MECHANISM OF THE HEART. 545 



gives origin to a median vein which runs to the liver the 

 anterior abdominal. By means of an anastomosing branch, 

 the anterior branch of the femoral is also connected to the 

 sciatic. 



The hepatic portal system, by which venous blood from 

 the posterior region and from the gut passes through the 

 liver on its way back to the heart is as follows : 



/"Anterior abdominal vein, from the union of 

 the two pelvics, receiving tributaries from 



Hepatic portal 

 system. 



the bladder, ventral body wall, and trun- 



cus artenosus. 



Hepatic portal vein, from the union of veins 

 from the stomach, intestine, and spleen. 



III. The pulmonary veins which bring back purified 

 blood from the lungs, unite just before they enter the left 

 auricle. 



Lymphatic System. 



The lymph is a colourless fluid, like blood without red 

 corpuscles. It is found in the spaces between the loose skin 

 and the subjacent muscles, in the pleuro-peritoneal cavity in 

 which heart, lungs, and other organs lie, in a sub-vertebral 

 sinus extending along the backbone, and in special lympha- 

 tic vessels which pass fatty materials absorbed from the 

 intestine into the venous system. There are two pairs of 

 contractile " lymph hearts " at two regions where the 

 lymphatic system communicates with the veins. A pair lie 

 posteriorly near the end of the urostyle ; the other two lie 

 between the transverse processes of the third and fourth 

 vertebrae. 



Mechanism of the Heart. 



We must now return to the heart to consider how it is that 

 the blood is propelled from the ventricle along the proper 

 channels. The right half of the ventricle being nearer the 

 right auricle contains more impure blood, and it is from the 

 right side of the ventricle that the truncus artenosus arises. 

 Therefore when the ventricle contracts, the blood which first 

 fills the truncus is venous. It passes along the left side of a 

 median longitudinal valve into the pulmonary arteries along 

 the path of least resistance. As the pulmonary arteries 

 become distended, the next quantum of blood that which 



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