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AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



union of the gastric vein, bringing blood from the stomach, with 

 the lieno-mesenteric vein returning it from the spleen and in- 

 testines. It runs to the left half of the liver, and is connected 

 by a cross-branch with the point of bifurcation of the anterior 

 abdominal. 



A pulmonary vein runs up the inner side of each lung, and 

 unites with its fellow to form a short common trunk, which 

 opens into the left auricle. 



The capillaries are excessively fine tubes forming networks in 

 nearly all parts of the body, by means of which the ultimate 

 branches of veins and arteries are united. A vein commences, an 

 artery ends, and a portal vein both commences and ends, in 

 capillaries. 



(2) Lymph System. The lymph is a colourless fluid which 

 resembles blood, differing, however, in the absence of red cor- 



Fig. 56. POSTERIOR LYMPH-HEARTS or FROG (after Ecker). 

 L, Lymph-heart. 



puscles. The lymph-system consists of lymphatic vessels of larger 

 and smaller size, which ramify in the skin, intestinal wall, and 

 other parts of the body, and lymph-spaces with which these vessels 

 are connected. The lymph-spaces include the irregular lacunae 

 between the tissues, and also larger cavities, such as the pleuro- 

 peritoneal, pericardial, subvertebral, and subcutaneous spaces. 

 The lymphatics of the intestine, which receive the special name 

 of lacteals, traverse the mesentery and open into the subvertebral 

 sinus. The lymph is propelled by two pairs of lymph-hearts, 

 small oval sacs, with rhythmically-contractile walls. The anterior 

 lymph-hearts are situated one on each side between the trans- 



