricle by the pulmonary veins, then enters the left ventricle 

 to reach the aorta, the branches which take it to all parts of the 

 body. 



(2) Lymph System. The lymph, as in other cases, resembles 

 blood, were it not for the absence of red blood-corpuscles. It is 

 contained in the minute lymph-spaces found in most of the 

 tissues, larger lymph-cavities, and lymph-vessels opening into a 

 thoracic duct. The lymphatic vessels resemble small veins in 

 structure, and are of two kinds (a) lymphatics, belonging to the 

 body generally; (/5) ladeals, belonging to the gut, and com- 

 mencing in blind branched tubules, one of which is found in 



[Fig. 87. MUSCLE-FIBRES FROM MAMMALIAN HEART (from Landois and 

 Stirling), much enlarged. 8, Side view; 9, cross-section. 



each intestinal villus. The thoracic duct, to which the lacteals and 

 most of the larger lymphatic trunks run, is a slender tube lying 

 above the dorsal aorta in the thorax, and opening anteriorly into 

 the junction between the left subclavian and external jugular 

 veins. Some of the lymphatics unite to form a much smaller 

 trunk, opening similarly on the right side. The most important 

 large lymph-space is the ccelom, here divided up into abdominal, 

 pleural, and pericardial cavities. 



