198 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



1, anterior mesenteric artery to front part of intestine, 2, posterior mesenteric 

 artery to back part of intestine, 3, splenic artery to spleen ; (b) 4 to 

 6 small urinogenital arteries which at once bifurcate, and run to urinogenital 

 organs and fat-bodies ; (c) small paired lumbar arteries to adjoining muscles ; 

 (d) a small hwmorrhoidal artery to the large intestine. (3) Each iliac 

 artery gives off (a) a hypoc/astric arteri/ to the bladder, and (b) epigastric 

 arteries to the ventral body- wall. It then becomes the sciatic artery of 

 the thigh, which divides into peroneal and tibial arteries for the leg and 

 foot. 



(3) The pulmo-cutaneous arches carry impure blood to the 

 lungs and skin. Each of them quickly divides into a pulmonary 

 artery running down the outer side of the lung and a cutaneous 

 artery which ramifies in the skin. 



The veins (Fig. 55) bring back blood from the various parts 

 of the body to the heart. They may best be considered under 

 the following headings: (1) Caval system; (2) Portal systems ; 

 (3) Pulmonary veins. 



The caval system consists of three caval veins with their 

 branches i.e., two precavals (anterior or superior venae cavse) in 

 front, and a postcaval (posterior or inferior vena cava) behind. 

 These pour their blood into the sinus venosus, which may be 

 regarded as formed by the fusion of their ends (see Fig. 55). 

 The precaval on each side is formed by the union of three veins 

 (1) The external jugular bringing back blood from the lower 

 jaw, floor of the mouth, and tongue ; (2) The innominate formed 

 by the union of the internal jugular and subscapular veins which 

 respectively return blood from the brain, and shoulder with back 

 of arm ; and (3) The subclavian. This is a large vein made up 

 of the brachial vein from the fore-limb, and the musculo-cutaneous 

 vein, which brings back blood from the muscles and skin of the 

 sides and upper surface of the body and head. 



The postcaval lies ventral to the dorsal aorta. It commences 

 between the kidneys, and is made up by the union of several 

 pairs of renal veins from those organs, and veins from the genital 

 glands and fat bodies. Just before entering the sinus venosus 

 it receives two hepatic veins from the liver. 



Portal Systems. A portal vein is one which, instead of pouring 

 its blood into a larger trunk, breaks up into capillaries within 

 the substance of some organ, supplying it with impure blood. 

 Both kidneys and liver in the frog possess such a supply, and in 

 accordance with this, (1) renal portal and (2) hepatic portal systems 

 can be distinguished. 



